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Today I made Pineapple smoothie with milk. It was tasty. But I kept it in fridge cause I wanted the drink to be even cooler. I forgot that pineapple will curdle the milk and turned the smoothie bitter. However, what I don't understand is that I always thought curdling happens with heat. And I used chilled milk with ice cubes and even frozen pineapple and I immediately put in the refrigerator. Is there any way to make pineapple smoothie with milk/curd and not let it get bitter?

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    On a side note, the average temperature of chilled milk + ice cubes + frozen pineapple is colder than the temperature inside your fridge. So putting it in the fridge will actually warm it up rather than making it "even cooler".
    – JBentley
    Jun 10 at 12:04

3 Answers 3

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Pineapple contains bromelain, which is a powerful enzyme that breaks down most animal proteins. As such fresh pineapple will always curdle milk. The bitter flavor is a side-effect of this curdling.

The only way to prevent this is to break down the bromelain by cooking the pineapple. So you can make a smoothie either by cooking the pineapple yourself, or by using canned pineapple.

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  • Chilli retards the enzyme a bit. I've always had success adding pineapple to jelly (which normally would break down the jelly) by cooking the pineapple with a bit of chilli - usually just one for me. It could work in this case
    – slebetman
    Jun 11 at 16:32
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In addition to @FuzzyChef's answer, pineapple contains acids, mostly in the form of citric acid, but also malic acid and a few others in lesser quantities. The citric acid content ranges from between 0.4% to 1.2%, depending on the report (e.g. this one, PDF, probably paywalled). Because of these acids, the juice of pineapples has a pH of around 3-4, making it substantially more acid than milk with a pH of 6.7-6.9.

Acidity can curdle milk, as you will have noted if you have ever added lemon juice or vinegar to milk to make a substitute for buttermilk.

So, even if you do manage to kill off the bromelain by cooking you will still have to contend with the acidity, which will still curdle your milk.

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    Pineapple is too high pH by itself to curdle milk in a practical use case - you need an overall pH of around 4.6 (see this for example) to curdle, which would require quite a lot of pineapple (pH is a log scale, remember, so the pH of pineapple juice (3.7) needs a very high amount to bring milk to 4.6 as compared to lemon juice (around 2). The point at which you'd have "curdled" milk with pineapple juice, you probably wouldn't care much.
    – Joe M
    Jun 9 at 22:32
  • @JoeM you certainly get curdled milk with addition of <30 ml of pineapple juice per cup (~250 ml) milk from my practical experience with them.
    – bob1
    Jun 9 at 23:01
  • @JoeM: How do you define "very high amount"? If we assume that the pH of pineapple juice is 3.7 and the pH of milk is 6.7, and if I've done my math right, then a mixture of 1 part pineapple juice to 7 parts milk will have a pH of almost exactly 4.6.
    – ruakh
    Jun 11 at 8:27
  • @ruakh that coincides quite nicely with my observation - 250/7 = 35
    – bob1
    Jun 11 at 8:56
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Not sure if this will work for you, but personally I tend to sidestep the issue by using a milk substitute - typically soy milk - if I'm going to be mixing it with fresh pineapple.

If you do go for the cooking route, I wouldn't worry about the acidity so long as you're mixing the milk and cooked pineapple cold - but if you mix them while the pineapple's still warm, it's likely to curdle.

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  • The bromelain breakdown is curdling; it's not doing it in the same way an acid does, but it still does (eventually) cause clumping (it's doing a similar thing, which is breaking down the caseins and bringing them out of suspension. See for example this article which describes doing this experimentally.
    – Joe M
    Jun 9 at 22:33
  • yeah I thought that too. Cause when I curdle milk with vinegar, the milk needs to be hot for it to work.
    – 4-K
    Jun 10 at 7:14
  • @Joe M - that experimental result disagrees with my experimental results, but I don't have the time to research it properly and it was an aside anyway, so I've removed that part.
    – Nye
    Jun 13 at 14:21

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