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My understanding is that lactose-free milk is made by adding the lactase enzyme to regular milk.

Is there enough lactase present in the milk to catalyse lactose in simultaneously-consumed regular dairy products, e.g. natural yoghurt?

(Assuming both are consumed in, say, the quantities you'd use on breakfast cereal, and that they're allowed to mix well.)

I am aware that lactase supplements exist; the question is whether these would still be necessary. I have anecdotal evidence that they may not be, but I am interested in a reasoned explanation one way or the other. In particular, whether the chemical mechanism by which lactase breaks down lactose consumes the enzyme as well, or whether it can continue catalytic actions on further lactose molecules afterwards.

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As you imply in your question, enzymes are catalysts, and as such, participate in a chemical reaction but remain unchanged after that reaction is completed. Therefore, once lactase has converted a lactose molecule into galactose and glucose, it is free to split another. According to this website, lactase can split up to sixty lactose molecules each second, so two lactase molecules can split 120; 4, 240; 16, 960. and so on. IOW, the more the lactase, the faster the conversion. This website provides a graphics that demonstrate how enzymes work.enter image description here

So to answer your question, yes, there is enough lactase present in a lactose-free product to further catalyze any lactose present in any added dairy products. Whether or not there is enough to catalyze that lactose before it passes into the large intestines is another matter, however. When lactase is added to regular dairy products, they're allowed to "ferment" for several hours. Since lactose is not cheap, only enough lactase is added to catalyze a given quantity of lactose in a certain amount of time. My guess is that there will still be a significant amount of lactose present before it's passed into one's large intestines, where it causes the symptoms of lactose intolerance.

There is a lactase enzyme sold on Amazon that can be added to regular milk. It requires five drops for sixteen ounces of milk. If my calculations are correct, it works out to 0.8 ml per gallon of milk.

FWIW, the product is excellent. It makes a gallon of lactose-free milk about a dollar cheaper, which adds up.

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  • Edited to include actual quantities.
    – BillDOe
    Jun 29, 2017 at 21:19
  • Nah, I was just too lazy to walk to the fridge and look at the label.
    – BillDOe
    Jun 29, 2017 at 21:32
  • Wow, thanks for the very detailed answer @BillOertell. Exactly the information I was hoping for.
    – hartacus
    Jun 30, 2017 at 8:40
  • @hartacus, the pleasure was mine. I actually learned something myself, as I was under the opposite impression (i.e. that lactase only split one lactose molecule), because of assumptions I made concerning the instructions regarding the lactase product I mentioned. I now realize that the recommended amount of lactase is based on it catalyzing the lactose in whole milk in 12 hours and not on it being used up.
    – BillDOe
    Jun 30, 2017 at 19:05

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