ÁREA: Alimentos
TÍTULO: Conditions to improve fermentation activity and viability of commercial dry yeast during the hydration process
AUTORES: THOMAZ, D. (IQ - UNESP) ; RAMOS, E.P. (IQ- UNESP) ; LALUCE, C. (IQ - UNESP)
RESUMO: Effects of several hydration procedures on the fermentative activity and viability of the dry commercial baker’s yeast were determined. The bread dough was transferred to a graduated tube, which was carefully sealed by a rubber stopper. The CO2 released from the dough during fermentation was collected in a flask containing salt solution. The gas pressure forced the outflow of the salt solution which was collected in a graduate cylinder its volume was proportional to the gas evolved form the bread dough. Viability was determined by colony counting on a solid medium. During hydration, variations in fermentation activity were less significant than the variations in viability. As indicated by the variations in the levels of internal glycerol, the best hydration solution was 0.08mol/L lysine.
PALAVRAS CHAVES: dry yeast, fermentation activity, additives used for cells hydration
INTRODUÇÃO: Nowadays, dry yeast in used in bread making and as starter for the fuel ethanol production process. Yeast cells are heat dried to residual moisture lower than 20% followed by alterations in membrane structure and permeability, particularly when the temperature of the rehydration medium increases up to 38°C to 43°C. Addition of CaCl2, glucose and polyethyleneglycol to the rehydration medium cause a decrease in cell permeability assessed as the losses of potassium ions, nucleotides, as well as a total loss of intracellular compounds (BEKER et al., 1984). The cell viability depends on the composition and temperature of the rehydartion medium (STEVENINCK and VAN LEDEBOER, 1974; KAZUHIDE and DAIJI, 1975).
Yeast organisms go into the anabiotic state as a result of leakage of all the free water and a part of the bond water from cells. As the extraction of bound water from cellular membrane elements causes a decrease in hydrophobicity and an increase in electrostatic interaction among cell components, the transition to the anabiotic state very possibly destroys the internal organization of phospholipid molecules and this may cause an increase in membrane permeability. A serious problem is the establishment of optimal conditions for the recovery of normal viability of organisms from the anabiotic state. The data obtained on the positive effect of increases in temperatures and the contents of the medium upon decrease in cell permeability may be of great industrial interest. The aim of the present work is to establish hydration conditions (solutions and hydrations temperatures) that would allow the obtaining of reconstituted cells showing increased viability and good fermentation activity, particularly during the storage of the frozen dough.
MATERIAL E MÉTODOS: Dry yeast: Commercial dry yeast distributed by Fermix, São Paulo, SP and produced by “PAK GIDA” in Istanbul, Turkey.
Hydration of the dry yeast cells for viability determination: suspend 0.05g dry yeast in 10mL water or hydration solutions. Hydrated cell suspensions were plated on YPD medium for viability determination. After 2-3 days growth at 30°C, viable cell number were expressed as cfu/mL (colony forming units/mL)
Hydration of dry yeast cells and determination of fermentation activity: 0.5g dry yeast was suspended in 10mL water or hydration solutions. After hydration, cells were collected by centrifuging (5min at 3000 rpm at room temperature) to be added to the dough formulation as described by MURAKAMI et al. (1996). A device similar to that described in literature (BURROWS and HARRISON, 1959; PERES, 2001) was used. The fermentation activity determined at 32°C was expressed as follows: a) fermentative activity expressed as the total volume of CO2 evolved from the bread dough; b) the fermentation rate (linear regression applied to the variations in the volume of CO2 formed x fermentation time, mL/min.); c) dough leavening activity expressed as increases in the volume (mL) of the dough/min/g of dry yeast added to the dough formulation.
Internal glycerol assay: Frozen cell were disrupted by glass beads in a vortex shaker. Cells debris were separated by centrifuging (5min at 7000 rpm at 4°C) and the internal glycerol was determined using an enzymatic kit from Doles, Reagentes e Equipamentos para Laboratório, LTDA, Goiânia G.O.
RESULTADOS E DISCUSSÃO: Time course curves of hydration were obtained as follows: viability curves of the rehydrate yeast cells, volumes of CO2 evolved from the bread dough and variations of bread dough were obtained at 27°C and 37°C. The hydration curves showed increases in viability above the incubation time (20 - 40 min) in which the maximal values of fermentative activity were obtained. During hydration at 28°C in water, drops in dough leavening activity were observed while constant values were obtained during hydration at 37°C. Thus, hydration at 37°C seems to be the best choice, since quite a high volume of dough was obtained for at least 120 min hydration during the fermentative assays. The yeast hydration for 40 - 80 min in water at 27°C or 37°C gave the highest values of viability. A few hydration solutions were used to replace water in the hydration process at 37°C. A concentration of 0.08 mol.L-1 lysine and glutamate led to the highest viability values during hydration when the pH of solutions was adjusted to 7.5. Elevation of the internal glycerol was minimized, when the pH of the water was raised to pH 7.5 by adding bicarbonate. The internal glycerol varied form one packet of dry yeast to another.
CONCLUSÕES: Rehydration media at pH 7.5 give greater viability values than at pH 5.5. Oxidative catabolism is favored at low pH values while synthetic processes are favored by higher pHs. The hydration of the dry yeast in lysine solution at pH 7.5 and 37°C led to the lowest values of internal glycerol. When yeast cells are submitted to stress situations, the levels of internal glycerol increase. In addition, the variations in glycerol content of the commercial of baker’s yeast are due to the different level of stress imposed to the yeast cells during drying or exposition to oxygen during packing.
AGRADECIMENTOS: The authors are grateful by FAPESP (proc. 2005/01498-6) and CAPES.
REFERÊNCIAS BIBLIOGRÁFICA: BEKER, M. J.; BLUMBERGS, J. E.; VENTINA, E. J.; RAPOPORT, A. I. 1984. Characteristics of cellular membranes at rehydration of dehydration yeast Saccharomyces cerevisiae. Eur. Journal Applied Microbiology Biotechonology, 19: 347-352.
BURROWS, S.; HARRISON, J. S. 1969. Routine method for determination of activity of backer’s yeast. Journal Inst. Brewing., 65: 39-45.
KAZUHIDE, Y.; DAIJI, O. 1975. The influence of osmotic pressure of rehydration fluid on the recovery of dried cells of a bacterium. Proceedings of the 1st International Congress of IMAS, 5: 641-645.
MURAKAMI, Y.; YOKOIGAWA, K.; KAWAI, F.; KAWAI, H. Lipid composition of commercial baker’s yeasts having different freeze-tolerance in frozen dough. Bioscience Biotechnology Biochem., 60: 1874-1876.
STEVENINCK, I.; VAN LEDEBOER, A. M. 1974. Phase transitions in the yeast cell membrane. The influence of temperature on the reconstitution active dry yeast. Biochim Biophys Acta, 352: 64-70.
