The application of plant sterols as functional ingredients in dairy products

Abstract

Since Iast century, food products have been fortified and enriched by addition of essential nutrients to satisfy needs of specific populations. A wide variety of dairy products is currently available, as a result of continued advances and improvements in the dairy industry over the past decade. Plant sterols have been recognised as a functional food ingredient due to their ability to lower systemic cholesterol levels. This current research was directed to the preparation of dairy products - milk, yoghurt and spread - with added plant sterols. The final plant sterol content of each of the formulated milk and yoghurt was 1% and of the spread 5%. Plant sterols interfere with the absorption of cholesterol in the gut but the exact details of how they do this are not yet well understood. Further research has thus been carried out in order to investigate the role of bile salts in assisting plant sterols inhibit the solubilisation of cholesterol and, in particular, whether the suppression of cholesterol absorption results from the lowering of the relative amount of cholesterol solubilised by bile salt micelles when plant sterols coexist with cholesterol. Bile salts are a family of steroidal compounds synthesized in the liver and they have a number of potential roles in the human body. Bile salt micelles have the ability to solubilise water-insoluble components in bile and lipolytic products, forming tiny water soluble packets called dietary mixed micelles in the small intestine. When the micelles make contact with the intestinal cell wall, they deliver their content. Since the cholesterol content of the mixed micelles is not water soluble, it forms a complex with proteins and triglycerides inside the intestinal cells before moving in to the body as chylomicrons. In this study, therefore, different types of bile salts and a mixture of bile salts which mimics that present in bile have been used in the presence of cholesterol, β-sitosterol and/or Stigmasterol. Increased hydrophobicity of the bile salts is a key factor in their ability to solubilise cholesterol and plant sterols. A solution of mixed bile salts which mimics that present in bile is very effective in this role. Subtle changes in sterol structure are also important, with β-sitosterol being much more effective than Stigmasterol in decreasing cholesterol solubilisation. Nutritional analysis of the formulated dairy products, performed using analytical techniques such as Kjeldahl digestion for protein content and high performance liquid chromatography for total sugar content, yielded reproducible results. The total lipid content was obtained through simple extraction procedures which yielded 1%, 1.4% and 15.5% of fat in the formulated milk, yoghurt and spread, respectively. The fatty acid profiles of the formulated dairy products were altered by an increase in monounsaturated and polyunsaturated fatty acids of approximately 50%. A serving size (200 mL) of milk or a standard punnet (150 g) of yoghurt and a serving of spread (10 g) would contribute 1.6 g, 1.4 g or 0.5 g of plant sterols, respectively, towards the amount required (2 g/day) to reduce cholesterol absorption.