ABSTRACT
A simple presence-absence test for detection of small numbers of osmotolerant yeasts in foods was developed. Yeast extract glucose 50 broth [consisting of 0.5% (w/w) yeast extract and 50% (w/w) glucose] was used as enrichment medium and was incubated with agitation at 30°C. The detection was done by (a) microscope and (b) streaking 0.03 ml of enrichment culture on selective yeast extract glucose 50 agar and incubation at 30°C for 5-7 d. If no yeast cells were observed under the microscope within 10 d of incubation, the product sample was judged as "free from osmotolerant yeasts." In accordance with this method 28 strains of osmotolerant yeasts were isolated from 27 spoiled high-sugar products. Twenty-four strains were identified as Zygosaccharomyces rouxii , 2 Zygosaccharomyces bailii and 1 each as Torulaspora delbrueckii and Debaryomyces hansenii .
ABSTRACT
To record the effects of prolonged culture storage on the sugar tolerance of Zygosaccharomyces rouxii and Z. bailii , the fermentation behavior of three freshly isolated strains in four different glucose broths (aw values; 0.963, 0.936, 0.909 and 0.858, respectively) was compared with that of the same strains after 2 years of storage (a) on high-sugar agar slants with repeated subculturing and (b) in Biomalt (liquid malt extract) without subculturing. The trials with stock strains resulted in large reductions of both ethanol yield and production rate. Cells stored in liquid malt extract showed a slightly faster and stronger fermentation than cells maintained on agar slants. Therefore, for storage of osmotolerant culture collections use of natural liquid products such as Biomalt, without subculturing, is suggested.
ABSTRACT
All three cardinal temperatures (Tmin, Topt and Tmax) for growth of 6 strains as well as Tmin and Tmax for growth of an additional 23 strains were determined in solutions of 10, 30, 50 and 60% (w/w) glucose at aw (20°C) of 0.990, 0.970, 0.922 and 0.868, respectively. The Topt for growth of Zygosaccharomyces rouxii and Z. bisporus were 24-28.5°C at aw >0.990 and 31-33°C at aw in the range of 0.922-0.868. Z. bailii showed Topt for growth of 29-31°C and 33-35°C at aw >0.990 and aw <0.922, respectively. The Topt for growth of Torulaspora delbrueckii was 27-28.5°C at aw <0.990 and 31-33.5 at aw in the range of 0.922-0.868. Debaryomyces hansenii showed a Topt of 24°C and 27-29.5°C at aw >0.990 and aw <0.922, respectively. The Tmin and Tmax for growth were also shifted toward higher values as the aw decreased; at aw<0.922 none of the tested strains grew at 4°C within 30 d. Several strains could grow at 42°C only in the presence of high sugar concentrations.
ABSTRACT
The influence of frozen storage at -25°C as well as repeated thaw-freeze stresses on the viability of osmotolerant yeast was investigated using cell suspensions in 20% (v/v) glycerol. Osmotolerant yeasts were particularly sensitive to both frozen storage and thaw-freeze stresses.
ABSTRACT
The influence of water activity (aw) on growth (lag-phase, mean generation time and cell yield) of osmotolerant yeasts was determined by culturing 7 strains in broths at 10 different aw values in the range of 0.998 to 0.626 and by counting the Colony Forming Unit (CFU) per ml. Broths were adjusted to the desired aw by means of fructose. None of the tested strains could grow at aw 0.701 and 30°C. During 60 d of incubation at this aw and temperature, slight reductions of the initial CFU/ml counts were noted. By incubation at aw <0.701 these reductions were larger. Six strains of Zygosaccharomyces rouxii grew at aw 0.760, whilst a strain of Zygosaccharomyces bailii could not grow at aw <0.858. For six strains the optimum aw for growth was in the range of 0.958 to 0.998. A single strain of Z. rouxii showed optimum aw for growth in the range of 0.913-0.958. Therefore, it was appropriate to redefine it as "osmophilic". Because of high dehydration, cells actively grown at aw 0.837 were approximately 30% smaller than cells actively grown at aw 0.998.
ABSTRACT
The heat resistance of vegetative cells and asci of two osmotolerant yeasts ( Zygosaccharomyces rouxii and Z. bailii ) was investigated in two different broths of aw 0.963 and 0.858, respectively. The highest heat resistance was observed with asci of Z. bailii LMZ 108, showing a decimal reduction time (D-value) at 60°C and aw 0.858 of 14.9 min. Asci of Z. rouxii LMZ 100 were less heat resistant (D60°C-value at aw 0.858 = 3.5 min). The heat resistance (D-values) of asci at aw 0.963 proved to be 20- to 50-fold and 5- to 8-fold higher than the D-valucs of the corresponding vegetative cells of Z. rouxii and Z. bailii , respectively. However, the lower the aw of the heating broth, the smaller the differences between heat resistance of asci and that of vegetative cells. Morever, different preparations of the same cell material were found to lead to different heat resistances.
ABSTRACT
To preserve high sugar products, the effectiveness of sodium benzoate and ethyl-paraben (para-hydroxybenzoic acid ethylester) against 18 osmotolerant yeast strains was investigated at different water activity values (aw). The influence of pH, acidulant, humectant as well as inoculum level on the tolerance limits for growth of selected strains has also been determined. The tolerance limits for growth of all 18 tested strains were only slightly affected by the aw of the substrate, provided that the preservative concentration reflects only the amount of water and not the whole volume of the substrate. At aw < 0.900 and pH<4.0 1500 ppm Na-benzoate was necessary to inhibit growth of all 18 tested osmotolerant yeast strains for 30 d, while in a similar medium but at higher pH-values Na-benzoate was less effective. Similarly, at aw <0.900 and pH 3.0 a 30-day-free shelf life was guaranteed by addition of 400 ppm ethyl-paraben, while a higher concentration of ethyl-paraben (900 ppm) was necessary if the medium was acidulated to pH 4.8 only. The activity of Na-benzoate or ethyl-paraben against osmotolerant yeasts was usually poor if the initial count of contaminants was high. Zygosaccharomyces bailii was the most preservative-resistant osmotolerant yeast among the tested genera and species.
