The Science of Food Preservation

It's midsummer and the crops on Canadian farm fields are ripening. Soon our markets will be overflowing with local produce apples, pears, plums, grapes and an abundance of colourful vegetables, all at once! Our harvest season is compressed into a few short weeks, yet our fields produce so much more than we can consume during this time.

Our ancestors invested considerable time and effort in preserving food for the winter months by canning, drying and fermenting the fruits of the harvest. Today, few households still do their own preserving. Most of it is done by commercial food processors whose products we later purchase in cans, jars or plastic packages. Modern refrigeration techniques have also added freezing to the variety of food preserving methods.

What are the relative merits of these different methods, and how do they affect the nutritional value of our food? Let's take a closer look.

Canning

The canning process involves placing food in jars and heating those jars to high temperatures to destroy micro-organisms that could cause food to spoil. The jars are then vacuum-sealed, preventing air and new micro-organisms from entering. When done properly, canning is a safe, time-proven method for preserving harvest-ripe food without the need for additives or preservatives.

Canned food does have a drawback, however: it is devoid of enzymes. Enzymes, which are naturally present in fresh and raw foods, are required for digestion and for all metabolic functions in the body. Enzymes are destroyed when foods are heated above 48°C. The rate of destruction accelerates with increased temperatures and longer cooking time. (Temperatures used for home-canning range between 100 and 116°C, higher for commercial canning.) If enzymes are not taken with food, the body must produce its own digestive enzymes. The long-term consumption of enzyme-deficient foods can exhaust the digestive organs and lead to various health problems, including chronic degenerative disease. If you frequently eat canned food, be sure to take a good enzyme supplement, available in health food stores.

How does canning affect vitamins and minerals? According to research by the University of Illinois Department of Food Science and Nutrition, vitamin and mineral levels in commercial canned foods are relatively stable and comparable to those of fresh foods. However, the research does not distinguish between naturally present nutrients and those added in the canning process. For instance, canning companies add calcium chloride to canned tomato pieces to keep them firm. Vitamin C is often added to canned fruit to maintain colour. Much of the fruit's own vitamin C content is destroyed by heating. Canning also reduces B vitamins, notably folic acid, which is especially important to women of childbearing age. Vitamins A, D and E, as well as minerals, are more stable in canned foods.

Freezing

This popular method is one of the simplest ways to preserve foods at home. Both fresh and cooked foods can be preserved quickly with minimal effort and time. Use rigid containers or flexible bags or wrappings that are moisture-resistant and leak-proof. It is not necessary to sterilize foodsfirst, as freezing significantly slows down enzymatic changes in food that would normally cause spoilage. However, commercial frozen foods are often blanched (scalded) prior to freezing, causing enzymes and vitamin C to be destroyed.

The vitamin and mineral content of unblanched frozen foods is relatively stable. However, some nutrition experts are concerned about the destructive effect of the molecular expansion that occurs with freezing, which causes the cells to burst and leaves frozen produce mushy and limp when thawed.

What are the relative merits of the different preserving methods, and how do they affect the nutritional value of our food?

Drying

Drying is one of the oldest methods of food preservation. Almost any type of food can be dried, but the most popular is fruit. Dried fruit makes a nutritious, tasty, quick-energy snack. Foods can be sun-dried or oven-dried, but best results are achieved with a dehydrator specifically designed for this purpose.

Drying concentrates the nutrient content of food. Many dried fruits are excellent sources of iron, potassium and magnesium. Remember, however, that any pesticides and preservatives present in fresh food will be concentrated in the dried version, so be sure to use organic produce for drying, and buy only organic dried foods. Non-organic dried fruits are also often preserved with sulphur dioxide to maintain the fruit's bright colour and "fresh" appearance. Sulphur dioxide can cause serious allergic reactions. It also destroys B vitamins in the body and has been linked to kidney malfunction.

Fermenting

Before the advent of refrigeration, people relied on fermentation with lactic acid bacteria (LAB) to prevent spoilage. Fermented foods are again popular today, as information about the considerable health benefits of LAB is coming to light. LAB are natural residents in the human intestinal tract. They promote the assimilation of nutrients, particularly the synthesis of B-complex vitamins and calcium. LAB have been shown to be effective in reducing serum cholesterol levels and lowering the risk of bowel cancer. The regular ingestion of sauerkraut and other LAB-fermented vegetables helps to recolonize the intestinal mucous membrane with beneficial bacteria.

Almost any vegetable can be fermented, but the most popular ones are cabbage, radish, carrot and beet. Sauerkraut, for instance, can be made at home by mashing shredded raw cabbage to squeeze out the juice, mixing the pulp and juice with unrefined sea salt and purified water, then packing it all tightly in a Mason canning jar and allowing it to stand at room temperature for several days. The naturally present LAB then initiate the fermentation process, which converts sugars and starches in the cabbage into lactic acid.

When purchasing commercial sauerkraut and other fermented veggies, be sure to choose a raw, unpasteurized product, found in the refrigerated section of natural food stores. Avoid canned, pasteurized products. The heat used in pasteurization to extend shelf life destroys enzymes and beneficial micro-organisms. Unpasteurized LAB-fermented vegetables are living foods that are naturally preserved and will keep fresh in the refrigerator for several months in an unopened package, and at least one month if stored in a closed container after opening.

In conclusion, harvest-fresh produce is always best, but natural food preserving methods offer a convenient way to keep our pantry stocked year-round with our favourite fruits and veggies.