For the last few years I've made my tomato sauces in a slow cooker ('crock pot'), or actually, in a machine that is sold as a 'plate warmer' but works great for low temperature cooking. I pre-heat the ingredients on my stove, then transfer it to the slow cooker for 10 or 14 hours to let all the flavors blend, then I puree and can the result. When I first learned this technique, I was told not to stir the sauce, because the long cooking time makes the sugars at the top caramelize and that would bring out a great sweet flavor. Indeed the top, after being in the cooker for that long, browns a bit, and the flavor is great.

However, recently I was learning a bit more about caramelization to understand my baking better, and it turns out that there are no sugars that caramelize at temperatures < 110 °C. So now I'm wondering - is this caramelization of my tomato sauce just a myth? The machine only goes up to 90 °C. I've checked the temperature at various depths in my sauce with an infrared thermometer, and indeed the temperature is nowhere higher than that. Anyone have more than anecdotal information on the chemistry of making tomato sauce?

up vote 21 down vote accepted

Short answer: if it doesn't get heated to the caramelization temperature then it does not caramelize. The science is here, and it says you need at least 110 °C for fructose.

Browning in your case is probably not caramelization, but a Maillard reaction, which

is a chemical reaction between an amino acid and a reducing sugar, usually requiring the addition of heat.

Maillard reaction can happen at lower temperatures if given enough time.

Lifted directly from this very useful answer that cites the excellent Harold McGee's book On Food and Cooking (emphasis mine):

There are exceptions to the rule that browning reactions require temperatures above the boil. Alkaline conditions, concentrated solutions of carbohydrates and amino acids, and prolonged cooking times can all generate Maillard colors and aromas in moist foods. For example, alkaline egg whites, rich in protein, with a trace of glucose, but 90% water, will become tan-colored when simmered for 12 hours. The base liquid for brewing beer, a water extract of barley malt that contains reactive sugars and amino acids from the germinated grains, deepens in color and flavor with several hours of boiling. Watery meat or chicken stock will do the same as it's boiled down to make a concentrated demiglace. Persimmon pudding turns nearly black thanks to its combination of reactive glucose, alkaline baking soda, and hours of cooking; balsamic vinegar turns nearly black over the course of years!

So in your temperature range:

  • ~212-300 °F (100-150 °C) - Maillard gets slower as temperature goes lower, generally requiring many hours near the boiling point of water
  • ~130-212 °F (55-100 °C) - Maillard requires water, high protein, sugar, and alkaline conditions to advance noticeably in a matter of hours; generally can take days
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    In an actual slow cooker, a little caramelisation may be possible at the edge (the pot can get enough above 100C above the waterline that stirring can cause splashes to sizzle so above that 110C) but this answer explains the vast majority of the browning even then. – Chris H Aug 17 at 12:31
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    @ChrisH I thought about that, but didn't want to mention since OP verified the temperature with an IR thermometer – Luciano Aug 17 at 12:41
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    Yes, it's probbaly better not in the answer. This is a rare example of slow-cooking that isn't in a slow-cooker, which was topic of discussion in meta recently – Chris H Aug 17 at 12:49
  • Ah thank you, for some reason I was under the impression that Maillard reactions only happen when baking, and I didn't realize their effect on flavor either. So this is then probably what explains the difference in flavor of this sauce when slow cooked vs when made on a stove top in say 45 or 60 mins? – Roel Aug 17 at 13:16
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    @Roel probably. Low temp + long time affects flavors differently than high temp + short time. – Luciano Aug 17 at 14:28

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