Microencapsulation of Streptococcus thermophilus STI – 15 in Soy Protein Isolate – Inulin matrix to enhance their viability under varied storage conditions and pH levels

Abstract

Probiotics must be safeguarded from severe environmental stressors such as harsh acidic conditions, high temperatures, excessive salt, and enzyme degradation, among others, to avoid a detrimental sensory influence on food incorporation. One of the potential ways for preserving probiotic bacteria appears to be microencapsulation. In the current study, Streptococcus thermophilus STI – 15 cells were microencapsulated in Inulin (IN), Soy protein isolate + Inulin (SPI + IN), Ultrasound treated soy protein isolate + Inulin (US-SPI + IN) as capsule matrixes and free cells (Unencapsulated cells) by freeze drying as the microencapsulation method. Activated STI – 15 cells were grown in MRS agar at 37˚C for 24 h, cells were harvested and mixed with either IN, SPI + IN, US-SPI + IN in 300 mL separately and freeze dried to microencapsulate. Microencapsulated (ME) ST (10%) were dissolved in sterilized skim milk and stored at 5°C, -18°C and enumerated at 28 days to assess the storage survivability. Different aliquots of distilled water adjusted at pH 1.5, 2.5, 3.5, 4.5 and 5.5 using HCl and NaCl were inoculated separately with ME ST kept for 60 mins and survivability was investigated. A comparison between the effects of different pH values resulted pH 1.5 is more lethal than the higher pH values. It was found that the survivability of the probiotic bacteria in their free form was significantly (p<0.05) lower than that of the encapsulated form, demonstrating the effective role of the wall components in shielding probiotics from acidic pH (Gastro-intestinal pH). Among three different wall material types of USSPI+ IN showed the highest survivability of S. thermophilus STI – 15 in all pH values. Probiotic entrapped within US-SPI+IN significantly (p<0.05) showed the highest survivability during both refrigerated (5°C) and freezing (-18°C) storage for 28 days of period by resulting the lowest reduction of the viable cell count (0.27 and 0.19 log CFU/mL for refrigerated and freezing storage respectively). Microencapsulation by freeze drying in soy protein isolate - inulin matrices, as well as soy protein isolate modification using power ultrasonography, showed to be a promising microencapsulation strategy for improving probiotic survivability to storage and stress conditions such as pH.