Date of Award

5-2012

Degree Name

MS in Agriculture - Dairy Products Technology

Department

Dairy Science

Advisor

Dr. Amy Marie Lammert

Abstract

In recent years, there has been an upsurge in medical research assessing the therapeutic benefits of probiotic bacteria and growing commercial interest in food fortification with these bacteria. Probiotic bacteria such as L. acidophilus are known to be predominant Lactobacilli species in the intestinal tract of healthy humans and suggested to provide clinical health benefits such as enhancement of immunity against intestinal infections, prevention of diarrhea and hypercholesterolaemia and improvement in lactose utilization. Many studies have demonstrated the possibility of incorporating probiotic bacteria in an ice cream matrix and shown its viability can be maintained throughout the shelf life of the ice cream. However, there is limited information about the protective effect of ice cream on viability of incorporated probiotic bacteria during simulated gastric digestion using an in vitro dynamic model stomach.

In phase one of this study, a preliminary study was conducted to determine the effect of air addition on the viability of L. acidophilus La-5. This was done by manufacturing low fat (4%) non-fermented ice cream mix supplemented with L. acidophilus La-5 to yield an initial population of 107cfu/g. The mix was processed with 60% and 100% overrun (OR) and stored at -10ᵒC for 90 days. The effect of air addition at different levels was tested post freezing and every 30 days throughout its shelf life of 90 days. The results showed less than one log reduction in the viable counts of L. acidophilus La-5 for both samples incorporated with 60% and 100% OR after freezing and the number of viable cells did not differ significantly (p>0.05) from day 1 to day 90.

In phase two of this study, a 22 full factorial experimental design was used to evaluate whether the viscous nature of ice cream mix plays an important role in improving the survivability of L. acidophilus La-5 during simulated digestion against low pH and presence of mechanical shear and to determine whether initial inoculation level has any effect on the viability of L. acidophilus La-5 at the end of 2 hr simulated digestion. Non-fermented low fat (5%) ice cream mixes with high and low viscosity were produced by changing the amount of stabilizer/emulsifier blend and each of the two mixes were supplemented with two levels of L. acidophilus La-5 to obtain an initial population of 108cfu/g and 106cfu/g before freezing. These mixes were frozen with 60% overrun. The ice cream samples were digested for 2 hr in an in vitro model stomach called Human Gastric Simulator (HGS). This model included factors such as gastric secretions, mechanical shearing due to peristaltic contractions and temperature and pH control. No significant effect (p>0.05) of different levels of viscosity on the survivability of L. acidophilus La-5 was found during and at the end of 2 hr simulated in vitro digestion, irrespective of the difference in initial inoculation level. The initial supplementation level of L. acidophilus La-5 had a significant impact (p<0.05) on its survivability during the simulated digestion of ice cream samples, irrespective of the difference in viscosity. The log survival of L. acidophilus La-5 was on an average 3.64 log cfu/g and 4.08 log cfu/g for ice cream samples supplemented with higher and lower amount of L. acidophilus La-5, respectively at the end of 2 hr. Nevertheless, this difference in overall survival was not statistically significant (p>0.05).

These studies demonstrated the efficacy of low fat non-fermented ice cream in maintaining high viable numbers of L. acidophilus La-5 throughout its tested shelf life of 90 days. In addition, protective effect of ice cream on the viability of L. acidophilus La-5 against harsh stomach conditions was observed, but this effect was not as a result of viscosity of ice cream. It was also found that an ice cream supplemented with 106cfu/g would result in a similar overall log reduction of L. acidophilus La-5 at the end of 2 hr simulated digestion compared to an ice cream supplemented with 108cfu/g.

The aggressive stomach conditions had a negative impact on the survivability of L. acidophilus La-5 during digestion of all the ice cream samples, but this detrimental effect can be reduced by incorporating L. acidophilus La-5 into an ice cream matrix which would increase the opportunity of bacteria to reach the small intestine and provide the desired health benefit.

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