Culture enumeration, lactose hydrolysis and sensory changes in stored frozen yogurt fermented with two culture systems

The objective of this study was to compare products fermented with two culture systems to two endpoints for the following characteristics: survival of the culture bacteria, changes in protein, lactose and galactose concentrations and sensory changes. Frozen yogurt was produced using a standard lowfat ice cream mix formulation, fermented with supplemented and traditional culture systems, and stored for 11 weeks at -20°C.

Three methods of recovery were employed: Bifid Glucose Agar with the Repair Detection System and Roll Tubes, Bifid Glucose Agar with the Repair Detection System on plates incubated in an anaerobe jar with a GasPak™, and Maltose/Galactose Reinforced Clostridial Agar incubated in an anaerobe jar with a GasPak™.

Statistical analysis indicated that the Repair Detection System provided significantly (p<.05) enhanced recovery of Bifidobacterium longum. Recovery of B. longum on BGA Plates and M/G RCA plates was approximately one-half log lower than recovery on BGA in roll tubes.

Culture bacteria in both systems survived at approximately 5x10⁶ cfu/mL during frozen storage. Lactose and protein levels showed no significant changes or differences between the two culture systems. Generally, galactose levels were significantly higher (p<.05) in the traditional culture system fermented to pH 5.6 compared to the supplemented system fermented to the same endpoint.

The manufactured products (supplemented and traditional) were not different from the commercial product with respect to flavor intensity of yogurt flavor, vanilla, sweetness and freshness. Acid flavor was usually more intense when product was fermented to a pH of 5.6. The commercial product was more smooth than the manufactured products. Consumers indicated a “like slightly” to “like moderately” response for the supplemented and traditional inoculated frozen yogurts.

The study concluded that the culture bacteria do survive the environment well enough to meet proposed standards of identity for frozen yogurt. The presence of probiotic bacteria in the supplemented system seemed to cause little to no difference in such attributes as protein and lactose levels, and sensory evaluation.