When it comes to the preparation of food, we owe a great deal to the experimentation of past generations. Successful recipes have been preserved by word of mouth and cookbooks, especially in the past century. As I continue to learn how to cook I prefer the advice of my wife and mother to that of cookbooks. And I especially like to think and dig into the science behind the sensibility of their methods.
In this essay, I’ll focus on the preparation of tomato sauce. Only popularised in the 1800s, tomato sauce was first made by Europeans in the 1700s from a fruit indigenous to Mexico and parts of South America. For hundreds of years before it was used on pizza and pasta, people from the New World ate tomato fruits from a wide variety of wild species.
Tomatoes have to be ripe to make a good sauce. Many of these compounds only form during the ripening process. But if you just squeeze ripe tomatoes, you will get tomato juice and not sauce. If tomatoes are cooked , even more compounds will be created, changing the taste of the juice, but it will still have an unappealing flavor. To get the full aroma and mouth-watering taste we need a non-random orchestra of compounds consisting of organic acids, soluble sugars, amino acids, pigments and over 400 compounds capable of interacting with our nasal receptors. This translates into a need for more food ingredients.
To address the acidity issue, which is often at a pH of below 4, essentially we have to fool our taste buds. Although the addition of sugar does nothing to the pH, it does mask the sourness. My wife and relatives, however, do not add the refined compound. Instead of a teaspoon of sugar, they blend in a carrot for every half kilogram of tomatoes. An 80 gram carrot has almost 4 grams of sugars, mostly sucrose and the remainder being an almost even-split of glucose and fructose. But the carrot’s fiber-content reduces its glycemic load to 3. The popular alternative ingredient, a teaspoon of sugar, has a mass of 4 grams of sucrose with zero-fiber and a glycemic load of 4. Granted, that’s not a prohibitively high number, but the pure compound also lacks the carrot’s vitamin A, lutein, potassium, small amount of protein and flavor. So for me it’s a no-brainer; carrots are a better alternative.
While tomatoes are being blended with herbs and carrots, a separate ingredient, the onion, is sautéed to further enhance the sauce’s flavor. Why? Onions are about 4% sugar with a glucose to sucrose to fructose ratio of 2:1:1. The high pan-heat is absorbed by the oil, which transfers some of the energy to the cut onions. The sucrose decomposes to fructose and glucose, temporarily adding to what’s already present. Then some of the latter pair of molecules dehydrate or go through other means of decomposition to yield pleasantly aromatic molecules such as butane-2,3 dione and several derivatives of furoic acid. Meanwhile other glucose and fructose molecules bond to produce a variety of oligomers responsible for the brown colour and unique taste of sautéed onions. What we have described is the process of caramelization, which is still not fully understood.
After letting the mixture of cooked onions and oil cool for five minutes, we place it into the blender with the carrot, herbs and tomatoes. Why? By doing so, the products of caramelization will mix more evenly with the bulk of the sauce, and we will also be creating a mixture that will at least be temporarily homogeneous.
Since the bonds between water molecules are stronger than those between water and oil, stirring oil and water does not create a thoroughly consistent mixture. But high speed-blending will break up the oil from the cooked onion into small droplets. Water will interact with these droplets but without abandoning them, water molecules will still be capable of forming hydrogen-bond interactions with their own kind. In other words we will have an emulsion.
An emulsion isn’t always stable. Even with the arrangement I described, there is still interfacial surface tension between the oil droplet and water. If strong enough it will get the oil to coalesce and separate again. But if emulsifying agents are present—-in other words, if we have molecules with an affinity for both the the oil and water, the tension will be reduced and the emulsion will be at equilibrium. In homogenized milk for example, the fat is broken up into tiny droplets and the casein itself within the milk acts as an emulsifier. Among the plant products of the tomato, is there a “built-in” emulsifier in our sauce? A day after preparing it, the leftover sauce had still not separated; we still enjoyed its consistency of taste. Was the emulsifier effective enough to act over two or more days? It tasted too good for us to find out. Besides, eating freshly prepared meals is a healthy habit.
Unraveling the Chemical Composition of Caramel . Journal of Agricultural Food Chemistry. School of Engineering and Science, Jacobs University Bremen, 28759 Bremen, German