Sheep Milk Symbiotic Ice Cream: Effect of Inulin and Apple Fiber on the Survival of Five Probiotic Bacterial Strains during Simulated In Vitro Digestion Conditions

Abstract

We conducted a study to determine the survival of bacterial cells under in vitro digestion. For this purpose, ice cream mixes were prepared: control, with 4% inulin, 2.5% inulin and 1.5% apple fiber and 4% apple fiber. Each inoculum (pH = 4.60 ± 0.05), containing 9 log cfu g^-1 bacteria, at 5% (<i>w</i>/<i>w</i>) was added to the ice cream mixes (<i>Lacticaseibacillus</i><i>casei</i> 431, <i>Lactobacillus acidophilus</i> LA-5, <i>Lacticaseibacillus paracasei</i> L-26, <i>Lacticaseibacillus</i><i>rhamnosus</i>, <i>Bifidobacterium animalis</i> ssp. <i>lactis</i> BB-12) and fermentation was carried out to pH 4.60 ± 0.05. The in vitro digestion method simulated the stages of digestion that occur in the mouth, stomach and small intestine under optimal controlled conditions (pH value, time and temperature). At each stage of digestion, the survival rate of probiotic bacteria was determined using the plate-deep method. As expected, in the oral stage, there was no significant reduction in the viability of the probiotic bacteria in any ice cream group compared to their content before digestion. In the stomach stage, <i>Bifidobacterium animalis</i> ssp. <i>lactis</i> BB-12 strain had the highest viable counts (8.48 log cfu g^-1) among the control samples. Furthermore, a 4% addition of inulin to ice cream with <i>Bifidobacterium</i> BB-12 increased gastric juice tolerance and limited strain reduction by only 16.7% compared to the number of bacterial cells before digestion. Regarding ice cream samples with <i>Bifidobacterium</i> BB-12, replacing part of the inulin with apple fiber resulted in increased survival at the stomach stage and a low reduction in the bacterial population of only 15.6% compared to samples before digestion. At the stomach stage, the positive effect of the addition of inulin and apple fiber was also demonstrated for ice cream samples with <i>Lacticaseibacillus</i><i>casei</i> 431 (9.47 log cfu g^-1), <i>Lactobacillus acidophilus</i> LA-5 (8.06 log cfu g^-1) and <i>Lacticaseibacillus paracasei</i> L-26 (5.79 log cfu g^-1). This study showed the highest sensitivity to simulated gastric stress for ice cream samples with <i>Lacticaseibacillus</i><i>rhamnosus</i> (4.54 log cfu g^-1). Our study confirmed that the 4% addition of inulin to ice cream increases the survival rate of <i>L. casei</i> and <i>Bifidobacterium</i> BB-12 in simulated intestinal juice with bile by 0.87 and 2.26 log cfu g^-1, respectively. The highest viable count in the small intestine stage was observed in ice cream with <i>L. acidophilus</i>. The addition of inulin increased the survival of <i>L. rhamnosus</i> by 10.8% and <i>Bifidobacterium</i> BB-12 by about 22% under conditions of simulated in vitro digestion compared to their control samples. The survival rates of <i>L. casei</i> and <i>L. paracasei</i> were also highly affected by the 4% addition of apple fiber, where the increase under gastrointestinal passage conditions was determined to range from 7.86-11.26% compared to their control counterparts. In comparison, the lowest survival rate was found in the control ice cream with <i>L. rhamnosus</i> (47.40%). In our study at the intestinal stage, only five ice cream groups: a sample with 4% inulin and <i>L. acidophilus</i>, a control sample with <i>Bifidobacterium</i> BB12, a sample with 2.5% inulin and 1.5% apple fiber with <i>Bifidobacterium</i> BB12, a control sample with <i>L. rhamnosus</i>, a sample with 4% fiber and <i>L. rhamnosus</i> reported bacterial cell counts below 6 log cfu g^-1 but higher than 5 log cfu g^-1. However, in the remaining ice cream groups, viable counts of bacterial cells ranged from 6.11 to 8.88 log cfu g^-1, ensuring a therapeutic effect. Studies have clearly indicated that sheep milk ice cream could provide a suitable matrix for the delivery of probiotics and prebiotics and contribute to intestinal homeostasis. The obtained results have an applicative character and may play an essential role in developing new functional sheep milk ice cream.