Dissertations / Theses on the topic 'Frozen dough'

Master of Science
Department of Grain Science and Industry
J.M. Faubion
The market for frozen goods is expanding and the frozen dough goods sector still has potential to expand its market. It is well known that deterioration in bread quality occurs during frozen dough/bread production. In addition, it is known that dough rheology influences bread quality. To prevent deterioration of bread quality, many additives have been used and researched. Combinations of oxidants (potassium bromate and ascorbic acid) are widely used worldwide. However, potassium bromate may be carcinogenic to humans, and it has been detected in bread after baking. Since it has been prohibited or strictly limited in many countries, many researchers have tried to find a replacement. Ascorbic acid is safe for human intake, and does not persist in bread. However, it is not as effective as potassium bromate. Possible replacements in frozen doughs include oxidant (ascorbic acid)-enzyme combinations. This study evaluated the effects of ascorbic acid-specific enzyme combinations as a replacement for the potassium bromate in frozen dough and related the effects to dough behavior (gluten network strength) as evaluated by dynamic oscillation rheometry. Bread quality was evaluated by test baking. Based on the results from fresh baking studies, potassium bromate can be replaced by an optimum level combination of ascorbic acid and hemicellulase/endo-xylanase. This combination clearly improved loaf volume, and crumb grain over both control and potassium bromate containing doughs. For frozen dough/bread production, the addition of all additives improved bread quality, but ascorbic acid and endo-xylanase containing dough resulted in higher volume, and better crumb structure than did dough containing potassium bromate. Dough rheology experiments show that rheology was affected by both the process and additives. Strain sweeps gave the information about dough stability. Both the additives and proofing improved dough stability. Dough behavior (gluten network strength) was assessed by frequency sweeps. Dough containing ascorbic acid and endoxylanase was most stable during frozen dough processing.

CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior
A tÃcnica de glazeamento à utilizada em produtos de confeitaria e pÃes doces, com a finalidade de promover a melhoria das caracterÃsticas externas do produto. O estudo teve como objetivo desenvolver soluÃÃes de glazeamento compostas de matÃrias-primas nÃo-convencionais como Ãleos vegetais, vegetais em pà e subprodutos do processamento de frutas e avaliar o processo de obtenÃÃo dos pÃes tipo forma atravÃs da tecnologia de massas congeladas verificando o efeito da estocagem por 60 dias nos parÃmetros de qualidade dos pÃes. As soluÃÃes de glazeamento contendo Ãleos vegetais foram: Ãleo de algodÃo/Ãgua, Ãleo de canola/Ãgua, Ãleo de girassol/Ãgua; Ãleo de milho/Ãgua e Ãleo de soja/Ãgua. As soluÃÃes de glazeamento contendo vegetais em pà foram: brÃcolis em pÃ/Ãgua; espinafre em pÃ/Ãgua; palmito em pÃ/Ãgua e tomate em pÃ/Ãgua. As soluÃÃes de glazeamento contendo subprodutos (cascas) de processamento de frutas foram: subproduto de abacaxi/polpa de abacaxi; subproduto de caju/polpa de caju; subproduto de goiaba/polpa de goiaba e subproduto de manga/polpa de manga. Cada uma destas soluÃÃes foi desenvolvida atravÃs de um Delineamento Composto Central Rotacional (DCCR) totalizando 11 ensaios, com trÃs repetiÃÃes no ponto central. A metodologia estatÃstica aplicada consistiu em anÃlise descritiva dos dados, testes de normalidade, anÃlise de variÃncia (ANOVA) e nÃo-paramÃtrica, teste de mÃdia Tukey, anÃlise de regressÃo mÃltipla, anÃlise de correlaÃÃo e superfÃcies de resposta. Os efeitos do perÃodo de estocagem congelada (0, 15, 30, 45 e 60 dias) foram avaliados sobre os parÃmetros fÃsicos de qualidade das massas. Para as soluÃÃes contendo Ãleos vegetais e Ãgua verificou-se que o maior volume obtido foi pela soluÃÃo de Ãleo de algodÃo e Ãgua com 4,23 mL/g, seguido da soluÃÃo com Ãleo de milho/Ãgua (4,03 mL/g). Atribuiu-se a menor viscosidade destes Ãleos a nÃo-formaÃÃo de pelÃcula externa à massa, nÃo prejudicando as transferÃncias de calor e massa durante a fermentaÃÃo e forneamento. O Ãndice de expansÃo foi superior a 2,00 para todas as soluÃÃes com exceÃÃo das compostas por Ãleo de canola/Ãgua e Ãleo de girassol/Ãgua. Com relaÃÃo Ãs soluÃÃes contendo vegetais em pà e Ãgua, observou-se que os volumes especÃficos, de modo geral, foram superiores aos obtidos pelas soluÃÃes adicionadas de Ãleo. Os vegetais em pà podem fornecer micronutrientes para as leveduras e melhorar o processo fermentativo, resultado devido ao maior nÃmero de alvÃolos apresentado, tendo obtido valores acima de 1.000 alvÃolos para todos os vegetais, exceto o palmito, portanto, apresentaram miolos com boa porosidade. Verificou-se que as soluÃÃes de glazeamento contendo subprodutos e polpa de frutas promoveram os menores volumes especÃficos para os pÃes, onde nenhum ensaio apresentou valores maiores que 4,00 mL/g. Este resultado pode estar relacionado com o pH Ãcido das frutas e a presenÃa de enzimas proteolÃticas que podem danificar a estrutura do glÃten, reduzindo a capacidade de retenÃÃo gasosa ou prejudicar a atividade das leveduras. AtravÃs das superfÃcies de resposta e modelos matemÃticos gerados foi possÃvel selecionar duas soluÃÃes de glazeamento para cada tratamento, totalizando 26 soluÃÃes que foram utilizadas em massas congeladas por atà 60 dias, onde foi avaliado o seu potencial para a manutenÃÃo da qualidade. Verificou-se que a estocagem congelada das massas reduziu o volume especÃfico, Ãndice e fator de expansÃo, pH, umidade total e volume produzido. Entretanto, as soluÃÃes de glazeamento, de modo geral, promoveram a reduÃÃo no descrÃscimo da qualidade ao longo do tempo de estocagem, quando comparados Ãs massas controle, ausentes da aplicaÃÃo das soluÃÃes de glazeamento. A aplicaÃÃo de soluÃÃes de glazeamento em massas congeladas para a produÃÃo de pÃes tipo forma, portanto, consiste em uma tecnologia que pode ser utilizada em processos industriais pois demonstrou eficÃcia na manutenÃÃo da qualidade das massas apÃs 60 dias de estocagem congelada, alÃm poder contribuir para a melhoria da qualidade nutricional dos produtos.
The glazing technique is used in bakery products and confectionery, in order to promote the improvement of the external characteristics of the product. The objective of this study was to develop composite glazing solutions of unconventional raw materials like vegetable oils, powdered vegetables and by-products of fruit processing through statistical tool Central Composite Rotational Design (CCRD) and study the process of getting the bread through the frozen dough technology evaluating the effect of storage for 60 days in the quality of breads parameters. The glaze solutions containing vegetable oils are: cottonseed oil/water, canola oil/water, sunflower oil/water; corn oil/water and soy oil/water. The glaze solutions containing powdered vegetables were broccoli powder/water; Spinach powder/water; palmito powder/water and tomato powder/water. The glazing solutions containing fruit processing by-products were byproduct of pineapple/pineapple pulp; byproduct of cashew/pulp cashew; Guava byproduct/guava pulp and byproduct mango/mango pulp. Each of these solutions was developed through a CCRD total of 11 samples with three replications at the center point. The applied statistical methodology consisted of descriptive data analysis, normality tests, analysis of variance (ANOVA) and nonparametric, average Tukey test, multiple regression analysis, correlation analysis and response surfaces, we used these tests to check the effect of different glazing solutions on the quality of physical parameters - specific volume, density, expansion factor of the dough, expansion index of breads, volume produced during fermentation, number and circularity of alveoli - to establish the conditions great to hold each of the study with frozen dough. The effects of frozen storage time (0, 15, 30, 45 and 60 days) were evaluated by the aforementioned parameters, and the total water content of the dough, acidity and pH. For solutions containing vegetable oils and water has been found that the greater volume was obtained by cotton seed oil and water solution with 4.23 mL/ g, followed by the solution of corn oil/water (4.03 mL/g) This result is related to the lower viscosity of these oils, attributing to non-formation of outer film to the dough and does not impair the heat and mass transfer during fermentation and baking. The expansion ratio was greater than 2.00 for all solutions except those consisting of canola oil/water and sunflower oil/water. With respect to solutions containing vegetable powder and water, it was observed that the specific volumes generally were higher than those obtained by the added oil solutions. In this case, the vegetable powder can provide micronutrients for the yeast and improve the fermentation process, results related to the higher number of wells shown (above 1000 for all vegetables except palmito) therefore presents brains with good porosity. It was found that the glazing solutions containing by-products and fruit pulp promoted, generally smaller specific volumes for the loaves, where no test showed values above 4.00 mL/g, this result may be related to the pH acid fruits and the presence of proteolytic enzymes that can damage the gluten structure and, consequently, reduce the gas retaining capacity, or impair the activity of the yeast. Therefore, before the results, we selected two glazing solutions for each treatment, totaling 26 glazing solutions to be used in frozen dough for up to 60 days and evaluate its potential as a quality preserving agent. Through the response surface and mathematical models generated it was possible to select two glazing solutions for each treatment, totaling 26 solutions that were used in frozen dough for up to 60 days, which was evaluated its potential as a quality preserving agent. It was found that frozen storage of the dough reduced the specific volume, expansion index, expansion factor pH, moisture and volume produced. However, the glazing solutions generally have promoted the reduction in the quality descrÃscimo over storage time, when compared to mass control, absent the application of glazing solutions. Application of glazing solutions frozen dough for producing bread-like form thus consists of a technology that can be used in industrial processes as it has demonstrated its effectiveness in maintaining the quality of the pasta after 60 days of frozen storage, in addition to contribute to improving the nutritional quality of products.

Fiber in frozen bread dough is thought to reduce the loss of yeast vitality and improve nutrition and quality of bread. The objective of this study was to determine the applicability of flaxseed fiber in frozen bread dough. Fiber was extracted from two sources of brown flaxseed and added to bread formulas at 0%, 1% and 3%. Dough was blast chilled and kept frozen until thawed at scheduled intervals. Thawed dough was proofed, baked into bread. Bread was tested analytically and by a sensory panel. Loss of overall quality was observed overtime. Breads containing flaxseed fiber had significantly larger loaf volumes (cc) and reduced firmness (g). A trained sensory panel detected significantly lower crumb firmness and stale flavors in breads containing flaxseed fiber. These results suggest flaxseed fiber has the potential to improve the perceived quality of frozen bread dough.

O conhecimento de propriedades termofísicas é importante na modelagem, na otimização e no projeto de equipamentos para processos como a fabricação de massa de pão congelada. Neste trabalho, as propriedades termofísicas condutividade térmica, difusividade térmica e calor específico foram determinadas experimentalmente ao longo do processo de congelamento da massa de pão, pois influenciam diretamente a cinética do processo que, por sua vez, determina a qualidade do pão produzido. Os resultados obtidos mostraram que algumas destas propriedades variaram em função do conteúdo inicial de água e principalmente devido à mudança da temperatura. O valor médio encontrado através da atividade de água das massas (aw = 0,950), para temperatura inicial de congelamento, pode explicar alguns desvios no ajuste dos dados experimentais a modelos disponíveis na literatura baseados na lei de Raoult, que são estritamente relacionados à fração de gelo formada. O uso combinado de goma guar e xantana reduziu a entalpia de fusão ao longo do tempo de armazenamento congelado (87,4 J.g-1 e 81,2 J.g-1 para a formulação sem gomas, não congelada e após 28 dias de armazenamento congelado; 84,4 J.g-1 e 76,9 J.g-1 para a formulação com 0,25% de goma guar e 0,25% de goma xantana na massa não congelada e após 28 dias de armazenamento congelado), indicando uma interação entre as gomas na capacidade de ligar água ao longo do tempo de congelamento. Porém, apesar do tempo de congelamento ter reduzido a entalpia de fusão, o processo de congelamento foi o principal fator na queda da taxa de fermentação e da produção de gás pelas leveduras independentemente do tempo de armazenamento congelado.
The knowledge of thermophysical properties is important to model and optimize equipment design for frozen food processes. In this work, the thermophysical bread dough properties, such as, thermal conductivity, thermal diffusivity and specific heat were experimentally determined during freezing process, due to their influence on the kinetics process, and therefore on bread quality. The results showed that some properties varied according to the initial water content and mainly due to temperature changes. The average value of initial freezing temperature found by water activity (aw = 0.950) explains the deviations between experimental data and available models in the literature based on Raoults law, which is related to the ice mass fraction. The combined use of guar and xanthan gums reduced the fusion enthalpy along frozen storage time (87.4 J.g-1 and 81.2 J.g-1 for formulation without gum, non-frozen and after 28 frozen storage days; 84.4 J.g-1 and 76.9 J.g-1 for formulation with 0.25 % of guar gum and 0.25 % of xanthan gum in non-frozen and after 28 frozen storage days). It indicated the interaction between gums and their ability to hold water along frozen storage. Despite frozen storage reduced the fusion enthalpy, the freezing process itself was the main factor in the decrease in proofing rate and gas production by yeast cells.

A qualidade global da massa de pão congelada por um longo período é um desafio para indústria de panificação. O uso de diferentes técnicas pode ajudar a explicar os danos sofridos à massa de pão durante o congelamento e armazenamento congelado. A presença de água na forma de cristais de gelo foi vista como a principal causa de defeitos à estrutura da massa. Gomas guar e xantana foram incorporadas às massas para contornar os danos causados pelo congelamento. Das análises térmicas conduzidas em calorímetro de varredura diferencial foi possível quantificar a quantidade de gelo presente na massa. A adição de (0,125 a 0,250) g/100g de goma guar e de (0,214 e 0,250) g/100g de goma xantana diminuíram os valores de entalpia na massa ao longo do armazenamento congelado. A fração de gelo foi menor e mais estável nas massas contendo (0,125 e 0,250) g/100g de goma guar ou xantana. O valor da atividade de água diminuiu nas massas contendo goma guar e com níveis mais altos de goma xantana, após o ciclo de congelamento e descongelamento e 170 dias de armazenamento. Alterações na estrutura das massas foram medidas e visualizadas pela análise de textura e por microscopia eletrônica de varredura (MEV). Adição de goma xantana e principalmente de goma guar às massas aumentaram a resistência à extensão após 170 dias de armazenamento congelado. A extensibilidade não foi influenciada pela adição de gomas e manteve estabilidade nos períodos de armazenamento estudados. A MEV ajudou a explicar os danos causados à rede de glúten pelos cristais de gelo ao longo do armazenamento congelado. A massa sem gomas apareceu com pequenos danos na rede de glúten com 21 dias de armazenamento e estes danos aumentaram progressivamente por até 170 dias, mostrando um glúten menos contínuo, mais rompido e separado dos grânulos de amido. As massas contendo gomas minimizaram, mas não evitaram os danos causados ao longo do armazenamento congelado.
The global quality of the frozen bread dough for long periods of frozen storage is the challenge to bakery industry. The use of different techniques could help to explain the damages caused in the frozen dough during the freezing and along frozen storage time. The presence of water as ice crystals was the main cause of damage on the dough structure. In order to minimize the freezing damages, guar and xanthan gums were incorporated in the dough. From thermal analysis by DSC technique, the amount of ice present in the dough was determined. The addition of (0.125 to 0.250) g/100g guar gum and (0.214 and 0.250) g/100g xanthan gum presented lower values of fusion enthalpy. The addition of (0.125 and 0.250) g/100g of guar gum decreased or stabilized the frozen water content, suggesting minimal damage on the dough structure along frozen storage time. The water activity in the dough samples with guar gum and higher quantities of xanthan gum decreased after the freezing-thawed cycle and after a period of frozen storage of 170 days. The incorporation of xanthan gum and mainly the incorporation of guar gum increased the maximum resistance of the dough after 170 days. The extensibility was not affected by addition of gums along frozen storage time. The MEV technical helped to explain the damaged dough structure caused by ice crystals along frozen storage time. Dough samples without gums presented structure damage only after 21 days and increased after 170 days, resulting in less continuous gluten, more disrupted and separated from starch granules. The doughs with gums suffered less damage in the gluten matrix, but did not avoid the problems caused by frozen storage.

Савремено пекарство у нашој земљи, још увек, у највећој мери подразумева производњу хлеба и пекарских производа in situ. Припрема пекарских производа у кућним условима своди се, готово искључиво, на мешење према традиционалноммоделу. Овакво понашање пекара и потрошача делимично је последица навика, али и изузетно лоше снабдевености нашег тржишта замрзнутим пекарским производима добијеним од квасног теста, као и сировинама намењеним запроизводњу истих.Циљ истраживања обухваћених докторском дисертацијом усмерен је ка дефинисању и унапређењу услова производње и финализације замрзнутих пекарских производа непосредно пред употребу, у близини корисника или од стране самог корисника. Добијени резултати требало би да дају допринос бољем разумевању феномена који се дешавају у поступкузамрзавања/одмрзавања теста. Модификација постојеће, код нас примењиване технологије, у смислу примене одабраних сојева квасца, одабраног режима замрзавања/одмрзавања и дужине чувања теста у замрзнутом стању, као и примене одабраних додатака тесту, требало би да резултује пекарским производима добијеним од замрзаваног теста чије су технолошке и сензорне карактеристике побољшане.Испитан је сировински састав теста и квалитет употребљених сировина карактеристичан за наше поднебље са аспекта њихове примене у производњи замрзаваног теста, о чему нема публикованих резултата. Доказана је могућност употребе комерцијалног пекарског квасца и комерцијалног брашна које по садржају и квалитету глутена не одговара у потпуности литературним препорукама.Предложене су математичке релације које омогућавају предвиђање трајања операција замрзавања и одмрзавања квасног теста у дефинисаним условима који су уобичајени у пекарској производњи, о чему нема литературних података.Дефинисани су оптимални услови замрзавања и одмрзавања квасног теста који омогућавају очување активности квасца у замрзаваном тесту на прихватљивом нивоу.На основу активности квасца у замрзаваном тесту са хидроколоидима, матурографских и екстензограмских показатеља квалитета као и његове микроструктуре доказан је позитиван утицај додатка хидроколоида у замрзавано тесто са аспекта његовог квалитета. Јасно изражене промене микроструктуре замрзнутог теста током његовог складиштења доприносе разумевању феномена који се дешавају у тесту услед његовог замрзавања, одмрзавања и складиштења у замрзнутом стању до 28 дана, као и интеракција између конституента теста и додатих хидроколоида.Доказано да је за припрему замрзаваног теста могуће користити сировине чији квалитет није оптималан, али да се тимезначајно скраћује период складиштења у коме не долази до неприхватљиве деградације његовог квалитета са неколико седмица колико препоручује литература, на свега 7 до 14 дана. Квалитет готовог пекарског производа добијеног од замрзаваног теста које се састоји од сировина уобичајено присутнихна нашем тржишту, а које је припремљено уз предложене минималне модификације традиционалног начина припреме хлеба, задовољавајући је из угла конзумента.
Savremeno pekarstvo u našoj zemlji, još uvek, u najvećoj meri podrazumeva proizvodnju hleba i pekarskih proizvoda in situ. Priprema pekarskih proizvoda u kućnim uslovima svodi se, gotovo isključivo, na mešenje prema tradicionalnommodelu. Ovakvo ponašanje pekara i potrošača delimično je posledica navika, ali i izuzetno loše snabdevenosti našeg tržišta zamrznutim pekarskim proizvodima dobijenim od kvasnog testa, kao i sirovinama namenjenim zaproizvodnju istih.Cilj istraživanja obuhvaćenih doktorskom disertacijom usmeren je ka definisanju i unapređenju uslova proizvodnje i finalizacije zamrznutih pekarskih proizvoda neposredno pred upotrebu, u blizini korisnika ili od strane samog korisnika. Dobijeni rezultati trebalo bi da daju doprinos boljem razumevanju fenomena koji se dešavaju u postupkuzamrzavanja/odmrzavanja testa. Modifikacija postojeće, kod nas primenjivane tehnologije, u smislu primene odabranih sojeva kvasca, odabranog režima zamrzavanja/odmrzavanja i dužine čuvanja testa u zamrznutom stanju, kao i primene odabranih dodataka testu, trebalo bi da rezultuje pekarskim proizvodima dobijenim od zamrzavanog testa čije su tehnološke i senzorne karakteristike poboljšane.Ispitan je sirovinski sastav testa i kvalitet upotrebljenih sirovina karakterističan za naše podneblje sa aspekta njihove primene u proizvodnji zamrzavanog testa, o čemu nema publikovanih rezultata. Dokazana je mogućnost upotrebe komercijalnog pekarskog kvasca i komercijalnog brašna koje po sadržaju i kvalitetu glutena ne odgovara u potpunosti literaturnim preporukama.Predložene su matematičke relacije koje omogućavaju predviđanje trajanja operacija zamrzavanja i odmrzavanja kvasnog testa u definisanim uslovima koji su uobičajeni u pekarskoj proizvodnji, o čemu nema literaturnih podataka.Definisani su optimalni uslovi zamrzavanja i odmrzavanja kvasnog testa koji omogućavaju očuvanje aktivnosti kvasca u zamrzavanom testu na prihvatljivom nivou.Na osnovu aktivnosti kvasca u zamrzavanom testu sa hidrokoloidima, maturografskih i ekstenzogramskih pokazatelja kvaliteta kao i njegove mikrostrukture dokazan je pozitivan uticaj dodatka hidrokoloida u zamrzavano testo sa aspekta njegovog kvaliteta. Jasno izražene promene mikrostrukture zamrznutog testa tokom njegovog skladištenja doprinose razumevanju fenomena koji se dešavaju u testu usled njegovog zamrzavanja, odmrzavanja i skladištenja u zamrznutom stanju do 28 dana, kao i interakcija između konstituenta testa i dodatih hidrokoloida.Dokazano da je za pripremu zamrzavanog testa moguće koristiti sirovine čiji kvalitet nije optimalan, ali da se timeznačajno skraćuje period skladištenja u kome ne dolazi do neprihvatljive degradacije njegovog kvaliteta sa nekoliko sedmica koliko preporučuje literatura, na svega 7 do 14 dana. Kvalitet gotovog pekarskog proizvoda dobijenog od zamrzavanog testa koje se sastoji od sirovina uobičajeno prisutnihna našem tržištu, a koje je pripremljeno uz predložene minimalne modifikacije tradicionalnog načina pripreme hleba, zadovoljavajući je iz ugla konzumenta.
Currently the baker's industry in our country is based on the production of bread and bakery products in situ. ln domestic conditions the preparation of bakery products is done solely using the traditional model. This is a consequence of baker and consumer habit in our country as well as the poor market supplies of frozen bakery products from yeast dough and raw material for this production.The aim of research for this dissertation is based on the definition and improvement of the production conditions as well as the flnalization of frozen bakery products prior to use. The results should contribute to the understanding of phenomena that exist in the process of freezing/thawing of dough. In order to preserve the technology commonlyused in our country, minor modifications were made using selected yeast types, selected regime of freezing/tharving, frozen dough storage time and additives to dough. These minor changes should result in the improvement of technological and sensor characteristics for baker products obtained from frozen dough. The raw material structure in dough and the quality of used raw material were analyzed from the aspect of the frozen dough production where there is nothing published upon this subject yet. The possibility of the use of commercial baker yeast and flour was proven even though their content and quality of gluten does not correspond to the literature recommendation.Mathematical relations that enable prediction of freezing/thawing of yeast dough operation durance in defined conditions were suggested and has no published results upon this subject yet. Optimal conditions of freezing/thawing yeast dough that preserves the activity of yeast in frozen dough on an accepted level were defined.On the basis of the yeast activity in frozen dough with hydrocolloids, the rheological characteristics and its microstructure, the positive affect of hydrocolloid addition to frozen dough was proven from its quality aspect. Major changes in microstructure of frozen dough during its storage contribute to the understanding of the phenomena thatappear in dough during freezing, thawing and storage in frozen state up to 28 days, as well as the interaction between the dough constituents and added hydrocollcids.The possibility of the use of raw material for the dough preparation that has not the optimal quality but shortens the storage time was proven. During this storage time the degradation of its quality does not appear as reconlmended by literature in the period of few weeks but in the period of 7 to 14 days.From the consumerrs point of view the quality of finished bakery product derived from frozen dough with available raw material and with minor modifications of the traditional method of preparation was satisfactorily.

Les pâtes surgelées sont relativement stables et peuvent être fabriquées à l’échelle industrielle, distribuées et cuites à la demande au moment de la vente ou de la consommation (point chaud). La congélation des pâtes sucrées induit une baisse de volume et une augmentation du temps de fermentation, ces conséquences sont dues à deux facteurs : la baisse de la production de CO2 (viabilité des levures) et la faible capacité de rétention de gaz du réseau gluténique. La perte de la qualité des pâtes congelées est accélérée durant le stockage. Cette thèse porte sur l’étude de l’effet de la congélation et de la conservation sur les levures et les propriétés techno-fonctionnelles des pâtes sucrées type Kougelhopf. Ce travail vise à l’étude de l’impact de la vitesse de congélation sur les propriétés microbiologiques, rhéologiques, structurales et sensorielles de ces pâtes. Elles ont été congelées à différentes températures (-20 °C, -30 °C, -40 °C et une immersion dans l'azote liquide) puis conservées à -40 °C pendant 9 semaines. Les principaux résultats de cette étude ont permis de mettre en évidence le rôle de la vitesse de congélation et de la durée de conservation sur les propriétés intrinsèques des pâtes sucrées surgelées. Il en découle que l’activité fermentaire et l’intégrité du réseau du gluten sont tributaire de la vitesse de congélation. En effet, cette dernière contrôle la taille et la localisation des cristaux de glace d’où la recherche d’un compromis entre une vitesse de congélation ni trop rapide pour diminuer la viabilité des levures ni trop lente pour former de gros cristaux pouvant perforer le réseau de gluten de la pâte. Ce travail a démontré que le surdosage de levure reste valable uniquement pour les pâtes sucrées surgelées destinées à être conservées au-delà de 4 semaines. Ce surdosage améliore ainsi la qualité globale du Kougelhopf en compensant la perte de l'activité des levures pendant la congélation et le stockage
The frozen doughs are relatively stable and can be manufactured on an industrial scale, distributed and baked on demand at the point of sale or consumption (Bake-off). Freezing sweet dough induces a decrease in specific volume and an increase in fermentation time, these effects are due to two factors: lower production of CO2 (yeast viability) and losing capacity to retain gas (gluten network integrity). The loss of quality of frozen dough is accelerated during storage. This study focuses on the freezing and frozen storage effects on Kougelhopf sweet doughs. The aim of this work is to study the impact of freezing rate on microbiological, rheological, structural, and sensory properties of sweet doughs. The sweet doughs were frozen at different temperatures (-20°C, -30°C, -40°C and an immersion in liquid nitrogen) and stored at -40°C for 9 weeks. The main results obtained showed an impact of freezing rate and frozen storage duration on the frozen doughs intrinsic properties. This study shown the dependence of fermentation activity and integrity of the gluten network with freezing rate, which controls size and location of ice crystals resulting in research of a compromise between freezing rate nor too fast to reduce yeast viability, nor too slow to form large ice crystals that could perforate gluten network. Added the yeast amount is necessary only for frozen sweet doughs to be stored beyond 4 weeks, which improves the overall quality of Kougelhopf by compensating for yeast activity decrease during freezing and frozen storage

碩士
國立臺灣海洋大學
食品科學系
95
Frozen dough was accepted by businessman who had been investing into the baking food industry. Owing to the shortage of manpower and well training technologist, difficult to control the quality of products and the required quantity increase of market day by day, it grows up sharply in recent two decades. However, how to offer a series and steady attractive products, the freezing and thawing technology of dough became importance and cannot be neglected. In order to understand the baking quality of bread made from frozen dough, four formulas of Toast, Sweet, French and Danish bread dough were prepared with different temperature condition of freezing, frozen storage and thawing and then baked, respectively, in the studies. The bread quality was monitored by fermented dough volume; firmness, height, volume, density and organoleptic estimation of baked bread. From results, reducing freezing burn and controlling quality easily of bread was found in quick freezing at -40oC, 30 min. The Sweet dough stored at -25oC had a good dough volume, the shortest time of fermentation and the best quality of bread after baking. Comparing the fermentation time of thawing condition, frozen dough thawing at 25oC, 60 min and un-thawing was 130 and 170 min, respectively, the former was shorter than the later. Meanwhile, the former also have a good softness of bread. The fermented dough volume of Toast, Sweet, French and Danish bread were found gradually decrease with their dough frozen storage time extension. However, the organoleptic estimation, taste feel and firmness of baked bread were similar each other. Four kinds of frozen dough were thawing and fermentation at 25, 3 and 0oC, respectively, and qualified its’ bread quality after baking. There were no significant difference in texture, firmness and color between all frozen dough after one month storage at -25oC and its’ fresh dough. After six months frozen storage at -25oC, the quality of baked bread could gain near it from fresh dough were found thawing at 25oC only. According to this study, the quality of bread made from frozen dough with thawing at 25oC, 30 min was closely to the fresh dough. There are no significant effect on the appearance of Danish bread during frozen storage, but need more time for fermentation with extending storage time

碩士
輔仁大學
餐旅管理學系碩士班
95
Abstract The aim of this study is to apply the frozen dough manufacturing technology to the Chinese steamed bread in order to increase production capacity, maintain stable quality of the products, and offer the traditional Chinese flour food with extra quality from the modern food processing technology. The primary concern of this research work was the production of the Chinese steamed bread dough through the frozen dough manufacturing process. The prescriptive production process adopts the 3x2x2 multi-factors experimental designs and the control factors and levels will be derived from the experimental processes. The compositions of the frozen dough consist of 1% instant yeast, 3% compressed yeast, 3% frozen yeast, 0.1% and 0.15 xanthan gum, 10% and 20% sponge. The manufactured dough pieces were packed and kept frozen in the freezer at -18˚C. According to sequential time intervals, dough pieces were sampled after 0, 14, 28, 42 days of frozen storage. After being removed from the freezer, completely thawed, fermented and steamed, they were made into steamed bread product. The present study contains two parts: The first part focused on the analyses of the frozen dough properties including dough proof time, yeast survival rate count, rate of dough expansion and dough color. The results indicated that as the frozen storage time increased the dough proof time increased significantly (p<0.05), the yeast survival rate count decreased significantly (p<0.05), and so did the rate of dough expansion (p<0.05). The shortest proof time of dough with 1% instant yeast frozen stored for 0 day was 86±3 minutes. The yeast survival rate count was low and there were no significant differences in dough expansion rate with 3% compressed yeast, 3% frozen yeast and in the three varieties yeast in dough color. The addition of xanthan gum of 0.1%, 0.15% and sponge of 10%, 20% also showed no significant differences in dough proof time, yeast survival rate count, rate of dough expansion and dough color. The second part of the study focused on the quality analyses of the Chinese steamed bread product which includes its specific volume, texture profile analysis, and sensory evaluation. In terms of specific volume, those made from dough with 3% frozen yeast had higher specific volume. In correlation analysis, the specific volume of the steamed bread was significantly negatively related to the storage days (p<0.01). In regards to the TPA test of the steamed bread, an increase in the number of days of storage time of the frozen dough resulted in significant increases in the hardness of the steamed bread (p<0.05), but revealed no significant differences with springiness, cohesiveness, gumminess and chewiness. The addition of 0.1% and 0.15% of Xantham gum, 10% and 20% of sponge revealed no significant differences in relation to texture profile analysis. The number of frozen days of the dough was significantly negatively related (p<0.05) to the springiness, cohesiveness, gumminess and chewiness of the steamed bread. The varieties of yeast was significantly positively related (p<0.05) to the color, appearance, texture, and overall acceptance of the steamed bread. The addition of sponge was also significantly positively related (p<0.05) to the texture of the steamed bread. Dough with 3% frozen yeast revealed a higher sensory evaluation score. Overall, the present experiment revealed that 3% frozen yeast and 0.1% xanthan gum with 10% sponge added was the best prescription for making good quality frozen dough of Chinese streamed bread.

碩士
大葉大學
生物產業科技學系碩士在職專班
93
The objectives were to evaluate effect of adding curdlan on frozen dough and bread quality. Adding different percentage of curdlan (0、0.5、1 and 2 %) on dough stored at -20 ℃ and -40 ℃ with various storage time (0、1、10、20、30、45 and 60). At certain intervals, dough was taken out and made bread for analysis. There were two parts in the research. The first part was to evaluate frozen dough affected by addition of curdlan. The second part was to analyze quality of bread with various storage time. The first part of results showed properties of frozen dough. The minimum proof time of dough with 0.5 % curdlan was 30.2±1.9 minutes. The highest yeast survival rate was 91.5±0.7 % which dough stored at -20 ℃、1 day. The dough stored at -40 ℃ had longer proof time and lower yeast survival rate than those stored at -20 ℃. Proof time significantly increased and yeast survival rate significantly decreased (p<0.05) with increasing storage time. Dough with 2 % curdlan stored at -40 ℃、1 day had the highest water activity, but the highest hardness until storage time of 30 days. Water activity and hardness of dough stored at -40 ℃ were higher than those stored at -20 ℃. Hardness of dough significantly increased (p<0.01) with increasing storage time. The second part of the results stated out analysis of bread quality. Bread with 1 % curdlan stored at -20 ℃、1 day had the highest specific volume (0.97±0.02 cm3/g). The specific volume of bread that dough stored at -20 ℃ was higher than those of stored at -40 ℃ and significantly decreased (p<0.01) with longer storage time. Water activity of bread that dough with various percentage stored at -40 ℃ was higher (p<0.01) than those stored at -20 ℃. In shelf-life analysis of bread, bread made form dough with 2 % curdlan stored at -40 ℃、1 day had the highest moisture percentage in one-day shelf-life. Bread made form dough stored at -40 ℃ got higher content of moisture than those stored at -20 ℃. Moisture content of bread were significantly different with various store time. In TPA analysis, bread made form dough stored at -40 ℃ had higher hardness than those stored at -20 ℃. Hardness of bread increased significantly with increasing stored time (p<0.01). There was the lowest total plate count and mold count in bread with 2 % curdlan. According to the results, dough with 0.5 % curdlan stored at -20 ℃ within 45 days had the shortest proof time , the lowest hardness and the highest yeast survival rate. Bread made from those dough would have the highest specific volume. It was good for longer shelf-life and fluffy texture bread. Dough with 2 % curdlan stored at -40 ℃ could increase water activity of dough, and moisture content and shelf-life of bread. It was good for longer shelf-life and more elastic texture bread.

碩士
國立臺灣海洋大學
食品科學系
94
The effects of yeasts and anti-frozen reagents on the qualities of frozen dough and breads made from the dough were investigated. The optimal frozen dough recipe and processing conditions for high quality bread making were also studied. Four kinds of yeasts including fresh yeast, Mauripan instant dry yeast, Bruggemen instant yeast and LHIS dry yeast were used at the level of 1.1% (flour basis) for making frozen dough. After frozen storage for 1, 2 and 3 weeks, the dough was competently thawed, fermented, and baked to make bread. The results of gas-production in dough showed that the product of LHIS dry yeast was the highest among four kinds of yeasts. Mauripan instant dry yeast was in the second order. The dough added with LHIS dry yeast had the lowest value of bread-density, and that of Mauripan instant dry yeast was in the second order. The cost of LHIS dry yeast was much higher than other yeasts; therefore, Mauripan was feasible for using in frozen dough. Four kinds of anti-frozen reagents including DONQU, S-Kimo 500 Long, Eisbar and Sofical Super were used to make frozen dough. After frozen storage for three weeks, the dough with DONQU had the highest volume in gas production and its bread had the lowest density. To control final temperature of dough under 20℃, the yeast powder was dissolved in water with half of ice. Under this condition, dough’s volume of 118 ml was the highest at 60 minutes. And final fermentation time was the shortest at 50 minutes during bread making. Taguchi’s experimental design method was used to obtain suitable formula. There was no significant difference in frozen dough gas-production and bread density between the concentrations of 1.1% and 1.4% of yeast added. Therefore, 1.1% of yeast was feasible for using in frozen dough. In order to control the dough temperature lower than 20℃, the suitable amount of ice added was 12%, and the amount of water in total could be reached 15%. More vita-gluten was added, the bread texture was better. The proposed amount of gluten added was 3%. The suitable amounts of anti-frozen reagents and sugar added in frozen dough were 2.5% and 20%, respectively. There was no significant difference in bread quality as shortening was added in dough at the levels of 6%, 8% and 10%. The addition of salt with 0.4%, 0.6% and 0.8% also showed no significant difference in bread quality. As the temperature difference between frozen dough and thawing was bigger, the surface condensed water on frozen dough would form more. The proposed condition for thawing temperature was 4℃ and the relative humility was 50% RH. As thawing time was longer, less condensed water was formed. The results indicated that the thawing time could be set at 16 hours, and the final fermentation time was 80 minutes.

碩士
國立高雄餐旅大學
餐飲創新研發碩士學位學程
102
In this research, Make the yeast soaked in 7% salt solution and 7% glycerol solution for 40 minutes, and lead it to produce glycerol. Finally investigate the influence of these two solutions on the process of making frozen dough and its quality. The result in final fermentation time phase of this research indicates that the salt-stressed and glycerol-stressed baker''s yeast in frozen dough developed better than controlled group in the thirtieth day.The glycerol-stressed one developed best than the others in the sixtieth day and the ninetieth day.Yet, the glycerol-stressed one had the hugest specific volume in the sixtieth day and ninetieth day. Regarding the bread softness, the glycerol-stressed had the softest condition in the first day and the thirtieth day.In addition, about the rate of volume expansion, gas production and length of toast, the salt-stressed and glycerol-stressed ones appeared better than controlled group. In this research, by this brand-new method, the sensory evaluation of toast made from salt-stressed and glycerol-stressed baker''s yeast in frozen dough is better than controlled group. Therefore, the natural glycerol from stressed yeast could improve the process of making frozen dough and its quality. Moreover, consumers are even more attaching importance to select the safe and high quality food. Thus, replacing the food additives with natural glycerol from stressed yeast is appropriate for the pursuit of natural, healthy and non-food additives trend.

Tese de doutoramento em Ciências (ramo de conhecimento em Biologia)
Bread is a central dietary item in most countries of the world. Currently, frozen dough technology is extensively used in the baking industry to supply oven-fresh bakery products to consumers and to improve labor conditions for bakers. Since freeze–thaw stress affects the viability and activity of yeast cells, one serious disadvantage of this technology is a significant reduction in leavening activity during frozen storage. To develop improved baker’s yeasts for use in frozen-doughs, yeast strains with high freeze tolerance as well as mechanisms of the freeze–thaw stress response in yeast cells, have been investigated with great interest. Torulaspora delbrueckii strains PYCC 5321 and PYCC 5323, isolated from traditional corn and rye bread are of potential industrial interest since they display high resistance to osmotic and Na+ injury and an exceptional freeze/ thaw tolerance, making them suitable for frozen dough technology. However, few reports exist on the genetics, biochemistry and physiology of T. delbrueckii in contrast to the vast knowledge on the traditional baker’s yeast Saccharomyces cerevisiae, constituting a draw back for their commercial application. Variability among T. delbruecckii strains PYCC 5321 and PYCC 5323 has been neither fully investigated nor reported by molecular typing. Therefore, we performed the molecular characterization of these yeast strains by both mitochondrial DNA restriction pattern analysis (RFLP's) and electrophoretic karyotyping, and showed that strain delimitation within the species T. delbrueckii by these methods is possible. In addition, we propose the use of RFLP's of mitochondrial DNA as an accessible molecular method to routinely discriminate T. delbrueckii strains. For a better evaluation of the potential offered by this yeast to the baking industry we have also characterized sugar utilization patterns, and respiration/fermentation rates. Our results show that T. delbruecki behaves very similarly to S. cerevisiae with respect to sugar utilization and regulation patterns. However, when compared to a baker’s yeast strain of S. cerevisiae, T. delbrueckii showed a higher contribution of respiration during aerobic fermentation of glucose, sucrose and maltose. This was evidenced by biomass yields determined in YP medium with either glucose, sucrose or maltose, which showed a very significant increase when high aeration rates were used (from 20% increase, in glucose or sucrose medium, to 80%, in maltose medium). This trait represents an advantage for the large-scale production of baker’s yeast. As shown for S. cerevisiae, we also have shown that sugar transport is the rate limiting step of sugar utilization in rich media in T. delbrueckii. In Chapter 4 we have cloned and functionally characterized a new transporter gene from T. delbrueckii, IGT1, which encodes an intermediate-affinity glucose transporter. IGT1, is located upstream of LGT1, the first hexose transporter described in T. delbrueckii, and displays a high homology to this gene and to other yeast glucose transporter genes. Functional characterization of Igt1p in a S. cerevisiae hxt-null strain revealed that it encodes a transporter able to mediate the uptake of glucose, fructose and mannose. Furthermore, similarly to S. cerevisiae Hxt2p, apparent Km of Igt1 transporter can be modulated by medium glucose concentration. Cells of S. cerevisiae hxt-null strain transformed with IGT1, when grown in 0.1% glucose displayed biphasic uptake kinetics with an intermediate- (Km = 6.5 ± 2.0 mM) and a high-affinity (Km = 0.10 ± 0.01 mM) component. Evidences that point to the existence of several hexose transporters with different glucose affinities and regulation in T. delbrueckii are also presented. Additionally, we have also established an improved gene disruption method for T. delbrueckii, and using this method constructed a Δlgt1 strain. Analysis of this mutant revealed that LGT1 disruption leads to a significant, although not severe, decrease in glucose transport in comparison with the wild-type strain. Finally, special attention was given to yeast freeze resistance. The mechanisms of freeze tolerance and freeze sensitivity in yeast are still poorly understood and are an important issue to be solved for the development of bakers' yeast strains that are more suitable for the frozen-dough process. In a previous work it was shown that the higher freeze resistance of the T. delbrueckii strains under study, could be attributed to their higher capacity to preserve plasma membrane integrity. In S. cerevisiae a decrease in temperature induces the expression of many genes, some of which result in a cold-sensitivity phenotype when deleted. However, little is known about the role played by many cold-responsive genes, and the regulatory mechanisms that control their response. HSP12 gene is one of these genes. Furthermore, it was shown that Hsp12p could be localized at the plasma membrane making it a good candidate for a role in the preservation of membrane integrity during freezing. Chapter 6 focuses on the cold-shock responses of a Δhsp12 mutant, emphasizing the Hsp12p contribution to freeze tolerance and its relation with trehalose. We show that Hsp12p plays a role in cryoresistance, although the hsp12 null mutant revealed to be more resistant to freezing than the wild type strain. We found that stationary-phase cells of the Δhsp12 mutant have a higher intracellular trehalose concentration than wild type cells that could account for its higher resistance. However, heat-induced trehalose accumulation is impaired in this mutant. Overexpression of HSP12 in a Δtps1 strain (not able to accumulate trehalose) allowed to demonstrate a clear increase in resistance to freezing storage and also to heat stress. Exploitation of yeast activities in the bread-making industry requires fundamental knowledge of their ecology, physiology, biochemistry and molecular biology. This knowledge, to which this work aimed to contribute, provides the base for genetic improvement strategies, and the new molecular methods for yeast identification and characterization, open up the possibility for future innovation in bakers’ yeasts.
O pão constitui um alimento essencial para uma dieta saudável a nível mundial. Actualmente, a utilização de massas congeladas na indústria da panificação apresenta várias vantagens, como o fornecimento aos consumidores de produtos de padaria e pastelaria sempre frescos e a melhoria das condições de trabalho, contribuindo para a sua expansão e aceitação. Os danos provocados pelo congelamento/ descongelamento afectam a viabilidade e a actividade das células de levedura, conduzindo a uma redução significativa na sua capacidade de levedação das massas. Com o objectivo de desenvolver leveduras de panificação melhoradas para aplicação em massas panares congeladas, têm-se estudado e procurado estirpes de levedura com elevada crioresistência, assim como investigado mecanismos envolvidos na resposta ao stress provocado pelo congelamento/ descongelamento. As estirpes de Torulaspora delbrueckii PYCC 5321 e PYCC 5323, isoladas do pão tradicional de milho e de centeio, possuem grande interesse com potencial aplicação na indústria da panificação. De facto, estas estirpes apresentam elevada resistência ao stresse osmótico e ao Na+ e uma tolerância excepcional ao congelamento/ descongelamento, tornando-as apropriadas para o uso em massas panares congeladas. No entanto, existem poucos estudos de caracterização genética, bioquímica ou fisiológica da levedura T. delbrueckii, contrastando com o vasto conhecimento existente sobre a levedura tradicional de panificação Saccharomyces cerevisiae, o que constitui uma desvantagem para a aplicação comercial desta levedura não convencional. As estirpes T. delbruecckii PYCC 5321 e PYCC 5323 foram anteriormente caracterizadas por estudos fisiológicos e bioquímicos, no entanto a sua variabilidade molecular não tinha ainda sido investigada. Por outro lado, também não se encontrava descrito um método expedito de tipagem molecular para diferenciação à estirpe de isolados de T. delbruecckii. Por essa razão, realizámos uma caracterização das estirpes PYCC 5321 e PYCC 5323, por análise de restrição de DNA mitocondrial (RFLP) e de cariotipagem electroforética, demonstrando que é possível a sua distinção/ diferenciação através destes dois métodos. Adicionalmente, propomos o uso de RFLP do DNA mitocondrial como um método molecular para a discriminação de rotina de estirpes de T. delbrueckii. Para melhor avaliar o potencial biotecnológico desta levedura caracterizámos os seus padrões de utilização de açúcares e respectivas taxas de respiração/ fermentação. Os resultados mostraram que T. delbruecki se comporta de uma forma idêntica a S. cerevisiae, no que diz respeito aos padrões de utilização e regulação de açúcares. No entanto, quando comparada a uma estirpe de panificação de S. cerevisiae, T. delbrueckii mostrou uma maior contribuição da respiração durante a fermentação aeróbia da glucose, sacarose e maltose. Este aspecto foi evidenciado pelo aumento significativo dos rendimentos em biomassa, determinados em meio YP suplementado com os diferentes açúcares e usando taxas de arejamento elevadas. Esta característica representa uma clara vantagem para a produção em larga escala de levedura de panificação. De acordo com o descrito em diferentes trabalhos para S. cerevisiae, os estudos aqui descritos mostraram que em T. delbrueckii o transporte constitui o passo limitante no consumo de maltose e glucose. No Capítulo 4 descreve-se a clonagem e caracterização funcional de um novo gene transportador de T. delbrueckii, IGT1, que codifica um transportador com afinidade intermédia para a glucose. Este gene, localiza-se a montante e na mesma cadeia do gene LGT1, o primeiro transportador de hexoses descrito em T. delbrueckii. Ambos os genes possuem elevada homologia com outros genes transportadores de glucose em leveduras. A caracterização funcional da proteína Igt1p na estirpe mutante hxt de S. cerevisiae revelou que este gene codifica um transportador capaz de mediar o transporte de glucose, frutose e manose. Tal como se verificou para o transportador Hxt2p de S. cerevisiae, o Km do transportador Igt1 pode ser modulado pela concentração de glucose no meio de cultura. Células da estirpe mutante de S. cerevisiae, transformadas com o gene IGT1 e cultivadas em glucose 0,1%, mostraram uma cinética de transporte de glucose bifásica, constituída por uma componente de afinidade intermédia (Km = 6.5 ± 2.0 mM) e outra de alta afinidade (Km= 0.10 ± 0.01 mM). Os resultados apresentados sugerem ainda a existência de outros transportadores de hexoses com diferentes afinidades para a glucose em T. delbrueckii. Adicionalmente, foi desenvolvido um método melhorado de interrupção de genes em T. delbrueckii, com o qual foi possível obter a estirpe mutante Δlgt1. A interrupção do gene LGT1 resultou num decréscimo significativo embora não acentuado do transporte de glucose comparativamente com a estirpe selvagem. Por fim foi dada especial atenção à resistência ao congelamento em leveduras. Os mecanismos de tolerância e sensibilidade ao congelamento em leveduras constituem um assunto importante para o desenvolvimento de estirpes de panificação mais adequadas ao processo de congelamento de massas panares. Num trabalho anterior, foi demonstrado que a resistência ao congelamento das estirpes de T. delbrueckii aqui em estudo, poderia ser atribuída à sua maior capacidade de preservação da integridade da membrana. Em S. cerevisiae, uma diminuição da temperatura induz a expressão de vários genes, alguns dos quais originam um fenótipo de sensibilidade ao frio quando removidos. No entanto, pouco se sabe acerca do papel desempenhado por muitos destes genes e dos mecanismos reguladores que controlam esta resposta. O gene HSP12 é um desses genes. Além disso, a proteína Hsp12 parece estar localizada na membrana plasmática o que a torna uma boa candidata para um papel na preservação da integridade membranar durante o congelamento. O Capítulo 6 descreve as respostas ao frio do mutante Δhsp12, dando ênfase ao contributo da proteína Hsp12p na tolerância ao congelamento e à sua relação com a trealose. Os resultados demonstram que a proteína Hsp12 desempenha um papel na crioresistência apesar do mutante nulo no gene hsp12 se ter revelado mais resistente ao congelamento do que a estirpe selvagem. De facto, a estirpe mutante Δhsp12, apresentou uma concentração intracelular de trealose mais elevada quando comparada com a estirpe selvagem, o que parece justificar a sua maior crioresistência. A sobreexpressão do gene HSP12 na estirpe Δtps1 (que não tem capacidade de acumular trealose) revelou um claro aumento na tolerância ao congelamento bem como na resposta ao stress induzido pelo aumento da temperatura. Apesar do mutante Δhsp12 apresentar uma maior acumulação de trealose em fase estacionária, esta acumulação encontra-se diminuída em resposta ao choque térmico, parecendo indicar que a proteína Hsp12 tem também um papel na resistência a temperaturas elevadas. A utilização de leveduras na indústria de panificação requer um conhecimento aprofundado da sua ecologia, fisiologia, bioquímica e biologia molecular. O trabalho desenvolvido no âmbito desta tese pretendeu contribuir para este conhecimento e para o fornecimento de novas estratégias de biologia molecular para a identificação e caracterização destas leveduras, abrindo novas possibilidades para a inovação em leveduras de panificação.