Valorisation of sweet whey by fermentation with mixed yoghurt starter cultures with focus on galactooligosaccharide synthesis

Abstract

Sweet whey was fermented with yoghurt starter cultures (YO-MIX 101 containing Lactobacillus acidophilus, Streptococcus thermophilus, and Bifidobacterium lactis and YO-MIX 532 containing Lactobacillus delbrueckii subsp. bulgaricus and S. thermophilus) as well as enzyme extracts thereof to synthesise galactooligosaccharides from lactose. The influence of fermentation temperature and initial lactose concentration was studied. The highest yield in sweet whey (34.2% including disaccharide GOS, 25.4% excluding disaccharide GOS) was obtained with the YO-MIX 532 culture at 50 °C and 100 g L−1 initial lactose. With YO-MIX 101, yields were considerably lower, but transgalactosylation was favoured in sweet whey compared with buffer (23.4% versus 8.5% total GOS at 100 g L−1 initial lactose). Of the isolated strains derived from the mixed cultures, only Lactobacillus bulgaricus and both strains of S. thermophilus showed notable GOS synthesis. Up-scaling to a stirred tank reactor system yielded 26.3% total GOS within 24 h of fermentation.