There are five kinds of food

There are five kinds of food.

That’s it. Five.

After studying food science it’s startling how much about food we don’t learn in school. I’m not talking about nutrition or making healthy choices. I’m talking about a basic understanding of how food is stored, distributed, and preserved. This is an introduction to food preservation. There are five kinds of food.

· Perishable foods

· Water activity controlled foods

· Acidified foods

· Low acid canned foods

· Chemically preserved foods

I’m going to devote a post to each one of these foods and explain what they are and what makes them different. To begin with though, I’d like to briefly discuss why foods spoil in the first place.

Foods are complex constructs of useful chemistry. They contain chemicals that can be broken down to release energy. If you remember your chemistry, you may recall that energy is stored in chemical bonds between atoms. It takes energy to break these bonds and when new ones form, energy is released. (Phrased more elegantly, energy is not stored in things but in the connections between them.)

This chemical reaction would consume some energy as bonds are being broken. Energy would be released when new bonds are formed. (Image:

Foods are grown in the real world and the real world is competitive. We aren’t the only thing that wants to eat. Microbes (tiny organisms that include bacteria and fungi) want to eat our food too. Microbes are naturally present in foods and plants, animals, milk, and eggs have inventive mechanisms to keep them in check. For most foods, mold is tenacious. Molds (a kind of microscopic fungi) are versatile, energetically thrifty, and creative. They when food spoils from neglect, mold is opportunistic. Microbes aren’t too different from us. They want to survive and procreate and they need certain things to do this — water, food, and a hospitable environment.

This lemon has seen better days. Mold is easy to spot on food. It requires oxygen to grow so it’s always found on the surface. Moldy foods won’t hurt you but they’ll usually taste pretty bitter from the breakdown of proteins into small chains called peptides. More about that later.

Foods can also spoil for another reason entirely. The molecules that make up food are somewhat unstable. Oxygen, which makes up about 20% of our atmosphere is very reactive and it likes to react with molecules in food, especially fats. Over time, foods can oxidize and while they may still be safe to consume, they’ll start to taste off. Sadly, nothing lasts forever. When we chill food, we slow its chemistry and biology, delaying the inevitable. This is why most perishable foods need to stay cold.

Oxygen affects foods in a lot of different ways. One half of this apple is oxidized and it’s caused the fruit to turn brown and develop some unpleasant flavors and textures. Different preservation techniques can help prevent oxidation in foods. (Image:

Preserving foods amounts to finding strategies to kill microbes, prevent them from growing, and prevent recontamination. It’s about cutting off their access to food, water, and shelter. Preservation techniques may also include ways to keep foods from oxidizing but microbial stability is usually the first concern. Preserved foods go back thousands of years in human history and encompass the other four categories of foods I’ve mentioned above. They’re of obvious importance because they allow us to store foods for long travel or for later consumption in times when perishable foods are unavailable.

Pickling is a very old kind of food preservation that allows us to store foods without refrigeration. Pickled foods are ‘acidified foods’ and prevent microbial growth by denying them a hospitable environment in which to reproduce.

I’ll discuss each kind of food in detail but here’s a few examples of each to whet your appetite for food science.

· Perishable foods — Fresh fruits, vegetables, meat, dairy, and seafood. Just about all foods start as perishable foods.

· Water activity controlled food — Dried fruits, nuts, beans, flour, soy sauce, salt-cod

· Acidified food — Pickles, mayonnaise (and most condiments), tomato sauce, most juices

· Low acid canned food — Canned fish, beans, olives, UHT processed milk

· Chemically preserved foods — Alcohol, eggs (well, sort of. Eggs are tricky)

There’s at least one food missing from this list and, to be honest I actually have no idea how its preserved. Can you guess what it is?

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