Growth kinetics of Streptococcus salivarious subsp. Thermophilus and Lactobacillus delbrueckii subsp. Bulgaricus starter cultures during fermentation of acha (digitaria exilis, stapf) based milk

Abstract

This study determined the growth kinetic (maximal specific growth rate (h-1), generation time (h), and viability (CFU/ml) of yogurt starter cultures; Streptococcus salivarious subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in acha (Digitaria exilis, Stapf) based milk was carried out. Acha-based milk was extracted, batch pasteurized in an Erlenmeyer flask, and inoculation with 3% yogurt cultures (Direct Vat type (DVS) Yo mix consisting of Streptococcus salivrarious subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in ratio 1:1 The result showed that Streptococcus thermophilus counts increased from 4.00 x 106 to 2.40 x 10⁸ (CFU /ml) at an extended time of 6 h, while Lactobacillus bulgaricus counts increased from 5.00 x 10⁶ (CFU /ml) to a maximum count of 1.42 x 10⁸ (CFU /ml) at 5 h during acha milk fermentation. Comparatively, Streptococcus thermophilus count increased from 6.80 x 10⁷ to 1.70 x 10⁸ (CFU /ml), while that of Lactobacillus bulgaricus increased from 1.20 x 10⁷ to 9.60 x 10⁷ (CFU /ml) after 6 h of fermentation in the dairy milk. The maximum specific growth rate of 0.406 h1 and doubling time (td) of 102 min was registered for Lactobacillus bulgaricus, while Streptococcus thermophilus exhibited higher growth with a specific growth rate of 0.416 h-1 at a doubling time of 100 min. The acidifying activity of the lactic acid bacteria showed a lower rate of 0.619 h-1 in the acha milk compared with 1.136 h-1 observed in the dairy milk. The curdling time for the acha milk occurred 1.5 hours after that of the control. These findings confirmed the possibility of producing yogurt from ‘acha’ based milk as an alternative to conventional cow milk-based yogurt.