Effect of starter culture and lipase type in probiotic yogurt and sour milk with the addition of hydrolyzed walnut oil on free fatty acid, fatty acid, and conjugated linoleic acid content during fermentation and storage

The aim of this work was to determine how the addition of hydrolyzed walnut oil, types of lipases (from Candida rugosa and porcine pancreas), and storage time affect the composition, acidity, microbiota, free fatty acid, and fatty acid content of sour milk and yogurt plus the monocultures of Lactobacillus acidophilus LA-5 and Bifidobacterium animalis ssp. lactis BB-12. Lipases were used to release free form of linoleic acid (LA) from walnut oil to create conditions favorable for CLA synthesis by the starter culture. On the identified parameters for titratable acidity and pH in fermented milk types, normal lactic fermentation was observed, despite the addition of lipolyzed oil. Furthermore, the storage time was found to have an effect on acidity in sour milk and the percent of lactic acid and pH of the yogurt. Results showed that the presence 0.03% of hydrolyzed oil increases the Bifidobacterium strain number. Storage time did not have any notable effect on the abundance of determined microorganisms in sour milk. Microbiological analysis indicated that the incorporation of hydrolyzed walnut oil to the probiotic yogurt and their storage have an effect on the abundance of all determined microorganisms. It was observed that the hydrolysis of walnut oil enhanced the levels of free LA in fermented milk. In sour milk with 0.1% of hydrolyzed oil addition, free LA was 22.34 ± 1.32 mg/100 g when C. rugosa (CR) lipase was used, and was ∼10 times lower (2.37 ± 0.03 mg/100 g) when porcine pancreas (PP) lipase was used. In yogurt with the same oil addition, the amount of free LA was determined to be 56.54 ± 1.72 mg/100 g when hydrolysis was carried out by CR lipase, and ∼4 times lower (14.91 ± 2.40 mg/100 g) when the oil was hydrolyzed by PP lipase. Free LA, obtained by the use CR lipase for oil hydrolysis and 0.1% walnut oil addition, increases the synthesis of CLA in probiotic sour milk (from 6.3 ± 0.1 to 7.8 ± 0.4 mg/100 g) and probiotic yogurt (from 9.0 ± 0.0 to 13.0 ± 0.3 mg/100 g). Using lipase from CR for the walnut oil hydrolysis and starter culture of probiotic yogurt proved to be more effective in terms of CLA synthesis. The addition of hydrolyzed walnut oil increases the share of PUFA and improves nutritional indices (index of atherogenicity, thrombogenicity, and hypocholesterolemic/hypercholesterolemic ratio), especially in products with the addition of 0.1% of oil. Moreover, probiotic yogurt with oil addition shows a more favorable proportion of the all fatty acid groups tested (SFA, MUFA, PUFA) and nutritional indices. So, the possibility of producing probiotic yogurt naturally enriched in CLA has been included, but upcoming studies must include data about the safety for the consumers and estimate the financial effect.