A Naturally Buffered Bulk Retentate Starter from Ultrafiltered Milk

Whole milk was ultrafihered to approximately 4:1 protein concentration, heated to 85C for 30 min, and cooled to 22C. It was inoculated with a commercial frozen concentrated lactic starter to give approximately 107 cfu/ml and incubated at 22C for 12 h. A commercial phage inhibitory medium and 11% nonfat dry milk were used as controls. After 12 h, retentate had significantly higher colony forming units per milliliter (3.2 x 10 9) and pH (5.21) than phage inhibitory medium (2.5 109 and pH 5.02) and nonfat dry milk (2.4 x 109 and pH 4.58). Retentate starter and phage inhibitory medium starter had equal activity in skim milk (.3% developed acidity in 4 h at 32C) whereas nonfat dry milk starter had significantly lower activity (.26% developed acidity). After a further 8 h incubation at 22C, retentate starter had the highest pH (4.95) compared with phage inhibitory medium (4.76) and nonfat dry milk (4.51). At this time retentate starter activity was higher (.3%) than phage inhibitory medium (.27%) and nonfat dry milk (.19%). In highly concentrated retentates (3.5:1 and 5:1), retentate starter lowered pH considerably quicker than nonfat dry milk starter.