Effect of ultrafiltration of bakers' and brewers' yeast extracts on their nitrogen content and turbidity

Five commercial yeast extracts (YE) were fractionated by ultrafiltration (U F) with 10 000, 3000 and 1000 Da molecular weight cutoff membranes in the a im of evaluating the effect of UF on the turbidity and total nitrogen conte nt of YE. Membrane pore size had much more influence on UF permeation rates than the source of the YE. UF filtration rates were on the average 4 times lower when the YE were treated with the 3000 Da membrane as compared to th e 10 000 Da filter, and the 1000 Da unit gave rates approximately 40% lower than those observed with the 3000 Da pore size membrane. Pre-filtration wi th a 8 mu m membrane reduced between 47 and 96% the original YE turbidities , while UF with a 1000 to 10 000 Da membranes gave filtrates having between 80 and 99.9% less turbidity than the original YE. On the average, UF with the 10 000 Da unit removed 12% of total solids, while UF with 3000 and 1000 Da cutoff membranes generated the retention of 20 and 23% of solids, respe ctively. Brewers' YE had lower total nitrogen content than bakers' YE, and UF increased the total nitrogen content of the dried yeast extract filtrate s (YEF) obtained. The powders obtained after freeze-drying of brewers' YEF tended to have higher moisture contents than bakers', and this was quite si gnificant with the YEF powders obtained with the filtrates generated with t he 1000 Da membrane.