My algorithm for selecting a butter/margarine-type-thing when shopping is simple, and not very scientific. I avoid butter, partly because I think it is bad for you, but mostly because it is a pain to spread straight from the fridge. I am not organised enough to take it out of the fridge early.

This leaves me selecting almost randomly from a bewildering variety of low and even-lower-fat spreads. So last time I went for the even-lower-fat spread and it was fine. Until I put it on bread I was toasting in the grill. When I did that a lot of the spread seemed to disappear into the bread.

Looking at the ingredients list, this is not surprising. The biggest ingredient is water. Vegetable oils are next, but then there is pork gelatin and rice starch. I don’t know how much of the spread is water, but as it is the main ingredient presumably at least around 30%, and maybe more than 50%.

We should not be surprised that water is used in low-fat foods, water is the perfect ingredient for makers of low-fat foods. It contains 0 calories, and is cheap. But of course, the discerning consumer is not going to be impressed if they open a pot of spread, and see what looks more like Evian that butter.

This is where gelatin and rice starch come in. They are both polymers, i.e., very big molecules, which have the handy property that small amounts of them can turn a liquid in a soft solid – which is what we expect a low-fat spread to be. Starch is what we use to make custard, and gelatin is what makes jellies jellies.

But gelatin melts at not much above body temperature, and then out comes the water. As the low-fat spread has so much water held together by gelatin, the spread only resembles butter/margarine at relatively low temperatures. The other problem with gelatin is that although water is of course vegetarian friendly, as well as being both halal and kosher, pork gelatin is none of those things. Those with dietary restrictions should be careful with low fat foods, they may contain more pork than you expect.