Well, that's because it contains vitamin C, which acts as an antioxidant, not the lemon itself or its acidity.
Let's imagine you want to make a fruit purée, for a sorbet or a fruit coulis, pears for example: you peel the pears, remove the cores and blend the whole thing, it's very simple.
The problem is that the pear purée will oxidize very quickly and change color, while the taste will not change (or only slightly), but it will be less appetizing.
Here's the trick: add a pinch of vitamin C to your fruit purée (as you blend), and the result is spectacular.
Here's a photo of a pear puree made 5 minutes ago: in the glass on the left, the puree on its own; in the glass on the right, the same puree with a pinch of vitamin C.
Vitamin C can be found in pharmacies, and you need to ask for some "for food use", i.e. non-aromatized lemon or orange, or in stores for the food trade. It's not very expensive, and you take a pinch each time.
Finally, a few examples of fruits and vegetables where vitamin C works wonders: avocados in guacamole, raw peeled purple artichokes (soaked in water with a pinch of vitamin C), strawberry or apricot coulis, etc.
As a general rule, whenever you have to soak something (vegetables or fruit) to prevent browning, systematically add a pinch or two of vitamin C to the water. You'll be amazed at how effective it is.To sum up
: a pinch of vitamin C in a preparation is highly effective in preventing the browning of fresh produce.
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