Vol50, No 1 February 1997 International Journal of Dairy Technology

Nutritional properties of fermented milk products

JUDITH BUTTRISS 5-7 John Princes Street, London WlM OAP, UK

Fermented milk products such as yogurts have been consumed for several thousand years and the belief that they are beneficial to health is probably equally ancient. But it is only in recent years that scientific support for these beliefs has begun to build. Fermented milk products, like the milk fr-om which they are made, are rich in protein, vitamins and minerals. However, in addition to these purely nutritional properties, there is increasing support for a number of other health advantages. The evidence for some of these is stronger than for others. For example, while the support for improvement in tolerance of lactose by maldigesters of this disaccharide is now very robust, evidence to support the alleged ability of fermented milk products containing particular bacterial cultures to reduce cuncer risk is still in its infancy. This paper focuses on the products recognized by Consumers as yogurts. It includes a comparison of the nutritional value of milk and yogurt, consideration of consumption trends and their impact on nutrition, and summarizes the strengths and weaknesses of the evidence associated with a number of suggested health benefits of hioferniented milks.

INTRODUCTION Fermented milk products have been consumed in Europe for perhaps 4000 years and yogurt is one of the oldest and most popular of these products worldwide. Many of the products available today are derived from the results of spontaneous lactic acid bacteria fermentation of milk that arose centuries ago through the influence of climatic conditions and local traditions in the handling of milk. A number of types of fermented milk products are listed in Table 1 but mostly this paper will focus on those recognized by consumers as yogurts.

Fermented milk products were originally developed as a means of preserving the nu- trients. Also, by fermenting milk with different microorganisms, it has proved possible to develop a wide range of products with different flavours, textures, consistencies and functions. Details of the different bacterial species used to make fermented milk products can be found in a review by Robinson. Yogurt is by far the most commonly found fermented milk in the UK. Yogurts are produced by a lactic acid fermentation process using thermophilic strains of bacteria. Other products such as viili, popular in Finland, are also made via a lactic acid fermentation process but mesophilic bacteria are used and a mould added to give viili its characteristic slimy consistency through a pro- cess which partially hydrolyses the fat. Other

products such as koumiss and kefir are made by processes in which lactic acid and alcohol fermentations are combined by use of a combi- nation of yeast and bacteria (thermophiles in the case of koumiss and mesophiles for kefir) (see Korolevo2 for details). Milk products incorpo- rating Lactobacillus acidophilus are discussed in detail by S e l l a r ~ . ~ Some of these are referred to as culture-containing rather than cultured because the former do not undergo a true fermentation process during their production.

Within the past few decades, yogurt has gained wide consumer acceptance in Britain, helped no doubt by its image as a healthy food. However, its popularity must also be due to its taste, the wide variety of types on the market and its convenience as a dessert or snack food. Yogurt is the most frequently consumed chilled dessert in households in the United Kingdom and, increasingly, consumers are becoming aware of its versatility in cooking.

Table 2 and Fig. 1 show overall consumption of yogurt in the UK and provide an indication of the most popular types purchased. Consistent

Paper given at symposium on The current status of bio- fermented milks-foc on Europe, London, 29 October 1996. 0 1997 Society of Dairy Technology

US

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Vol50, No I February 1997 International Journal of Dairy Technology

with consumers strong interest in health, a variety of low fat and virtually fat free yogurts are now available, as are products with a reduced calorie and sugar content through the use of artificial sweeteners.

NUTRITIONAL ATTRIBUTES Nutritionally, plain yogurt has a similar compo- sition to that of the milk from which it is made and so is an excellent source of high quality protein, calcium, phosphorus, magnesium, zinc and the B vitamins riboflavin, B12 and niacin. However, there are a few subtle differences, particularly with respect to yogbrts vitamin content (Table 3).

Nutritional properties remain similar regard- less of the method of production, eg, stirred

The UK Yoghurt Market by Sector, 1992

!t 21%

Fig. 1. Segmentation of the yogurt market, 1992. Based on Milk Marketing Board Trade Estimates for 1992.

compared with set yogurts. However, the composition can be modified by changes brought about by the bacteria in the starter culture during the fermentation process, by synthesis or release of nutrients or other substances by the bacteria in the starter culture, by addition of other ingredients during manu- facture, eg, skimmed milk powder, fruit or fruit juice, and by storage conditions. There is also the suggestion that the health properties of fermented milks may be determined by the nature of the culture used in their production.

Lactose An example of how the nutritional composition is modified by the presence of the culture concerns the milk sugar, lactose. During fer- mentation, 20-30% of milks lactose is hydro- lysed to its component sugars, glucose and galactose, by the culture ba~ te r i a .~ Conse- quently, the lactose levels in yogurt can be lower than in milk, although this is not always the case as skimmed milk powder or non-fat milk solids are sometimes added during manu- facture. A lower lactose content would pre- sumably help tolerance of the product by those with a reduced ability to digest lactose. But clearly the explanation is not this simple because yogurts with a fairly high lactose content also seem to be better tolerated by such individuals than the equivalent amount of lactose as milk (see below).

Lactic acid is produced as a byproduct of the fermentation process. This may act as a pre- servative by reducing pH. It has also been suggested that it influences the physical proper- ties of the casein curd (eg, to induce a finer suspension) so as to promote digestibility, to improve utilization of calcium and other miner- als and to inhibit the growth of potentially harmful bacteria (see G ~ r r , ~ and Alm,6 for reviews).

Protein Protein is composed of amino acids. Although the overall amino acid concentration is similar in yogurt and the milk from which it is derived, there is a greater proportion of free amino acids in yogurt because of the proteolytic activity of the bacterial culture which partially digests the protein during fermentation. However, this characteristic is unlikely to be of major im- portance as most people already digest protein efficiently.

Fat It has been proposed that the digestibility of fat is improved via the action of the bacteria present during the fermentation process, but whether this effect really exists is highly debatable.5,7 The overall energy (calorie) con- tent of yogurt reflects both the fat content of the milk from which it was made and whether or not there has been the addition of ingredients such as cream or sugar.

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Vol50, N o 1 Fehruary 1997 lnrernurionrrl Joirrnol of Dairy Technolo,qy

Vitamins The vitamin content of yogurt varies with the type of milk used (particularly the fat content of the milk, which influences the amount of vitamin A and other fat soluble vitamins present), with the strain of bacteria and with . fermentation conditions. In addition, whenever milk is processed there is likely to be an effect on the concentration of the more labile water soluble vitamins, concentrations of which are influenced by heat and/or light. Yogurt manu- facture is no exception. Vitamin C is heat and light labile, but it is more stable in the acid conditions of fermented milk than in nmnal milk.7

Levels of some B vitamins are reduced due to the requirement of some lactic acid bacteria for vitamin B12, for e ~ a n i p l e . ~ . ~ Losses of up to 90% of vitamin B12 have been reported with specific bacterial strains, but losses are not always this great and can be reduced con- siderably by use of a supplementary culture capable of synthesizing significant amounts of vitamin B 12.

On the other hand, several cultures are able to synthesize the B vitamin, folk acid, increasing the level of this vitamin present in the final p r ~ d u c t . ~ In the current climate in which there are concerns about the folate intake of women in the early stages of pregnancy, this aspect may prove to be a nutritional advantage.

Minerals The fermentation process has little effect on the mineral content of milk. Yogurt is an excellent source of a number of minerals, especially calcium, zinc, phosphorus and magnesium (see Table 4). Not only are the concentrations of these minerals high, but the bioavailability is also generally good (ie, the proportion available for absorption and utilization within the body). Table 5 demonstrates this point. It lists the number of servings of calcium containing foods which would need to be consumed to match the amount of calcium available from a single small pot of yogurt (or from a glass of milk).

Some researchers have speculated that bio- availability of minerals from yogurt might even be enhanced, compared with milk, because of the lower (more acid) gastric pH induced by yogurt, but scientific support for this is incon- clusive and, generally, findings do not support superior bioavailability.9

SUGGESTED HEALTH PROPERTIES The belief that yogurt consumption is beneficial to health is ancient and part of folklore, having been passed from generation to generation in many parts of Europe. Recently, the term probiotic has been coined to describe some of these benefits.

An oral probiotic was defined at a special workshop in Frankfurt as living micro-organ- isms which upon ingestion in certain numbers, exert health benefits beyond inherent basic nutrition. Probiotics may be consumed either as a food component or as a non-food prepara- tion.

Many health benefits have been attributed to fermented milk products over the years, some of which are listed in Table 6. For some of these eg, improved lactose tolerance, a considerable amount of evidence has been amassed as discussed below, but others remain little more than speculation and are far from well-estab- lished from a scientific point of view.

The job of attempting to make sense of the available evidence is confounded by a number

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of factors9 Not all the data are derived from well designed studies and not all interpretations are based on statistically significant data. Much of the data concerns animal models and in vitro experimentation, and this cannot be extrapo- lated directly to humans. Furthermore, it is difficult to generalize because of the large number of strains of microorganisms involved which may differ considerably in their proper- ties and effects.

Central to a number of these is the ability of viable bacteria in 'live' yogurt and other fermented milk products to survive passage through the stomach. The term 'live' refers to products which retain live bacteria after manu- facture because the milk from which they were made was heat treated prior to the addition of the bacterial culture (heat treatment kills the bacteria). This applies to the majority of the brands of yogurt available today.

Complexity of the gut microflora Many of the suggested health benefits are associated with the concept that the gut micro- flora is involved in resistance to disease,"" and that its balance can become deranged by environmental factors and stress, thus com- promising resistance. It has been suggested that under such conditions probiotics have the potential to manifest a beneficial effect.

This concept serves to emphasize the role of the intestinal microflora in human health and the complexity of the microflora ecosystem. It is now evident that the intestinal microflora can influence health in a number of ways-both positive and negative. l2 These include impacts on nutrition, physiological function, eg, one of the short chain fatty acids produced by colonic bacteria (butyrate) is important in determining the rate of colonic cell growth and differentia- tion,I3 drug efficacy, carcinogenesis, immuno- logical responses, resistance to infection and responses to endotoxins and other stresses. The several hundred types of bacteria which com- prise the microflora each contain a series of different enzymes capable of a range of differ- ent functions, eg, conversion of substances present in the gut to different compounds, which may be beneficial or detrimental. Also toxins and other substances produced by bac- teria present in the gut can modify the host's response, eg, to infection or stress. However, beneficial microorganisms can protect against the proliferation of harmful pathogenic ones.

So any bacteria-harmful or otherwise- entering the intestine via food are arriving in an already very complex system. To be potentially influential in the gut, bacteria need to be able to survive passage through the stomach with its extremely acid pH, and to survive the actions of bile acids and the immune system. Other factors that affect the ability of bacteria to grow successfully in the colon (large bowel) include availability of substrates and competition for these, substrate versatility and efficiency of use,

ability to adhere to the gut epithelium or particle surfaces in order to remain in the environment, and ability to produce substances, eg, antimicrobial agents, which adversely influ- ence the growth of bacteria already present in the gut. There is evidence that bacteria that are indigenous to the human gut are highly adapted and so well able to tolerate these obstacles. l4

The extent to which diet and items within the diet can modify microbial ecology is a question on which little general agreement exists. This is in part due to the difficulties associated with defining the indigenous flora and the variability from one person to the next. The theoretical effects of diet are via provision of substrate, via antibacterial or bacteriostatic effects of food components, via an influence on gut physi- ology, eg, bile and acid secretion, motility etc and, fourthly, the introduction of microorgan- isms via the diet may influence the existing ecology.

Which microorganisms are involved? The belief that fermented dairy products confer health benefits is associated with various fer- mented products made using a diversity of cultures. Most cultures used to produce yogurt contain strains of bacteria from either or both of the lactic acid generating genera known as Lactobacillus spp and Lactococci spp, eg, streptococci and enterococci. Standard cultures contain mainly Lactobacillus bulgaricus and Streptococcus thermophilus. Other species are used to impart flavour (see Robinson' for a review), It is thought that these classic yogurt bacteria are likely to have little or no probiotic potential as they are probably killed in the intestine. l S

However, recently there has been particular interest in the possible health benefits of yogurts produced using cultures containing Lactobacillus acidophilus and bifidobacteria, both of which are found naturally in the human gut. This interest probably arose initially from the finding that infants who are entirely breast fed have a totally different microflora in their colons compared with bottle fed infants. In breast fed infants up to 99% of the bacteria present are bifidobacteria, whereas infants receiving formula have a much more diverse microflora.16 Gum suggests that an explanation of this might be linked to the presence in human milk of antibacterial substances which inhibit pathogens but not bifid~bacteria.'~ Another explanation, also discussed by Gurr, is that the difference in composition between human and cows' milk (eg, differences in protein) might induce differences in transit time of the milk through the gut. The more rapid transit time of human milk may result in conditions ideal for the fermentation of lactose in the colon, providing an acid environment inhibitory to the growth of pathogens such as Escherichia coli but tolerated by bifidobacteria. Whether or not bifidobacteria are a significant advantage to the

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baby has yet to be vigorously tested, but it is well established that breast fed babies are less susceptible to infection than those who have been solely formula fed. Compared with in- fants, the gut and faecal microflora of older children and adults seem to be relatively unaffected by diet.

IMPROVED LACTOSE INTOLERANCE Numerous studies appear to indicate that yogurt, particularly products containing live bacteria, is well tolerated compared to milk by individuals who have lactose intolerance. Such individuals experience gastrointestinal symp- toms due to a reduced ability to digest lactose, the sugar (a disaccharide) in milk, into its component sugars-glucose and galactose- which can then be absorbed readily in the small intestine. Undigested lactose cannot be ab- sorbed and travels to the large bowel (colon) where it is digested by the resident micro- organisms, causing excess gas production, intestinal discomfort and diarrhoea. In humans lactase levels peak in early infancy to enable digestion of human milk to take place effi- ciently. In most white Europeans levels remain high during childhood and into adulthood, perhaps stimulated by cows' milk consumption. However, in as many as 70% of the world's population (mainly non-Europeans), activity of the enzyme, and hence ability to digest lactose effectively, declines rapidly after weaning. This is genetically determined and is considered to be linked with the absence of a tradition of dairying in Africa, India and Asia because of adverse climatic conditions.

Lactose intolerance is not an all or nothing phenomenon and the condition can exist to varying degrees, depending on the extent of loss of enzyme activity. The most likely explanation for an improved tolerance of lac- tose when it is consumed as part of yogurt is the presence of microbial (3-galactosidase derived from the bacterial culture used in yogurt production, which like intestinal lactase can break down lactose to its component sugars. Being intracellular, this enzyme survives its passage through the stomach so long as the cell wall of the bacteria remains intact, and so potentially reaches the small intestine in its active form. Kuhn and colleagues'8 suggest that the enzyme can survive passage into the small intestine even in non-viable but intact bacteria. Pasteurization (and freezing) destroy the en- zyme, but usually yogurt cultures are added to the milk after it has been heat treated rather than before.

However, the hypothesis is confounded by the finding that lactose maldigesters tolerate yogurts with varying levels of (3-galactosidase equally well.I9 This suggests that factors in addition to lactase activity, including rate of gastrointestinal transit, may contribute. It has been suggested that the presence in the gut of bacterial lactase might stimulate residual in-

testinal lactase activity. However, the results of a double blind study provide no evidence to support this.*'

Gurr' proposes that cultured products such as yogurt, because of their acidity and the con- sequent finer dispersion of protein in the stomach, retard the emptying of the stomach's contents into the small intestine. Any capacity to breakdown lactose, whether it be of micro- bial or indigenous origin, would then have longer to take effect and consequently lactose digestion would theoretically be more efficient, even when the specific activity of the enzyme was low.

Recent work has also suggested that the regular consumption of gradually increasing amounts of lactose might actually stimulate changes in the microflora of the colon, thus improving tolerance.*'

OTHER SUGGESTED BENEFITS A number of other claims of health or probiotic effects of fermented milks have been made (Table 6).

Protection against gastrointestinal infec- tions Gastrointestinal infections, including diar- rhoea, can result from a change in the gut microflora caused by an invading pathogen. Before the problem manifests itself, however, the pathogen usually needs to establish itself in sufficient numbers within the gut.

At the beginning of this century it was suggested that consumption of fermented milk products containing viable lactic acid bacteria might interfere with the colonization and sub- sequent proliferation of foodborne pathogens, thus preventing manifestation of L hulgaricus, S therrnophilus, L acidophilus6 and also Bifdobacteriurn b i f d ~ r n ~ , ~ ~ have all been implicated in this effect. In addition, it has been suggested that lactic acid bacteria can protect against antibiotic induced diarrhoea in patients taking antibiotics capable of destroying natural antagonists to disease producing organ- isms.26 Various mechanisms have been sug- gested including the ability of these cultures to lower intestinal pH, which favours growth of lactic acid bacteria, and their ability to produce acids such as lactic, acetic, formic and pro- pionic acids, which create an unfavourable environment for the multiplication of patho- genic bacteria.

However, most of the evidence supporting a role for cultured or culture containing dairy products is derived from animal experiments and in vitro s t ~ d i e s , ~ , ' ~ > * ~ and findings tend to be i n c o n ~ i s t e n t . ~ ~ ~ ~ But one promising line of research concerns treatment of young children with severe diarrhoea associated with various conditions including rotavirus infect i~n.*~.~ ' In several studies, ingestion of fermented products has reduced the duration of diarrhoea asso- ciated with rotavirus infection. There are also

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reports of benefit to sufferers of travellers' d i a r rh~ea .~ .~ ' Proposed mechanisms include immunomodulatory effects, influence on bac- terial enzyme activity and competition for receptor binding sites.3'

Relief of constipation The administration of fermented milk products containing lactobacilli to severely constipated elderly patients has been reported to have beneficial effects on bowel movement in sev- eral s t ~ d i e s . ~ , ~ '

Improved immunity Studies in experimental animals and in vitro have suggested that consumption of live, active bacteria via products such as yogurt enhance specific and non-specific immune mechanisms, and as a consequence provide protection against pathogens and viruses.'5 It has been suggested that gamma-interferon may be in- strumental in this But to date, evidence is not sufficiently strong to justify claims that fermented milks enhance the im- mune response or reduce the risk of infection in humans. In particular, it is not clear what the implications of the experimental findings in animal models and in vitro are for normal healthy people.

Cholesterol reduction Having a high blood cholesterol concentration is considered to be one of the four major risk factors for coronary heart disease. Interest is growing in the possibility that some fermented milk products may facilitate a reduction in blood cholesterol level, specifically the low density lipoprotein (LDL) fraction. This inter- est initially derived from the observation that Masai tribesmen in East Africa consume large quantities of yogurt like foods and have low blood cholesterol in spite of a diet largely based on animal products. A number of studies have since provided support for a hypocholester- olaemic effect, although others have failed to provide confirmation. The conflicting reports in the literature may be explained by factors such as differences in experimental design and in the species and strains of bacteria used. For example, Agerbaek and colleagues34 have re- ported a 10% reduction in LDL-cholesterol after regular consumption of a fermented product carrying a specific bacterial culture (which included Enterococcus faecium) for 6 weeks. There was criticism of a number of aspects of this short study, however, including the type of placebo used-it was not a fer- mented product. There was also a high initial level of LDL-cholesterol in the treated group of middle aged men. Other studies reviewed by Renner9 and Schaafsma" have also demon- strated reductions in LDL-cholesterol of a similar magnitude4.2% and 5-6%. Addi- tional studies are underway, in which some of the shortcomings of earlier studies are being

addressed. But further work is needed to attempt to identify the mechanism of the hypocholesterolaemic response, whether a dose-response relationship exists and whether these effects persist or are merely transient. Such work will help assess the clinical im- portance of these findings."

Protection against cancer There are reports suggesting that fermented milk products may protect against certain types of cancer.y There is a small amount of epi- demiological support for this,3s although the data are inconsistent. In animal models yogurt, lactobacilli and bifidobacteria have all been reported to inhibit the growth or cause regres- sion of tumours which have been transplanted or chemically induced.

Again, various mechanisms have been sug- gested including the potential of some strains of lactic acid bacteria to reduce the activity of faecal (bacterial) enzymes known to promote the synthesis of carcinogens from available substrates, or stimulate the host's immune ~ y s t e m . ~ However, there is little or no evidence to show that these mechanisms operate in humans, although some findings from experi- mental animals and in vitro studies support these ideas. It can be concluded that, although there are some indications of possible underly- ing mechanisms, evidence to date is still very preliminary and is restricted to an indication of changes in bacterial enzyme activity. As yet there is no evidence that this results in a reduction in cancer incidence and much more work is needed before such claims can be made.

Adverse effects There is much interest in the potential role of probiotics such as dairy cultures in promoting human health, but it is important to recognize that there might also be some adverse effects.36 It is as yet unclear whether probiotics might be disadvantageous in young children (under the age of 2) who are yet to establish fully an adult gut flora. There may also be detrimental effects in patients with autoimmune disease or certain other underlying diseases. There is additionally a concern about the potential to transfer to the host the property of antibiotic resistance which some bacterial strains carry.37

CONCLUSIONS Regular consumption of cultured and culture containing milk products have been claimed to confer a wide range of health benefits. It can be concluded that a number of investigations support the view that yogurt (particularly 'live' yogurt) is a well tolerated alternative to milk for those with a reduced ability to digest lactose. This improved tolerance is considered to be partly due to the presence of the enzyme (3- galactosidase in the bacterial culture within the

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yogurt, and partly due to other factors such as effects on transit time.

There is also growing evidence of a role for fermented products, perhaps containing spe- cific strains, in reducing the duration of diar- rhoea in conditions such as rotavirus infection. There is also the possibility, which still needs substantiation, that cholesterol reducing poten- tial exists. However, data relating to most of the other alleged health related benefits, such as beneficial influences on cancer and immunocompetence, are conflicting. Differ- ences in experimental design and in the strain and species of bacteria used may help explain these inconsistencies, but as most of the work to date has been conducted in animal models and in vitro, the applicability of these findings to humans remains to be established. In particular, it is important to establish whether probiotics can improve the health of already healthy individuals.

Regardless of the current deficiencies in our knowledge, fermented milk products provide a wide range of important nutrients, and are palatable, widely available at an affordable price and versatile. In addition to these quali- ties, evidence of health benefits associated with the presence of specific strains of live bacteria is no longer anecdotal, and is gradually gaining established scientific credibility.

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