Spent brewers yeast - analysis, improvement, and heat processing considerations.

Ingledew, W.M., Langille, L.A., Menegazzi, G.S. and Mok, M.H.

Abstract
The paper discusses differences in the chemical composition of brewers' yeast, a typical secondary yeast from an anaerobic fermentation, and primary yeasts specifically manufactured for food and feed. The vitamin content of brewers' yeast is very similar to that of primary yeasts but the crude protein content (N x 6.25) of both brewers' and bakers' yeast (36 to 42%) is much lower than that of primary yeasts of the inactive dry type (52 to 59%). Anaerobically grown brewers' yeast has considerable levels of cell carbohydrate, especially the reserve carbohydrate glycogen which may be up to 40% of the dry weight, in addition to 20% structural carbohydrate. Brewers' and bakers' yeasts contain less nucleic acids than primary yeasts. This is a nutritionally favourable feature, high intake of nucleic acid purines leading to excessive levels of uric acid. Improvement of brewers' yeast for food or feed purposes may be achieved by a period of storage. Using degassed, homogenized brewers' yeast slurry at a concentration of 130 mg/ml stored at 40 degrees C, it was shown that utilization of the endogenous carbohydrate reduced the dry weight/ml and thereby increased the crude protein content on a dry weight basis. This reached a maximum (41 to 53%) after 140 h starvation. In the same time carbohydrate decreased from 41% to 27%, falling to 21.5% after 350 h. Nucleic acid content showed little change over this period. At higher temperatures the increase in protein content occurred in a shorter time. Yeast for human food as a protein rather than a vitamin source may require various processing treatments, including debittering, chemical or autolytic breakdown and precipitation or extraction to increase protein to nucleic acid ratios. Consumption of live yeast may lead to gastro intestinal upset so some form of inactivation, usually by heat, is practised which also improves digestability. Yeast rendered inactive by drying to 90% solids is relatively expensive and interest has revived recently in inactivated yeast slurry as a supplement for liquid feed. Yeast protein could provide part of the crude protein component currently provided in some cases by 100% urea. Experiments have shown that little heat is required to kill yeast effectively at the concentrations (108 cells/ml or higher) in yeast slurry. Less than 1 min at 60 degrees C using heat exchangers or direct steam injection is relatively inexpensive.
Keywords: brewers' yeast by-product carbohydrate protein waste