Master of Science (MS)


Animal Science (Animal, Dairy, and Poultry Sciences)

Document Type



Streptococcus salivarius ssp. thermophilus and Lactobacillus delbrueckii ssp. bulgaricus are dairy cultures widely used in the dairy industry. Low sonication intensity condition is a non-destructive technique that uses sound waves to cause cavitation in aqueous solutions and may improve the permeability of membranes, speed up the transfer of substrates and promote cellular growth and propagation. The objective of this study was to determine the effect of low sonication intensities at different temperatures on acid tolerance, bile tolerance, protease activity and growth of the two dairy cultures. The cultures were freshly thawed and suspended in 0.1% peptone water and 18 ml of sample was sonicated using horn (diameter 13 mm) set at a maximum acoustic power output of 750 W, frequency 24 kHz. The treatments were four sonication intensities of 8.07, 14.68, 19.83 and 23.55 Watts/cm2 randomized at three different temperatures (4, 22 and 40°C) of inoculated peptone water before sonication. The energy input (1500 Joules) was kept constant in all treatments. Control samples did not receive any sonication treatment. Growth and bile tolerance of samples were determined hourly for 12 hours of incubation. Acid tolerance was determined for Streptococcus thermophilus every 5 minutes for 20 minutes of incubation and for Lactobacillus bulgaricus every minute for 5 minutes of incubation. Protease activity was determined at 0, 12 and 24 hours of incubation. The experimental design was a completely randomized design (CRD). Three replications were conducted for each experimental condition. Data were analyzed using Proc Mixed Model of Statistical Analysis System (SAS®). Differences of least square means were used to determine significant differences at P<0.05 for main effects (low sonication intensity, time and temperature) two way interaction effect (low sonication intensity * temperature and low sonication intensity * time) and three way interaction effects (low sonication intensity * time * temperature). Low sonication conditions include a) low sonication intensities, b) temperatures and c) times, all three of which played a role in influencing the desirable attributes of both microorganisms. Of all the low sonication intensities studied, 14.68 watts /cm2 had the best overall influence at certain time points for Streptococcus thermophilus improving its acid tolerance, bile tolerance and growth at 4°C, growth at 22°C, bile tolerance and growth at 40°C and improving the Lactobacillus bulgaricus bile tolerance and growth at 4°C, its acid tolerance and protease activity at 40°C. Low sonication intensity of 19.83 Watts/cm2 had the overall best influence at certain time points for acid tolerance of both microorganisms at 22°C. Low sonication intensity of 23.55 Watts/cm2 had the overall best influence at certain time points for protease activity of Streptococcus thermophilus at 40°C and Lactobacillus bulgaricus at 22°C. Some low sonication conditions improved certain characteristics of culture bacteria.



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Committee Chair

Aryana, Kayanush J.