The activity of micro-organisms and enzymes is halted when food is frozen allowing food and nutrients to be preserved for long periods of time. In the case of refrigeration microbial and enzymatic activity is considerably reduced in temperatures below 5 degrees centigrade, which is why refrigeration is such an effective means of food preservation. Whilst some cultures sought out caves to preserve food and aristocrats built cool houses the vast body of everyday cooks had access to neither. Refrigeration is an excellent modern invention and an ideal way to keep raw food fresh.
Many associate traditional cooking with glass-jar canning. Is there nothing more homely than a glass jar of chutney resting on a shelf in the kitchen. Do not glass jar evoke images of grandmother cooking jam over a huge pot? On continental Europe the practice of canning home-grown produce in glass jars continued well into the 1960’s and 1970’s. In eastern Europe and Russia many did and still do have small gardens where they grow most of their own produce relying on canning to preserve the glut of seasonal vegetables.
For those reliant on canning seasonal produce cellars, garages, outhouses soon become equipped with shelves packed with glass jars of peas, carrots, beans, onions, potatoes, peaches, cherries, sausages …. In the UK people used Kilner jars. In Germany they use Weck. In France they use Parfait. In the United States Mason jars. All, bar design, operate on the same method – high heat sterilisation of cooked food in glass jars, vacuumed sealed with a rubber band following a water bath.
Canning is a relatively modern invention dating back to the Napoleonic wars, which is why it is slotted under modern preservation methods. Prior to the invention of canning, food preservation was entirely dependant on the methods outlined under Traditional methods of preserving food.
Canning works by placing cooked food in a heat-sterilised glass jar (or tin) sealing the lid and placing the glass jars or tin in a hot water bath to create an air-tight vacuum. When done correctly it is highly effective at preserving food for long periods of time. The high-heat/high-pressure sterilisation of the food during the water bath process is effective at killing off any pathogenic microbes that may have been present on the food when jarred. The rubber band and oxygen-free vacuum created by the high-pressure is effective at preventing fresh contamination of the cooked food. The combination of both methods results in cooked food being preserved for months, even years on end.
The downside to canned food is the increased risk of contamination from clostridium botulism since this particular pathogen is anaerobic and relatively heat-resistant. Contamination of canned food with clostridium botulism is thankfully rare. It arises mostly with the canning of low acid fruit and vegetables such as asparagus, lentils or garlic. For those interested in canning produce it is therefore essential that the home-cook follows the canning instructions carefully.
The second down-side to home-canning is the amount of time, heat and equipment required. From a nutritional point of view many of the food’s nutrients and fibre are preserved by the canning process – though the high heat required of sterilisation, cooking and hot water bathing means that enzymes and beneficial microbes will be destroyed so don’t expect to replenish your micro-flora with canned foods. For that the eater has to eat unheated fermented foods.
(See also Chapter on Sugars and Sweetness and Jams)
In high concentrations sugar binds with the free moisture in food, thereby preventing microbial growth. For this to be effective, however, large amounts of sugar are required. In the case of jam 1 kg of sugar to 1 kg of fruit. A relatively modern method of food preservation given that sugar was traditionally a rare and expensive ingredient that few would have been able to use or afford for regular food preservation.
More recent attempts to preserve food for long enough to stay fresh on the shelves include irradiation and thousands of new chemicals. I am no expert in any of these and do not wish to engage in a lengthy discussion as to whether or not they are safe based on sound science. Suffice it to say that they are non-traditional ways in which to preserve food. Following the general principle tried by time, tested by generations none of these new methods have enough of track-record to be deemed safe according to traditional standards.
No home cook has an irradiation chamber in their kitchen, nor do they generally stock their spice cupboards with bottles listing unfathomable E-numbers. Sadly, industrial food is heavily reliant on these new means to preserve food and enhance flavour. For the everyday cook preparing everyday food based on time-honoured methods none of these food preserving techniques are required. All that is needed is some natural sea-salt, plenty of produce and a some basic knowledge of how food can be preserved.