Enhancing survival of lactic acid bacteria in ice cream by natural encapsulation and gene transfer

Viable culture bacteria in frozen dairy desserts can be a source of {dollar}beta{dollar}-galactosidase for lactose malabsorbers. However, these cultures are susceptible to death during freezing.; Cultures selected for high {dollar}beta{dollar}-galactosidase activities and survival rates in the presence of bile were examined for survivability during freezing in reduced fat ice cream. Encapsulated Streptococcus thermophilus strains survived better than their non-encapsulated mutants in reduced fat ice cream after freezing and frozen storage at {dollar}{lcub}-{rcub}29spcirc{dollar}C for 16 d (28% vs. 19%). However, an encapsulated strain of Lactobacillus delbrueckii spp. bulgaricus failed to survive as well as a non-encapsulated strain in reduced fat ice cream.; Factors that improved survival of S. thermophilus ST1068 in ice cream were (1) harvest of cells in the late log phase of growth at 37{dollar}spcirc{dollar}C rather than {dollar}rm40spcirc, 42.5spcirc, or 45spcirc C,{dollar} (2) keeping overrun at 50% rather than 100%, and (3) storing at {dollar}{lcub}-{rcub}17spcirc{dollar}C rather than {dollar}rm{lcub}-{rcub}23spcirc or {lcub}-{rcub}29spcirc C.{dollar} Survival of strain ST1068 was unaffected by (1) neutralization of acid during growth or (2) substitution of nitrogen for air in building overrun.; Possible involvement of a plasmid with capsule production was tested. Sequencing and computer analysis of plasmid pLC88 from Lactobacillus casei ssp. casei NCIB4114 failed to reveal any gene that might code for exopolysaccharides. This suggested that the gene(s) previously claimed to be involved in capsule production may not exist in plasmid pLC88 or may have been lost during mutagenesis at elevated temperature.; Homology comparisons of sequences of pLC88 (DNA and putative proteins) showed that pLC88 replicates by the rolling circle mechanism. pLC88 has potential as a cloning vector for "food-grade" lactic acid bacteria.