Industrial interest of yeast extracts

The autolysate of yeast cells is a valuable source of various substances which can be used as ingredients in foods, feeds and fermentation. Many workers have shown the growth-promoting properties of yeast extracts (YE) on bacterial growth. Although YE are generally less expensive than protein hydrolysates, as fermentation media ingredients, they still remain a relatively expensive material for large-scale fermentations process. In most cases YE are autolysates of Saccharomyces cerevisiae, as brewers' or bakers' yeast. The YE obtained from brewers' yeast are generally less expensive than those obtained from bakers' yeast. This is primarily because biomass for the manufacture of brewers' yeast extracts is usually obtained from breweries as surplus brewer's yeasts rather than from sources of primary-grown yeasts such as bakers' yeast produced on molasses. There is thus an economic interest in using brewers' YE in microbiological media, but little is known of the comparative growth-promoting properties of bakers' or brewers' YE. There is little information on how variations in the composition of YE influence their growth-promoting properties, and if mixtures of various sources of YE can be beneficial.

In examining the effects of blends of brewers' and bakers' YE on their biological properties two avenues are possible: mixing commercial YE made from pure cultures of brewers' or bakers' yeast, or producing a YE by co-autolysis of the two strains. The aim of this study was to determine the effect of the sources of bakers' and brewers' YE, as well as their mixtures, on the growth of four lactic acid bacteria. Furthermore, YE were produced by co-autolysis of bakers' and brewers' yeast and the biological value of the products was examined. These data are aimed at providing information to laboratory and industrial scientists that formulate microbiological growth media, as well as to producers of YE.

Blending bakers' and brewers' YE in fermentation media

In the first portion of this study (blends of commercial YE), three brewers' and three bakers' yeast extracts (YE) were obtained from five commercial suppliers. They were added to microbiological media and their growth-promoting properties were examined using four lactic cultures (Lactobacillus casei EQ28 and EQ85, Lactobacillus acidophilus EQ57,Pediococcus acidilactici MA18/5-M). The best overall product in this study was a bakers' YE. However, a systematic preference of bakers' YE over brewers' YE was only encountered with Lb. casei EQ85. Therefore, results from this study show that this statement cannot be generalized, and that LAB reacts differently to the source of YE.

Bakers' YE have a higher total nitrogen content (between 10.8 and 11.9%) than brewers' YE (between 7.1 and 9.8%), but there was not always a correlation between the nitrogen content and growth. A systematic preference for bakers' YE over brewers' YE was only encountered with Lb. casei EQ85, but the other lactic cultures had variable reactions to the source of YE. With Lb. casei EQ85 and Pc. acidilactici 17/5M, mixing of the two sources of YE gave progressively higher growth as a function of the content of the better YE. With Lb. acidophilus EQ57 and Lb. casei EQ28, however, there were instances where a mixture of 75% brewers' YE with 25% bakers' YE gave biomass levels higher than those obtained with the pure products. Industrially speaking, these results show that producers of LAB or of other microbial cultures could find it worthwhile to not only examine the replacement of bakers' YE by brewers' YE, but also to consider mixtures of the two sources. Such an examination of mixtures of various sources is very tedious if a laboratory can only rely on fermentation equipment, and the automated spectrophotometry technique described in this study can provide a useful tool in this aim. Scientists who work with fastidious strains and who must enrich existing commercial media or formulate their own culture media could also adopt this strategy.

Preparing novel YE by co-autolysis of bakers' and brewers' yeast

In the second series of assays, autolyses were conducted in the laboratory with mixtures of brewers' and bakers' yeast, to see if the YE obtained differed from those obtained from autolysis of the individual yeast cultures. Preliminary studies on the autolysis process were conducted. Autolysis yields of the two types of yeasts at 45, 48 and 51ºC were not significantly different, so a range of 45 to 51ºC could be used for the autolysis procedure. The effect of solids levels on autolysis yields were also examined. Between 10 and 18% solids of yeast solids at the beginning of the autolysis process, lowering the initial yeast solids resulted in increased autolysis yields. However, the cost associated with drying of yeast autolysates increases as a function of water content. For this reason, it was decided to dilute the yeast suspensions to 14% dry matter before imitiating the autolysis procedure.

Brewers' yeast autolysates had higher turbidity than those of bakers' yeast. The maximum yield was obtained with the co-autolysis of a combination of 60% bakers' yeasts and 40% brewers' yeasts. Co-autolysis was beneficial to protein hydrolysis, since the α-AN proportion of TN was highest for the 60% bakers' 40% brewers' yeasts mix.  Growth of Lb. acidophilus EQ57 was best in the autolysate obtained from 100% brewers' yeast, in spite of the higher nitrogen content of YE produced when bakers' yeast was used during co-autolysis. From an industrial perspective, these results indicate that producers of YE could increase yields and protein hydrolysis by conducting co-autolyses. In some cases this could also improve the biological value of the YE, particularly with strains having a preference for amino acids over peptides. However, the growth-promoting value of such novel YE would still need to be tested with each target strain, as is the case with mixtures of bakers' and brewers' YE.