Effect of incubation temperature on yield and molar mass of EPS during fermentation of milk by Streptococcus thermophilus DGCC 7785 and the impact on the rheological properties of acid milk gels

Abstract Text: Some strains of Streptococcus thermophilus produce exopolysaccharides (EPS) during milk fermentation. It is unclear if there is any change in the yield, or properties, of EPS when milk is fermented at different temperatures. We investigated the yields of both ropy and capsular forms of EPS, and the physical properties of ropy EPS that were isolated during the fermentation of milk by S. thermophilus strain DGCC 7785. Reconstituted skim milk was fermented at 33, 39 and 45°C until pH reached 5.2, 4.9, 4.7 and 4.5. Fermented milk was then heated to 80°C for 10 min and whey (containing bacterial cells) was obtained by decantation. Whey was ultrafiltered with sufficient diafiltration using 100 kDa membrane at 45°C to remove soluble sugars and proteins. The UF retentate was centrifuged and the supernatant and pellet were analyzed for ropy and capsular EPS, respectively. Ultrafiltration of whey from a non-EPS producing strain was also carried out as a control for estimating the amount of capsular EPS. Molar mass of ropy EPS samples were analyzed using size exclusion chromatography multi-angle laser light scattering (SEC-MALLS). Rheological properties of fermented milk gels were analyzed using small-strain dynamic oscillatory measurements. The yield of ropy EPS was 102, 108 and 55 mg/L when milk was fermented at 33, 39 and 45°C respectively, whereas the yields of capsular and total EPS were 102, 132 and 102 mg/L, and 204, 241 and 157 mg/L for these fermentation temperatures (33, 39 and 45°C). Significantly higher (P < 0.05) yields of capsular and total EPS were produced at 39°C. Total EPS content significantly (P < 0.05) increased from 150 mg/L to 257 mg/L when the pH of milk decreased from 5.2 to 4.5. Molar mass of ropy EPS ranged from 2.0×10⁶ to 2.8×10⁶ g/mol, exhibiting no significant (P > 0.05) effect of temperature or pH values during fermentation. Gelation pH (~ 5.3) of milk did not change with incubation temperature, whereas storage modulus values of the final gel (at pH 4.5) significantly (P < 0.05) increased (58, 135 and 410 Pa at 45, 39 and 33°C, respectively) with a decrease in fermentation temperature.  

Keywords: Exopolysaccharides, fermented milk, Streptococcus thermophilus