Preservation of non-Saccharomyces yeasts: Current technologies and challenges

There is a recent and growing interest in the study and application of non-Saccharomyces yeasts, mainly in fermented foods. Numerous publications and patents show the importance of these yeasts. However, a fundamental issue in studying and applying them is to ensure an appropriate preservation scheme that allows to the non-Saccharomyces yeasts conserve their characteristics and fermentative capabilities by long periods of time. The main objective of this review is to present and analyze the techniques available to preserve these yeasts (by conventional and non-conventional methods), in small or large quantities for laboratory or industrial applications, respectively. Wine fermentation is one of the few industrial applications of non-Saccharomyces yeasts, but the preservation stage has been a major obstacle to achieve a wider application of these yeasts. This review considers the preservation techniques, and clearly defines parameters such as culturability, viability, vitality and robustness. Several conservation strategies published in research articles as well as patents are analyzed, and the advantages and disadvantages of each technique used are discussed. Another important issue during conservation processes is the stress to which yeasts are subjected at the time of preservation (mainly oxidative stress). There is little published information on the subject for non-Saccharomyces yeast, but it is a fundamental point to consider when designing a preservation strategy.

Visual and simple determination of glucose-induced acidification by yeast cells: application to rapid cytotoxicity test

This study demonstrated that glucose-induced proton release from yeast cells was more sensitive to various inhibitors than cell proliferation. In this study the inhibition of glucose-induced proton release was determined on the basis of color change of pH indicator, methyl red, from pH 5 to pH6 at cell density of 2.5 × 10⁷ cells/ml. When yeast cells were incubated with the inhibitors of glucose intake, glycolysis, and plasma membrane H ⁺ -ATPase for 1 h, these cytotoxic effects were observed by following the change in absorbance at 527 nm due to methyl red for 5 min. The cytotoxic effects of heavy metal ions, detergents and quinones were observed in the same manner. The above method was superior in sensitivity and measurement time to cell proliferation measurement that required 9 h. This visual cytotoxicity test (methyl red test) is expected to be useful as simple and rapid cytotoxicity test with yeast cells.

The microbiology of malting and brewing

Brewing beer involves microbial activity at every stage, from raw material production and malting to stability in the package. Most of these activities are desirable, as beer is the result of a traditional food fermentation, but others represent threats to the quality of the final product and must be controlled actively through careful management, the daily task of maltsters and brewers globally. This review collates current knowledge relevant to the biology of brewing yeast, fermentation management, and the microbial ecology of beer and brewing.

Net effect of wort osmotic pressure on fermentation course, yeast vitality, beer flavor, and haze

The net effect of increased wort osmolarity on fermentation time, bottom yeast vitality and sedimentation, beer flavor compounds, and haze was determined in fermentations with 12 degrees all-malt wort supplemented with sorbitol to reach osmolarity equal to 16 degrees and 20 degrees. Three pitchings were performed in 12 degrees/12 degrees/12 degrees, 16 degrees/16 degrees/12 degrees, and 20 degrees/20 degrees/12 degrees worts. Fermentations in 16 degrees and 20 degrees worts decreased yeast vitality measured as acidification power (AP) by a maximum of 10%, lowered yeast proliferation, and increased fermentation time. Repitching aggravated these effects. The 3rd "back to normal" pitching into 12 degrees wort restored the yeast AP and reproductive abilities while the extended fermentation time remained. Yeast sedimentation in 16 degrees and 20 degrees worts was delayed but increased about two times at fermentation end relative to that in 12 degrees wort. Third "back-to-normal" pitching abolished the delay in sedimentation and reduced its extent, which became nearly equal in all variants. Beer brewed at increased osmolarity was characterized by increased levels of diacetyl and pentanedione and lower levels of dimethylsulfide and acetaldehyde. Esters and higher alcohols displayed small variations irrespective of wort osmolarity or repitching. Increased wort osmolarity had no appreciable effect on the haze of green beer and accelerated beer clarification during maturation. In all variants, chill haze increased with repitching.