Over the years preserved foods have gained an unfortunate stigma. This is mainly due to the fact that preserved foods are often associated with modern, mass produced preserves that contain high amounts of salt, sugar and artificial ingredients. With a faster pace of life nowadays and hectic lifestyles there is no denying that meals that can be poured out of a tin in seconds are convenient and it explains the popularity of such items. That because of the high amounts of salt, sugar and artificial ingredients used to preserve them and in this hectic life style, food that can be pulled out of a can and turned into a delicious meal is very convenient.
However, is there a way to preserve food using natural ingredients? If so, what is the science behind making safe, delicious and shelf-stable preserves at home?
Water Activity
The art of making preserves at home is all about learning how to control water, or at least controlling the amount of free water that bacteria and moulds can live off. Purified water has a water activity of aw 1.0 meaning that all the water is available for hydration of materials. Put simply this means that moulds and bacteria can grow rampantly as all bacteria and moulds need a certain amount of water to live. (and for bad things to live off) and in fact all bacteria and moulds need a certain amount of free water to live off. Honey, for example, is an ingredient that is very shelf-stable and does not require refrigeration. This is partially because its water activity is roughly aw 0.6; meaning that the amount of free water available is not enough for most potentially harmful bacteria and fungi to live off. Romans, although not really aware of the science behind it, took advantage of this and began to store food in honey to preserve it for times when fresh food would be scarce. Honey isn’t the only natural preservative we can add to food to preserve it; we can also alter the amount of free water by adding salts, sugars and alcohols. By adding these solutes to water, we can reduce the amount of free water available to other organisms, making homemade preserves safer to eat and far more shelf-stable. Some of these preserves include candied peel, olives and capers in brine, salted fish and several cured meats, all of which have been being produced for centuries.
One of the best ways of naturally storing foods is dehydration; by removing water from an ingredient we can make it shelf-stable for a very long time. Foods which are dehydrated usually have an aw of between 0.2-0.3, so in this case there is almost no free water for any living organism to live off. However, this alone isn’t enough, we have to store dehydrated foods in very air tight environments as due to humidity they tend to rehydrate using the free water in the air. Some dehydrated foods can still harbour spores of harmful fungi and bacteria so watch out!
Here is a great little chart showing how water activity works on microorganisms
Acid and Alkaline
Unfortunately, calculating the water activity of your preserves isn’t enough to make sure the food you are preserving is safe to eat. If you want to start preserving food at home, you’ll also need to calculate the pH of you preserves. This is a far less daunting task as all you will need is a pH meter or some litmus paper and you’ll be ready to go!
The main reason pH is important is because bacteria and moulds cannot grow in an extremely acidic or alkaline environment. To adjust the pH of a preserve one can add a few drops of something acidic such as lemon juice, vinegar and citric acid. Alternatively, you can add something alkaline such as bicarbonate of soda or lye to salted water or a sugar syrup. Next, check the pH using your pH metre and as long as that liquid falls into the safe pH ranges your food will be shelf-stable.
HInterestingly, when making jams and marmalades one rarely needs to add any form of acid or alkaline as most fruits tend to naturally contain enough acid to preserve them. Due to this, just adding sugar to strawberries or citrus fruit, heating the mixture and reducing it creates a jam safe enough to preserve without adding any extra acid. The interesting thing comes when you make a jam, marmalade or a chutney, when doing these product one rarely needs to add some form of acid as most fruit actually contain enough acid to naturally preserve it, for example strawberries and citrus fruit are usually acidic enough, so just adding sugar and reducing them will create a jam safe enough to preserve without the need to add any acid. Chutneys, although often thought of as being made of vegetables may also be made using fruit such as tomatoes (really and truly a berry). In this case, as they contain a relatively low pH, additional acid, as with jams is not usually needed to render them stable. However, chutneys made from vegetables require the addition of an acid, often vinegar as very few vegetables contain enough acidity to preserve the overall product. Chutneys are very similar to jams in their form however they are usually made with vegetables and flavoured with sugar and vinegar as very few vegetables contain enough acidity to preserve them. One of those vegetables is the tomatoes (although they are a berry), they contain a relatively low p making them safe to can without any additives!!
Alkalinity is rarely used to preserve ingredients but when it does it makes for a very useful preservative. Romans used to use ash to marinate their olives both in order to preserve them for longer and also for the brine to work faster. In Nordic European countries alkaline was used to preserve cod in creating lutefisk, a somewhat slimy fish that has a very long shelf life.
Here is a very useful list of some pathogens and their pH scale
Herbs, spices and other additives
There are some other additives that also limit bacterial growth. Alcohol is one of the best preservatives around and by storing ingredients in alcohol or adding ethanol to water one can adjust the water activity content to a safe level. There are also some other interesting food items that can be added to your preserves to kill some of the harmful pathogens.
Nicin a protein found in fermented milk products, such as kefir, has been proven to kill a number of bad bacteria such as Bacillus, Listeria and Staphylococcus. Some herbs such as thyme and oregano have also been shown to kill off harmful bacteria when used in conjunction with wine. Studies show that by adding either thyme, oregano or both to red wine killed off salmonella and E.coli after some time. Garlic juice in addition to wine has also shown great results in reducing bacteria and moulds.
Oil acts in a different way but may also be used as a method of preservation. By acting as a seal against air entering, the food item is protected to a higher degree than without, as most bacteria find it hard to survive without air. As it does not let air effect the ingredients being preserved and without air most bacteria find it hard. However, some bacteria do not require air at all and actually thrive when no air is available; Botulism is one of these deadly bacteria. Initially because of this factor Botulism will reproduce at a much faster rate. However, because Botulism, like other bacteria also depends on other factors besides air to survive it will eventually die. It is the ramifications of this bacterial death that are of more concern to us, as, when Botulism bacteria die they release toxins. These toxins are incredibly heat resistant and in fact deadly. When Botulism dies it releases a deadly toxin that is incredibly releasing a deadly toxin that is very heat stable and very, very toxic. Some foods such as herbs are generally fine to store in oil as there are very few nutrients available for Botulism to grow and multiply. However, root vegetables, in particular garlic, DO contain enough nutrients for Botulism bacteria to live off meaning that even storing homemade Pesto, can potentially be deadly.
Heat and pressure
When preserving, all items should be treated with some form of heat or pressure as this ensures that most bacteria will be killed off. Boiling ingredients in a jar or a can for an allotted period of time exerts pressure, which in turn creates a vacuum. This vacuum keeps air out thereby limiting bacterial growth. One can expect that the pressure in the can will create a vacuum keeping air out and again limiting bacterial growth. With highly acidic items it is safe enough to just place the jar or can containing them in boiling water i.e.. reaching a temperature of 100 C. However, when it comes to non-acidic items the containers will need to be taken to a temperature above 121 C as some bacteria, including Botulism can still survive up to this temperature. Items in boiling water and seal them off, however in most cases boiling the product will not be enough as some bacteria such as botulism can still survive.
This knowledge came about in 1810 with the introduction of canned food when canned foods started being produced to feed the French and the British armies who were at war with each other. Despite the precautionary measure of canning, soldiers were still falling sick from eating the canned food. Although the food still smelt and tasted good it was actually infested with bacteria. Fast forward to 1851 when a French engineer created the world's first pressure cooker and due to this, canned foods where able to reach temperatures of 121°C meaning that all bacteria could be killed off. This finally rendered canned products both very shelf-stable and safe to eat.
Because of the internal temperatures that they can reach, pressure cookers, in combination with the other factors we mentioned in this post, are the best way to make foods shelf-stable at home. This internal temperature ensures that not even Botulism spores will survive, ensuring that any product you prepare at home is safe to eat.
Note: It is important to note that a number of variables exist when making any preserved item at home. Please do further research prior to trying this out.