Designer Milk – Exclusively on Dieticle
“Designer Milk” is (anonymously submitted) series of articles about today’s Milk production – exclusively on Dieticle.
Reports of prolific and successful research in the areas of biotechnology and genetic engineering have unleashed potential ideas that were previously inconceivable in the subject of dairying. It is now firmly established that novel value-added products can be derived from milk and milk products with nutritional and biotechnological interventions. While until recently, breeding policies have aimed at producing more milk, attempts are now directed toward enhancing the value of milk and studying its health implications. This has found more support with clinically established epidemiological linkages between diet and chronic diseases that encourage search for new links between food and disease.
The extra nutritional therapeutic attributes of milk and milk products have also been brought into this broad network of research. Milk composition can be altered by nutritional management or through the manipulation of naturally occurring genetic variation among cattle. The possible channels of influencing milk composition to suit specific needs can be investigated with the help of a thorough comprehension of the biochemistry, genetic traits, and factors in the animal diet that affect milk synthesis and composition. By an intelligent combination of the two approaches— nutritional and genetic—a milk designed to suit consumer preferences can be developed. This ‘‘designer milk’’ may be rich in specific milk components that may have influence on well-being or on processing. This article examines the potential that exists in altering milk composition by nutritional and genetic approaches in order to achieve specific health benefits and/or processing opportunities.
MILK ‘‘DESIGNING’’: THE PROSPECTS
Man has been taming and manipulating other species for his own benefits for thousands of years. Several breeds of cattle that produce large quantities of milk exist today as a result of selective breeding adopted by farmers over centuries. The global appeal of milk as a healthy beverage that is good for adults as well as infants has prompted much investigation on the commodity.
Research on animal breeding, husbandry, and feeding conditions has always had a profound impact on the quality of milk, its constituents, and the subsequently manufactured products. Altering the composition of milk in a manner that suits health and processing needs forms the basis of the current research interests in the area. For example, a greater proportion of unsaturated fatty acids in milk fat, reduced lactose content in milk for lactose-intolerant people, and/or milk free from b-lactoglobulin (b-LG) would benefit human diet and health. From a technological point of view, there exist vast opportunities in altering the primary structure of casein to improve the technological properties of milk and producing milk high in protein content. Engineering milk that clots in less time leads to increased yield and/or more protein recovery during cheese manufacture. Milk that contains nutraceuticals and replacement ingredients for infant formula are other interesting avenues.
Genetic manipulation (GM) also offers the prospect of healthier animals with improved resistance to diseases such as mastitis or to the ticks that can infest cattle, thus reducing the need for antibiotics and pesticides. Medicines may be produced in the milk of cows. For example, GM cows could produce milk with a clotting factor for hemophiliacs, milk containing human serum albumin for blood transfusions, or milk with a hepatitis vaccine. Several of these medicines could be produced much more efficiently than with the technologies currently used.
MILK FAT MODIFICATION
The quantity of milk fat is a determinant of its value and hence a major indicator of the revenue accrued from milk. As a recent trend, advanced knowledge about the chemistry of milk fat and human physiology encourages product developers to modify the milk fat to counter the changing functional and nutritional challenges. Dairy products provide less than 15% of the total fat available in the diet (O’Donnell, 1993). Milk fat provides 25% of the saturated fat, which is still not as high as that in the two groups of fats and oils (29%) and meat, poultry, and fish (39%). The contribution of dairy products to the total cholesterol is 16%, much less than that in eggs (39%) and meat, poultry, and fish (43%).
Modifications of the composition and quality of fodder result in different milk fat compositions and influence the nutritional and technological value of fats. A sophisticated trend in the ‘‘health market’’ today is to modify the milk fat composition by either adopting suitable feeding strategies or by genetic modes. It is now almost possible to achieve the ideal composition of milk fat for human health and well-being recommended by O’Donnell (1989), after the Wisconsin Milk Board 1988 Milk Fat Roundtable. The combination suggested at the meeting was less than 10% polyunsaturated fatty acids (PUFA), less than 8% saturated fatty acids (SFA), and more than 82% monounsaturated fatty acids (MUFA).
Altering the fatty acid chain length and level of saturation in milk fat
The long-chain fatty acids of milk fat are derived from the diet via blood. The short-chain fatty acids (C10 and below) of milk fat are first synthesized in the mammary gland and then elongated to C12–C16. If the mechanism for elongation is blocked by genetic technology, the ratio of medium-chain fatty acids (C12–C16) to short-chain fatty acids in milk fat should reduce. Since the C12–C16 fatty acids are generally regarded by nutritionists as less desirable, milk fat with reduced content of medium length fatty acid chains would garner more value due to greater consumer demand.
There is ample experimental evidence to suggest that nutritional modifications can cause significant changes in milk fat composition. The degree of unsaturation of the serum lipids, tissue fat, and milk fat may be increased promptly by feeding unsaturated fats in an encapsulated or protected form to lactating animals (Ashes et al., 1997). It is established that MUFA (C18:1) content can be increased by 50–80% and may approach 50% of milk fatty acids by feeding lipids rich in 18-carbon fatty acids (Grummer, 1991). Feeding low-roughage diets increases the proportion of MUFA in milk fat, the effects of feeding low-roughage diets and lipid being additive. The SFA content (palmitic acid, C16:0) of milk fat can also be reduced by 20–40% unless the supplemented lipid is rich in palmitic acid. SFA particularly palmitic and other medium-chain fatty acids tend to increase levels of blood cholesterol (O’Donnell, 1993).
Feeding highly unsaturated oils (e.g., soybean oil) caused depression in milk fat, but increased the proportion of unsaturated fatty acids to SFA in milk (www.extension.iastate.edu). A study at the University of Alberta (Mason, 2001) revealed that feeding canola oil in the encapsulated form (to protect it from biohydrogenation by the rumen microorganisms) led to higher increases in linoleic (18:2) and linolenic (18:3) acids than while feeding unprotected oil seeds. As the melting point of milk fat containing unsaturated fatty acids is more, the spreadability of butter made from this milk improved tremendously. An Australian study involving the feeding of a special blend of canola and soybean meal in the protected form resulted in doubling the spreadability of butter (CSIRO, 1999). When taken out of a refrigerator at 5 C, the butter was nearly as spreadable as margarine, without losing its special eating qualities. Clinical trials revealed that consumption of dairy products made from this milk led to decrease in low density lipoprotein (LDL) levels in the blood of the consumers.
Chouinard et al. (1998) compared the results of feeding to Holstein cows, a control total mixed ration (TMR) with TMR supplemented with calcium salts of three fatty acids from oils with progressive degree of unsaturation— canola oil, soybean oil, or linseed oil. The digestibility of nutrients was higher for rations containing calcium salts than for the control ration. The milk yield increased in proportion to the degree of unsaturation in the feed supplement. The fat content in milk reduced in all the experimental diets as compared to the control. The addition of calcium salts to the ration decreased the proportions of SFA that contained C6–C16 and increased the proportions of C18:0, cis-9-C18:1, and trans-11-C18:1 in milk fat. These findings were confirmed later by Aigster et al. (2000) who reported that feeding calcium salts of high-oleic sunflower oil (HOSO) containing more than 86% oleic acid at the rate of 7.5% of diet dry matter weight to Holstein cows increased the oleic acid content of milk fat from 26% to over 40% and decreased the cholesterol-raising saturates from 41% to 33%.
Increasing CLA levels in milk fat
Milk fat is a good source of the putative anticancer agent, conjugated linoleic acid (CLA), a product synthesized in the rumen during the biohydrogenation of linoleic acid (LA). Research has shown that it is possible to influence the extent of ruminal biohydrogenation and the concentration of CLA absorbed and incorporated into milk fat. There is evidence that the concentration of CLA in milk influences its pharmaceutical properties (Kelly and Bauman, 1996). The level of CLA could, therefore, influence the value of the milk as a commodity, although it is not at present a criterion for deciding the price of milk.
CLAs reportedly suppress carcinogens, inhibiting proliferation of leukemia and cancers of the colon, prostate, ovaries, and breast. They are the only natural fatty acids accepted by the National Academy of Sciences of United States as exhibiting consistent antitumor properties at levels as low as 0.25–1.0% of total fats (Eynard and Lopez, 2003). The other reported beneficial health effects of CLA as supported by biomedical studies with animal models are antiatherogenic effect, altered nutrient partitioning, improved lipidmetabolism, antidiabetic action (type II diabetes), immunity enhancement, and improved bone mineralization (Bauman et al., 2001; Bell and Kennelly, 2001).
Reports suggest that feeding lipid sources rich in linoleic and linolenic acids either as seeds or free oil increases the CLA content of milk when oil is accessible to the rumen microorganisms for biohydrogenation (Dhiman et al., 2000). The scientists found that supplementing the dietary dry matter with 2% or 4% soybean resulted in a 237% or 314% increase in CLA content of milk compared with the control.
Stanton (2000) and her team worked on the supplementation of cow’s diet with ingredients such as full fat rapeseed, full fat soybean, and pulp-n-brew (by-product of brewers’ grains rich in LA) to study their effect on the CLA levels in milk. When diets of pasture-fed cows were supplemented with full fat rapeseed and full fat soybean, the CLA levels in milk fat increased by 53% and 34%, respectively, after 18 days of feeding when compared to the unsupplemented group of cows on pasture which served as control. The yield and proximate composition of milk were unaffected by the supplementation.
Milk from a grass-fed cow can have five times as much CLA as milk from a grain-fed animal (Robinson, 2003). An experiment supplementing either silage, autumn grass, or spring grass over three periods with pulp-n-brew revealed that the CLA levels increased in case of supplementation of silage and autumn grass, but was less effective in the case of spring grass (Stanton, 2000). Spring grass feeding led to a 2.1-fold increase in CLA content of milk. The CLA-enriched milk fat exhibited cytotoxicity toward mammary and colon cancer cells.
Incorporating CLA along with soy oil in the diet of cows increased the CLA levels, simultaneously decreasing the SFA in milk fat (Pszczola et al., 2000). In an attempt to increase the CLA content in milk via the cow’s diet, Bell and Kennelly (2001) divided 28 Holstein cows into 4 groups and fed them different diets—control diet (CTD), low-fat diet (LFD), high-fat diet 1 (HF1), and high-fat diet 2 (HF2). The animals were kept on CTD for 8 days before starting the different diet regimen. All experimental diets resulted in lower fat percentage in the milk when compared to CTD, whereas other parameters such as milk yield, protein, and lactose were unaffected. The CLA concentration in milk fat was 0.49%, 0.56%, 3.7%, and 5.63% in the group fed CTD, LFD, HF1, and HF2, respectively. Thus, increasing the fat content in the diet increased the CLA content up to 9–12 times, despite lower total fat content.
Abu Ghazaleh et al. (2003) found that feeding lactating dairy cows a blend of fish oil and MUFA and PUFA resulted in an increase in the concentrations and yields of CLA in milk, the greatest increase being with a blend of a high LA source (e.g., regular sunflower seeds). Beaulieu and Drackley (2004) reported similar results where a diet rich in LA led to increasing the CLA levels in milk fat twofold. Supplementing nonluminous green fodder with mustard cake in the feed of buffaloes resulted in 6.18-mg CLA per gram of fat as compared to 6.05 mg/g when the supplement was groundnut cake (Tyagi et al., 2004). The total CLA in buffalo milk and milk products increased significantly when the animals were fed berseem and wheat straw in the ratio 87:13.
Tsiplakou et al. (2006) examined the CLA content in the milk fat of sheep and goat milk segregated into two groups. Animals in Group 1 were totally on pasture from April onward with supplementary feeding during winter, whereas animals in Group 2 served as the control group and were kept indoors without grazing. The study revealed that the CLA content in milk fat of Group 1 increased in April and May, during the availability of early grass and declined thereafter, whereas that in Group 2 remained more or less constant. The CLA content (cis-9, trans-11) in sheep milk was 2% of the total fatty acids fat content and was much higher than that in goat milk (0.62%).
Animal variation is also a major source of differences in the CLA content of milk fat. Bauman and Perfield (2002) discovered that the 9,11 isomer of CLA in milk fat is synthesized by the cow and not rumen bacteria as had earlier been reported. Synthesis involves a mammary enzyme, delta-9 desaturase, which acts on a trans-fatty (vaccenic) acid produced by rumen bacteria. Several genetic factors that regulate the expression of the delta-9 desaturase gene have been identified. In a line of transgenic goats that contained a rat stearoyl-CoA desaturase gene targeted at converting medium- and long-chain SFA to their monounsaturated forms, Reh et al. (2004) found that the desaturase enzyme also converted the rumen-derived MUFA C18:1 trans-11 to the C18:2 cis-9 trans-11 isomer (CLA) in the milk fat of one of these animals.
The omega fatty acids
Omega-6 and omega-3 are essential fatty acids, but the body requires them in a ratio that is not normally achieved by the typical diet of today’s developed nations. It is reported that the current average intakes of essential fatty acids expressed as ratios of o-6 to o-3 fatty acids are 8:1 in United Kingdom, 10:1 in United States, and 12:1 in Australia (www. omega-3info.com). Health bulletins indicate that the proportion of o-6 to o-3 fatty acids should be equal or close to 5 for cardiovascular health (Simopoulos, 1999).
At present, the average PUFA content in modern diets (nearly 30% of calories) is too high. It is suggested that our PUFA intake should not be much greater than 4% of the caloric total, in approximate proportions of 2% o-3 linolenic acid and 2% o-6 linoleic acid (Fallon and Enig, 2000). The intake of total o-3 fatty acids in the United States is 1.6 g/day (Kris- Etherton et al., 2002). Of this, a-linolenic acid (ALA) accounts for 1.4 g/ day, whereas eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) together account only for 0.1–0.2 g/day. DHA is required by the brain and nerve cells and is essential for normal visual and neurological development in infants (Tomlinson, 2003). The major food sources of ALA are vegetable oils, principally canola and soybean oils. Oily fish are the richest source of EPA and DHA. EPA and DHA can be made by the body from ALA, but sometimes this capacity is impaired, so oily fish remains the best source. The recommendations for intake of o-3 fatty acids range from 0.5 to 2 g/day. ISSFAL (International Society for the Study of Fatty Acids and Lipids) recommend 0.65-g EPA and DHA per day (Willumsen, 2006). Of this, the content of each should be at least 0.22 g. Omega-6 is the essential fatty acid that is in ample supply in oils, nuts, and seeds.
Too much o-6 in the diet creates an imbalance that can disrupt the production of prostaglandins leading to increased tendency to form blood clots, inflammation, high blood pressure, irritation of the digestive tract, depressed immune function, sterility, cell proliferation, cancer,water retention, and weight gain. On the other hand, deficiency in o-3 is associated with asthma, heart disease, and learning deficiencies. It is established that o-3 fatty acids have a hypolipidaemic action in human, reducing harmful cholesterol levels, particularly plasma triglycerides (Tomlinson, 2003). It also has an anti-inflammatory action and helps to reduce platelet aggregation. Essential fatty acids have proven to be effective in the treatment of several other ailments including eczema, rheumatoid arthritis, asthma, Alzheimer’s disease, and AttentionDeficitHyperactivityDisorder (ADHD). There are reports that approximately equal amounts of these two fats in the diet will result in lower risk of cancer, cardiovascular disease, autoimmune disorders, allergies, obesity, diabetes, dementia, and some mental disorders.
Dietary manipulation in cows is a practical way to maintain a desired ratio of o-6 to o-3 fatty acids in milk. Milk from pastured cows contains an ideal ratio of essential fatty acids. Dhiman et al. (1999) reported equal quantities of the omega fatty acids (16.5 mg/g fat) in the milk of cows entirely on pasture. Reducing the proportion of grass to two-third of the ration increased the o-6 fatty acids to 31.4 and decreased o-3 fatty acids to 13.5 mg/g milk fat. Further reduction in the dietary proportion of grass to one-third resulted in 42.7 and 8.2 mg/g fat of o-6 and o-3 fatty acids, respectively. There are reports that organic milk contains almost 70% more o-3 fatty acid than nonorganic milk (Cheek, 2006).
Mammals are dependent on dietary sources of essential fatty acids as they lack the desaturase enzymes necessary to synthesize them. Kao et al. (2006) engineered transgenic mice expressing the o-3 fatty acid desaturase enzyme from the nematode Caenorhabditis elegans, which synthesizes a wide range of PUFA and possesses the only known example of an o-3 desaturase enzyme in the animal kingdom. The milk from these mice had more o-3 and less o-6 PUFA, and hence had showed an overall decrease in the o-6:o-3 PUFA ratio in the milk. The milk phospholipids from the transgenic mice had an o-6:o-3 ratio of 1.78 as compared to 9.82 in the control animals. The authors anticipate that this may be a suitable method to improve the nutritional profile of dairy-based diets.
Reducing fat content in milk
It has long been recognized that the yield of milk fat can be altered through nutritional interventions. Several workers have reported that supplementing normal diet with fats in different forms and concentrations decreases the yield of fat in milk (Baumgard et al., 2000; Bell and Kennelly, 2001; Chouinard et al., 1999; Peterson et al., 2002). Genetic studies also pointed to the power of hereditary traits in influencing the quality of milk. Genetic markers for milk quality of dairy cattle were discovered and reported by the Iowa State University in the United States in 1996 (http://www.biotech.iastate.edu/database-of-researchers/). Laboratory experiments with the marker revealed that animals with the ability to produce low-fat milk could be accurately identified. Such genetic testing was aimed at improving dairy herd performance by identifying animals with the potential to produce low-fat milk. A herd producing low-fat milk was seen as a means to reduce milk-processing costs as the low-fat milk eliminated the necessity to separate fat from milk.
As a variation to altering the fat composition, Wall et al. (1997) suggested that modifying the cow’s genetic makeup to enable it to produce milk with 2% fat would reduce the cost of feed per kilogram milk by 22%. In changing the fat composition, targeting enzymes that influence the synthesis of fat is important. As an example, reduction of acetyl-CoA carboxylase that regulates the rate of fat synthesis within the mammary gland would translate to a drastic reduction in the fat content of milk and reduce the energy required by the animal to produce milk (Ntambi et al., 1999).
Type of fatty acids versus product quality
The type of fatty acids present in milk fat can influence the flavor and physical properties of dairy products. There are reports that butter produced from cows fed high-oleic sunflower seeds and regular sunflower seeds were equal or superior in flavors to the control butter (Middaugh et al., 1988). The experimental butter was softer, more unsaturated and exhibited acceptable flavor, manufacturing, and storage characteristics. Other workers (CSIRO, 1999; Mason, 2001) have also reported the increase in the unsaturated fatty acids content in milk fat, leading to an improvement in the spreadability of butter even at refrigerated temperatures.
Extruded soybean and sunflower diets yielded a Cheddar cheese that had higher concentrations of unsaturated fatty acids while maintaining flavor, manufacturing, and storage characteristics similar to that of control cheese (Lightfield et al., 1993). It is also beneficial from a safety point of view as the accumulation of fatty acids, namely C12, C14, C18:1, and C18:2, enhanced the safety of cheeses against Listeria monocytogenes and Salmonella typhimurium (Schaffer et al., 1995).
Increasing the oleic acid content of milk fat from 26% to over 40% by feeding calcium salts of HOSO containing more than 86% oleic acid at the rate of 7.5% of diet dry matter weight to Holstein cows did not affect the sensory and physicochemical properties of Latin American white cheese (Queso Blanco). There was also no difference (as a result of the modified and improved fatty acid profile) between the firmness of the product from modified milk and that made from normal milk (Aigster et al., 2000).
To be continued…