Effects of additives on the survival of lactic streptococci in frozen storage

Three single-strain cultures, Streptococcus lactis C(2), S. cremoris R(1), and S. diacetilactis DRC(2), were frozen and stored in skim milk, in skim milk containing apple juice, and in skim milk containing one of the following additives: glycerol (10%, v/v), dimethyl sulfoxide (10%, v/v), l-malic acid (0.5 and 2.0%, w/v), acetamide (0.5 and 2.0%, w/v), or succinimide (0.5 and 2.0%, w/v). Cultures were frozen and stored at -23.3 C, frozen and stored at -196 C in liquid nitrogen, or frozen at -196 C and stored at -23.3 C. Cultures frozen and stored at -196 C in liquid nitrogen gave the greatest recovery of viable cells. The number of cells surviving after storage at -23.3 C was greater when the cells had been frozen in liquid N(2) than when they had been frozen at -23.3 C. All strains stored at -23.3 C showed a decrease in numbers of surviving cells; additives, particularly l-malic acid and apple juice, were advantageous in preserving the viability of the S. lactis C(2) and S. cremoris R(1) strains, but had little or no effect on the survival of S. diacetilactis DRC(2). l-Malic acid and apple juice stimulated acid production for all cultures in activity tests following incubation after thawing, whereas glycerol and dimethyl sulfoxide retarded its development.