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Hot Topic: Heat Treatment Influences Milk Protein Digestion

    Dairy factory with milk pasteurization tank for milk heat treatments

    Written by: Lauren Milligan Newmark, Ph.D. | Issue # 121 | 2024

    • Standard treatments that increase cow milk’s microbial safety can influence the structure of milk proteins but do not reduce the protein content of milk.
    • Female adults in a randomized, controlled trial digested ultra-high temperature milk more slowly than pasteurized milk, however ultra-high temperature milk was associated with a more rapid release of amino acids into the bloodstream.  
    • For most consumers, these digestive differences may not be significant, but there are some populations where a rapid release of amino acids would be beneficial.

    When it comes to pasteurization, cow milk proteins can take the heat—to a certain degree. Although boiling cow’s milk can reduce protein levels, neither high-temperature short-time (HTST) pasteurization nor ultra-high temperature (UHT) processing decreases the protein content of cow’s milk. 

    “Heat treatment doesn’t destroy milk protein,” explains Dr. David Dallas, Associate Professor in the Nutrition Program of the College of Health at Oregon State University. “It isn’t that proteins are nutritionally damaged, it is more about their bioactivity and structure.” 

    For human consumers, these structural changes could influence the way milk proteins are digested, including how they are broken down, absorbed, and utilized by the body [1-3]. 

    Heat treating milk isn’t new—pasteurization techniques have been used for over a century to increase milk’s microbial safety and shelf life. It seems surprising, then, that there have only been a handful of studies addressing how different milk heat treatment methods affect nutrient delivery of protein in humans [3, 4]. 

    “Heating intensity is very interesting for bovine milk proteins,” explains Dr. Kasper Hettinga, Professor of Food Sciences and Agrotechnology at Wageningen University. Whey proteins, which make up 20% of milk protein, are heat sensitive and denature when heated. Although denaturing sounds like a negative, Hettinga shares that “it can actually make the proteins easier to digest.” 

    Milk’s whey proteins have a three-dimensional structure—imagine a ball of yarn that has been quickly wound up and has twists, turns, and tangles. To digest whey protein requires unwinding and untangling this three-dimensional structure into one long string of amino acids, then snipping apart the peptide bonds that link the amino acids. Denatured whey proteins arrive in the digestive tract already a bit untangled, allowing the digestive enzymes to release the individual amino acids in a shorter amount of time [2].

    Casein proteins, which make up the other 80% of cow milk protein, respond differently to heating than most food proteins [2]. “Caseins are more flexible molecules and do not lose their structure when heated,” says Hettinga. Instead of a tangled ball of yarn, casein proteins are more spherical in shape. These small spheres, called casein micelles, are usually digested more slowly than whey proteins because they form a curd in the digestive tract when enzymes start to break them apart. However, in vitro models of digestion suggest that increasing the temperature of milk, as is done with UHT processing, can result in a more crumbled or fragmented curd, which would promote more rapid casein digestion [2].  

    A new study [1] from a team of researchers in New Zealand provides one of the most comprehensive and systematic studies on the digestive kinetics of heat-treated milk proteins to date. Combining MRI technology and blood biochemistry, the researchers compared digestive rates and nutrient delivery of cow milk proteins from two standard heat treatments. Based on results from in vitro digestion simulations and animal models, they predicted that the higher temperature used by UHT treatments (preheated 95° C, 90 seconds, processed 140° C, 4 seconds) compared with HTST pasteurized milk (75° C, 15 seconds) would result in faster gastric emptying and thus a faster delivery of nutrients to the bloodstream.

    In the study, each of the 20 female adult participants consumed both HTST pasteurized milk (PAST-M) and UHT treated milk (UHT-M). Participants were randomly assigned which milk they would consume first, and both participants and researchers were blinded as to the type of milk participants were drinking. There were three to 28 days of “washout” between consumption of each milk type to minimize any cross-over effects of study procedures. Participants consumed identical dinners and then fasted starting from 10pm on the night prior to consuming each intervention milk sample. On the morning of the intervention, participants drank 500ml of refrigerated milk and then had MRI scans of their gastric contents after 5, 10, 20, 30, 60, 90, 120, and 180 minutes and venous blood draws after 20, 30, 40, 60, 90, 120, 240, and 300 minutes. 

    As predicted, amino acids showed up more quickly in blood samples after UHT-M consumption compared with PAST-M. However, this faster release of amino acids was not because of faster milk digestion; contrary to results from in vitro models, gastric contents emptied more slowly after consuming UHT-M, particularly during early digestion [1]. MRI scans suggest that casein proteins from UHT milk formed a clot with less dry matter (and more water), which stayed in the stomach longer than the dense clot formed by PAST-M caseins [1]. The study authors suggest that the rapid release of nutrients associated with UHT-M, including branched chain amino acids and essential amino acids, were part of the liquid (whey) phase that was not restricted within the casein curd [1].  

    HTST pasteurized milk, which is the most consumed milk in the U.S. [4], was associated with a slower release of amino acids but faster gastric emptying, whereas UHT milk, which is more shelf stable, had a faster release of amino acids but slower gastric emptying—but is one digestive pattern more optimal than the other? 

    “Milk is the gold standard when it comes to protein nutrition and digestibility,” explains Dallas. “Compared with soy protein which has low bioavailability and low digestibility, 100 percent of the milk protein you consume will be digested by humans, with or without heat treatments.” 

    However, there may be clinical applications where a faster delivery of amino acids could be advantageous [1]. “For the average human adult drinking either [pasteurized or UHT] milk, it isn’t that one milk type is going to be much better,” says Hettinga. “But there may be benefits for specific target groups, like the elderly.” Hettinga explains that as humans age, their digestion slows down. A more rapid release of branched chain amino acids, particularly leucine, that help stimulate protein synthesis in skeletal muscle could be especially beneficial for this population. 

    Cow milk is an important source of high quality protein for humans of all ages [2]. The results from this study suggest that commonly used heat treatments can be an effective way to control physiological responses and enhance the delivery of essential nutrients [1]. 

    References

    1. Milan AM, Barnett MP, McNabb WC, Roy NC, Coutinho S, Hoad CL, Marciani L, Nivins S, Sharif H, Calder S, Du P. The impact of heat treatment of bovine milk on gastric emptying and nutrient appearance in peripheral circulation in healthy females: a randomized controlled trial comparing pasteurized and ultra-high temperature milk. The American Journal of Clinical Nutrition. 2024 May 1; 119(5): 1200-15.
    2. van Lieshout GA, Lambers TT, Bragt MC, Hettinga KA. How processing may affect milk protein digestion and overall physiological outcomes: A systematic review. Critical Reviews in Food Science and Nutrition. 2020 Aug 5;c60(14): 2422-45.
    3. Fatih M, Barnett MP, Gillies NA, Milan AM. Heat treatment of milk: a rapid review of the impacts on postprandial protein and lipid kinetics in human adults. Frontiers in Nutrition. 2021 Apr 30; 8: 643350.
    4. Lacroix M, Bon C, Bos C, Léonil J, Benamouzig R, Luengo C, Fauquant J, Tomé D, Gaudichon C. Ultra high temperature treatment, but not pasteurization, affects the postprandial kinetics of milk proteins in humans. The Journal of Nutrition. 2008 Dec 1; 138(12): 2342-7.