Tag Archives: carbohydrates

The building blocks of food

So now that we’ve talked a little about what it means to do science and some of the basic principles of food chemistry, the next question that might be reasonable to ask is: what is food? The answer to this question is complex, but it turns out it is also deceptively simple. Because most food is comprised almost entirely of four substances: water, fats, proteins, and carbohydrates. And if we understand what these four components of food include, it turns out that a lot of basic recipes can be interpreted as simple ratios of these four ingredients. So, what sort of properties can be attributed to these substances?

Water

As discussed in the last post, an important part of understanding how recipes work is determining whether something is acidic or basic, a characteristic which requires the presence of water. Another important feature of water is that water is a polar molecule, which means that one side of the water molecule (the side nearest the oxygen) has a slight negative charge (red) and the other side (nearest the two hydrogen molecules) has a slightly positive charge (blue). This is illustrated below and has important consequences for the chemical properties of water:

The polarity of the water molecule means that water is good at dissolving other substances. Because many proteins and carbohydrates have similarly uneven distributions of charge, the polarity of water molecules causes them to cluster around the charged regions of  these larger molecules. This mechanism works to pull apart the larger molecule, resulting in the breakdown of the molecule when exposed to water.

Fats

Something we all know about fats (even if we may never have thought about it much before) is that they don’t mix well with water. This you know if you’ve ever tried putting olive oil in your pasta water or tried to emulsify a vinaigrette. (Emulsification is the process of mixing two liquids which are normally unmixable, vinegar and oil in the case of a vinaigrette). The oil and the water don’t mix readily and if you want to emulsify your vinaigrette you’d best be prepared for some vigorous whisking. (You may know that there are tricks that you can utilize to help emulsify your vinaigrette and we’ll talk about the science of emulsification at some point).  The difficulty in mixing water and oil is due to the structure of the fat molecule. A fat or oil molecule is non-polar (unlike water which is polar) and therefore the two substances repel each other.

Another thing to note about fats is that some fats are what we call saturated fats, while other fats are unsaturated. A fat molecule is comprised of essentially two parts, a carboxylic acid  (\text{COOH}) bonded to a fatty acid chain comprised of carbon and hydrogen. The number of hydrogen molecules bonded to the carbon determines how saturated the fat is (saturated with hydrogen, that is). As you may know, the saturation of the fat effects its properties: because of the regular structure of the saturated fatty acid chains, saturated fats form a regular (solid) structure much more readily than unsaturated fats. Hence saturated fats are solid at room temperature (butter, vegetable shortening, etc.) while unsaturated fats remain liquid (olive oil, fish oil, etc.). Rancidity occurs when the carbon chain of the fatty acid is broken by a reactive molecule (such as oxygen in the air), producing small fragments. In saturated fat, the carbon backbone is protected by its bonds with the hydrogen atoms and is therefore less easily broken. This is why unsaturated fats go rancid much more quickly than saturated fats.

Proteins

Proteins are some of my favorite food molecules because they tend to change the most dramatically during cooking. A protein is a polymer, a long  chain (or possibly several connected chains) of molecules (in this case amino acids). So what’s an amino acid? An amino acid is comprised of an amine group (\text{NH}_2) and a carboxylic acid group (\text{COOH}) which are bonded to another structure, called the side-chain. The structure of the side-chain distinguishes between the different amino acids and is generally composed of carbon, oxygen, and nitrogen. Because there is variety in the structure of the amino acid side chains, the proteins they make up have interesting properties. A single protein will be comprised of a variety of amino acids and the way the side-chains interact determines the structure of the protein. This structure has three levels: the order of the amino acids themselves, the spiral structure formed by the bonding between the amino acids, and folding caused by bonding between amino acid side-chains located at different places in the amino acid chain. So what’s important about protein for the home cook? Proteins are important in the browning reactions which give foods flavor when cooked (we’ll discuss this in detail in the next post), also many amino acids and short chains of amino acids have flavors of their own and contribute to the taste of foods that have had their proteins partially broken down (say due to curing or fermentation processes). Furthermore, in addition to flavor, proteins are also important for texture. Many proteins can absorb at least some water, but only some proteins are water soluble (i.e. they dissolve in water). And as we discussed last time, proteins can denature (i.e. lose their structure) when heated, exposed to acid, or agitated which can be very important in cooking. And in addition to these interesting behaviors, some proteins are even more interesting and they’re called enzymes. Enzymes are a group of proteins that serve to catalyse specific reactions (i.e. help them to occur) and sadly most of these reactions in cooking have to do with spoilage. It’s enzymes which cause the oxidation (discoloration) of cut fruits (think apples or avocados), cause fish to turn mushy, and green vegetables to brown with age. However, enzymes also do some good things such as tenderizing meat or aiding  in fermentation.

Carbohydrates

Carbohydrates encompass a large family of molecules which we know colloquially by a variety of names: sugar, starches, and gums are all examples of carbohydrates. So named because they were initially thought to be made up of carbon and water, this later proved to be untrue. While carbohydrates are comprised of carbon, hydrogen, and oxygen, the hydrogen and oxygen do not form water in the carbohydrate structure. Now there’s a lot that could be said about carbohydrates, but I just want to stick to the basics for now, so let’s start with some terminology. A sugar is the simplest carbohydrate and is formed of a single molecule. There are many different types of sugar: sucrose, fructose, glucose, deoxyribose, etc. and when we eat them, sugars taste sweet. One step up from sugars we have oligosaccharides (i.e. “several-unit sugars”). “Saccharide” is another word for “sugar”, which you may have guessed if you know that “saccharine” is a synonym for “sweet”. These sugars are short groups of only a handful sugar molecules; however, they are too large to trigger our taste buds, and therefore don’t taste sweet to us. Interestingly, the human digestive system doesn’t break down oligosaccharides itself, instead they pass through to the large intestine where they’re broken down by bacteria. Next up we have polysaccharides (i.e. “many sugars”) or sugar polymers. Whereas the oligosaccharides were comprised of a handful of sugar molecules, a polysaccharide may contain as many as several thousand individual sugar molecules. Polysaccharides are pervasive in both biology and in cooking and are produced in plants (amylose, amylopectin, cellulose, etc.) and in animals (glycogen), and as the home cook is probably aware polysaccharides are commonly used for thickening. Flour, corn starch, pectin, and plant gums are all polysaccharides.

Putting it all together:

And now we get to the interesting part. It turns out that many recipes can be reduced to simple ratios of these four building blocks of food. For example lets consider different doughs, here are some simple ratios that could be used to make a variety of different baked goods:

  • Bread: 5 parts flour (carbohydrate) : 3 parts water : (teeny bit of yeast and salt)
  • Pasta: 3 parts flour (carb) : 2 parts egg (protein + fat)
  • Pie crust: 3 parts flour (carb) : 1 part water : 2 parts fat
  • Biscuits: 3 parts flour : 2 parts liquid : 1 part fat
  • Cookies: 3 parts flour (carb) : 2 parts fat : 1 part sugar (carb)

Note that these ratios generally omit small ingredients, such as chemical leaveners, which can have a big effect on the character of the final product. Though not all starches and sugars are made equal (you couldn’t go replacing the flour with cornstarch) these ratios are a good place to start and a good way to think about proportions in cooking. And a fun way to experiment! You have to admit, it’s pretty fantastic that simply changing the ratios of the same ingredients can have such a varied result.

References:

  • McGee, Harold. “The Four Basic Molecules of Food.” On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. 792-809.
  • Ruhlman, Michael. Ratio: The Simple Codes behind the Craft of Everyday Cooking. New York, NY: Scribner, 2009.