Nutritional aspects of fermented milk products

FEMS Microbiology Reviews 46 (1987) 337-342 Published by Elsevier 337 FER 0075S Nutritional aspects of fermented milk products * M.I. G u r r Key words: Lactic acid bacteria; Yoghurt; Kefir; Cultured buttermilk; Lactose digestion; Protein quality; Vitamin 1. S U M M A R Y Fermented milk products are a palatable and economical source of a wide range of nutrients. The nutrient composition is similar to that in milk, but concentrations of vitamins are in general a little lower, with the possible exception of folic acid. Concentrations of lactic acid, galactose, free amino acids and fatty acids are increased as a result of the fermentation. Lactose-intolerant individuals tolerate lactose when it is consumed in yoghurt better than when it is taken in the equivalent quantity of milk. The mechanism of this effect has not been clearly established. By far the greatest proportion of published material freely available in the West concerns yoghurt; reference is made to other cultured products where results are available to indicate interesting differences. 2. I N T R O D U C T I O N Since the time of Metchnikoff [1], fermented milk products have had an image of possessing almost magical health-giving properties but it is * Presented at the Second Symposiumon Lactic Acid Bacteria Genetics, Metabofism and Applications, 22-25 September 1987, Wageningen, The Netherlands. Parts of this paper have been published in: Milk - The Vital Force, (Proceedings of the 12th International Dairy Congress, The Hague, 29 Sept.-3 Oct. 1986), edited by the Organizing Committee, D. Reidel Publ. Comp., Dordrecht, The Netherlands, 1987, pp. 641-655. Correspondence to: M.1. Gurr, Nutrition Consultant, The Milk Marketing Board, Thames Ditton, Surrey KT7 0EL, U.K. - only in the last two decades or so that some of these ideas have been subjected to rigorous scientific test. This review will concentrate on two aspects: the nutritive value of fermented milks compared with natural milk and the digestion of lactose in relation to the benefits of cultured products for lactose-intolerant individuals. The major emphasis will be on yoghurt, but other cultured milks will be referred to for comparison. 3. N U T R I T I V E V A L U E The products discussed here are based on cow's milk and therefore it can be expected that their nutrient composition will be broadly similar to that of the milk from which they were made. The composition will be modified by (a) changes in milk constituents brought about during the fermentation by the action of the microorganisms upon them, (b) the addition of nutrients and other chemical substances supplied b y the organisms during the fermentation, (c) the presence of the microorganisms themselves and their associated enzymes, and (d) materials added in manufacture. - 3.1. E n e r g y The chief sources of energy in milk are fat and lactose. The energy value of yoghurt is very similar to that of the milk from which it is made. When the solids-not-fat are increased in the basic yoghurt 0168-6445/87/$03.50 © 1987 Federation of European Microbiological Societies Nutrition Consultant, The Milk Marketing Board, Thames Ditton, Surrey, KT7 0EL, UK. and Visiting Professor in Food Science & Technology, University of Reading, Reading RG6 2AP, UI~ 338 3.2. Lactose and its metabolic products During the fermentation of milk, the microorganisms generally use lactose as a substrate, converting it into lactic acid. As a result, the lactose concentration in yoghurt is lower than in unfermented milk, provided that no supplementation with skim milk powder was made during manufacture. Whereas fresh milk contains a negligible quantity of lactic acid, the fermentation process results in the conversion of some lactose into lactic acid [5]. Lactic acid may be beneficial by (a) acting as a preservative for the product; (b) contributing a mildly sour and refreshing taste; (c) influencing the physical properties of the casein curd to promote digestibility; (d) improving the utilization of calcium and other minerals and (e) inhibiting the growth of potentially harmful bacteria in the gut. Its energy value is 15 k J / g compared with 16 k J / g for lactose. Lactic acid occurs in yoghurt as two isomers; L ( + ) and D(--) lactic acid. The D(--) isomer is metabolized only very slowly in man compared with L( + ) lactic acid [7] and if taken in excess can lead to metabolic disturbances. The World Health Organization recommended not more than 100 mg D(--) lactic acid per kg body weight should be consumed daily, although Giesecke and Stangassinger [8] claim that only 60 m g / k g / d a y can be metabolized by man. A large part of the D(--) lactic acid intake is metabolized by the liver or is excreted in the urine by adults. D(--) Lactic acid normally represents about 40-50% of total lactic acid in yoghurt and there is no evidence for metabolic problems associated with the amounts normally consumed. Other fermented milk products contain a much smaller proportion of o ( - - ) lactic acid because of the different metabolism of the starter bacteria used [14]. Thus the proportion of D(--) lactic acid as a percentage of total lactic acid is: Kefir, 2-5; cultured buttermilk, 3-6; soured milk, 4-12; fromage frais, 4-14. Because of the breakdown of lactose during fermentation, the concentration of galactose is higher in cultured products than in unfermented milk. Galactose is normally absorbed very rapidly from the gut and metabolized to glucose in the tissues. A rare inborn error of metabolism, galactosaemia, in which the patient is unable to convert galactose into glucose, resulting in excessively high blood galactose concentrations is associated, among other symptoms, with cataracts [9]. Richter and Duke [10] observed that rats fed an exclusive diet of yoghurt developed cataracts, an effect they attributed to the galactose content of the yoghurt. It should be emphasized, however, that the extrapolation of these animal results to man is invalid because, in the first place, no human beings eat diets consisting entirely of yoghurt and secondly, rats have a limited ability to convert galactose into glucose, whereas, apart from rare cases of galactosaemia, human beings have an abundance of the enzyme needed to metabolize galacrose. It can certainly be excluded, therefore, that normal consumption of yoghurt will be associated with cataracts as has sometimes been suggested. In the fermentation of milk to make kefir, the presence of yeasts as well as bacteria leads to the formation of ethanol from the milk sugar. The final concentration is about 0.17-0.19% [11]. 3.3. Proteins and amino acids The total amino acid content and composition of yoghurt and other fermented milks does not mix, however, then on a weight for weight basis, yoghurt may provide the consumer with a higher intake of protein, carbohydrate, calcium and certain B group vitamins than milk [2]. It has frequently been claimed that the fat is more digestib (...truncated)