Phytosterol antioxidant activity and effects on shelf life of fluid milk and yoghurt quality

Phytosterols play a major role in the area of functional foods. Their ability to reduce serum cholesterol in humans has been widely proven and they are now being added to various foods world-wide. Several studies have also reported on their antioxidant activity, and as such phytochemicals may have the potential to impart other benefits to food. The present study investigated the antimicrobial activity of several phytosterol mixtures produced by Forbes Medi-Tech Inc. (Vancouver, BC) against milk microflora including Pseudomonas as well as yoghurt starter cultures and the antioxidant activity of various free and esterified sterols in oils. The effect of phytosterols on the rheology of yoghurt was also investigated.; As indicated by the results, a commercial phytosterol preparation (CPP) containing beta-sitosterol, campesterol, sitostanol and campestanol, exhibited no antimicrobial activity at a concentration of 1.8% (w/v) towards the SPC (standard plate count) or psychrotroph population in milk stored at 4°C. A challenge study employing various Pseudomonas spps in milk at 4°C also confirmed that the CPP was not antimicrobial. Homogenization of the milk subsequent to CPP addition, in order to increase dispersal, did not increase its antimicrobial effect. However, the addition of dispersible forms of phytosterols designated ADP (dispersible phytosterol mixture A) and BDP (dispersible phytosterol mixture B), which contained sodium stearoyl lactylate (SSL, 0.03%; w/v) appeared to produce an antimicrobial effect against SPC and psychrotrophic bacteria in refrigerated milk. The ADP and BDP (0.72% w/v) did not, however, inhibit the growth of Pseudomonas. An investigation into the antimicrobial activity of SSL alone revealed that it did not inhibit the growth of SPC in milk, indicating that it did not posses any intrinsic antimicrobial activity that added to that of phytosterol. The CPP, as well as several other phytosterol mixtures had no effect on the growth of Lactobacillus bulgaricus and Streptococcus thermophilus during yoghurt production at 33°C. The phytosterols also had no effect on acid development as evidenced by pH values. Acid and starter culture levels were similar to those of the controls during storage at 4°C for 30 d. This is seen as a beneficial feature since growth of and acid development by these organisms are crucial for yoghurt quality. Saccharomyces cerevisiae and Aspergillus ochraceus, inoculated into yoghurt samples to simulate spoilage by yeast and mold, were not inhibited by the inclusion of the CPP either during production or storage at 4°C for 30 d. Rheology results indicated characteristic flow and viscoelastic properties for both control and CPP containing yoghurts. Overall, the results indicated that the CPP did not contribute to an increase in shelf life, however, it also exhibited no negative effects on yoghurt quality. Investigation into the antioxidant activity of phytosterols revealed that free phytosterols (3.4% w/w) exhibited greater activity than phytosterol esters (5.44% w/w) in MCT (medium-chain triglyceride) and canola oils. Also, only free phytosterols were shown to reduce lipid oxidation in oils stored for 12 months. The phytosterol esters offered no protection against lipid oxidation for oils heated to 100 and 190°C but, instead, appeared to increase the propensity to lipid oxidation. Free phytosterols reduced the thermal degradation of canola oil when held at 190°C for 7-12 h and reduced lipid oxidation in oils heated to 100°C. The loss of linolenic but not linoleic acid was also reduced by addition of free phytosterols to the oil. The increased activity of the free versus esterified phytosterols was likely due to the availability of a terminal hydroxyl group in the former compounds, for interaction with peroxide radicals.