Enzymes are the very actors for all metabolic processes. They are relatively large proteins with very specific secondary and tertiary (folding) structures. They participate as catalysts with extremely specific substrates and products, but they are dependent on specific co-factors (often metal ions) for activity and each enzyme has a fairly narrow range of pH and temperature over which it is active. Enzymes can be denatured in many ways - these are typically irreversible changes to the tertiary structure which render the enzyme ineffective.
Human and bovine body temperatures aren't very different, but the difference between the natural pH of milk, that of a young calf's first stomach the pH of the human stomach are each so different that enzyme activity in one environment will likely be virtually non-extant in any of the others. You can't expect any particular enzyme to have continued activity when consumed without understanding the temperature and pH and even the co-factor issues of that specific enzyme. Many fungal and bacterial enzymes can have activity at human stomach pH & temp, for example "Beano" is galactosidase from Aspergillus niger fungus and is active in the digestive tract. "Lactaid" lactase enzyme is also a galactosidase from Kluyveromyces or Aspergillu fungi, active at human digestive tract pH.
Plant enzymes often have a wide range of temperature, but typically require a higher pH than the human digestive tract. So the heathfood contention that consuming plant enzymes to aid digestion is very doubtful. For example the idea that bromelain (tropical fruit protease) can have any effect at stomach pH is woo - yet this sort of claim is common:
http://www.vitaminshoppe.com/store/en/browse/sku_detail.jsp?id=VS-3006
Not necessarily. The enzyme could be (nearly) inactive, yet not denatured at milk pH. Still this seems wrongheaded. The evolutionary logic of milk containing lactose as an energy media instead of the component glucose and galactose, or even lactate is likely that even though creating lactose in the parent cow costs extra energy it makes the energy less available to opportunists. You need an organism capable of hydrolyzing lactose, and that excludes a lot of fungi, bacteria and even most adult mammals. So it doesn't make sense for milk - at least in the parent cow - to hydrolyze the lactose. A young calfs first stomach pH is much higher than an adult humans' so even if the enzyme is *intended* to become active in a calfs gut, it's far from certain it would have any effect in a human gut.
In fact most adult humans can't produce lactase, and therefore the lactose in milk is digested by intestinal flora causing stomach upset. This was certainly true before pasteurization and homogenization, and demonstrates that any residual lactase in cows milk is ineffective in the human gut. Interestingly, milk fermentation into yoghurt, koumiss, kefir, buttermilk with several types of Lactobacillus or Lactococcus removes nearly all of he lactose eventually and makes this high nutrition food accessible to non-lactase producing humans.
Some bacterial lactases will work in the human gut, but understand that enzymes common names refer to their catalytic function and not to a specific molecular structure. Aspergillus lactase is certainly different from human lactase which differs from bovine lactase despite similar activity. They each have potentially different pH, temps and co-factors.
Yes - possible, tho' quite unlikely. A skeptic would require evidence for that rather extraordinary claim.
No - wrong on that last point. The shear forces created in homogenization are very likely to denature enzymes. I am familiar with this topic from the brewing industry. The "mash" is a term used for an enzymatic degradation of malt starch, proteins and cellulosic materials. During the mash the crushed malted grains are mixed with water and held around 60-70C, at a controlled pH, for an extended period of enzymatic degradation. The type of mixing pumps used have an impact on the rate of enzyme denaturing because shear forces within the pumps disrupt the protein tertiary structure.
So there is very good reason to believe that homogenization mixing - with vastly higher shear forces - causes significant enzyme denaturation. Whether this is a 10% decrease or a 99.9% decrease is not in evidence. Pasteurization is even more likely to cause nearly complete denaturing of milk enzymes. The idea that these enzymes can be effective in the human gut, even when not denatured, is doubtful.
Well, fruit and meats contain enzymes that tend to cause something like digestion spontaneously, but obviously that is not the goal/reason. Milk probably contains enzymes strictly as a byproduct of the production metabolism. I see no evidence otherwise.
There is a real unanswered question here. Not only are the (probably ineffective) milk enzymes denatured by homogenization and pasteurization, but all proteins in milk are effected. Are these disrupted proteins less easily digested ? I think this is unlikely since gastric acids are sufficient to cause much greater changes in structure and condensation of proteins than homogenization and pasteurization. All the protein we digest undergoes this acidic treatment.
Yes, there is a lot of woo and non-skeptical thinking out there regarding milk, enzymes and foods generally.
