Food Processing

Most of the food eaten by humans is perishable. Food consistency, appearance and flavor deteriorate rapidly. It can even be spoiled or become unwholesome when stored for too long periods. But if appropriate methods of preservation, such as canning, freezing, salting, smoking or drying, are used, meat and crop harvests can be saved for future utilization. These methods preserve food efficiently because they kill or at least restrict the growth of microbes and slow or inactivate enzymes that change the characteristics of the food. These preservation methods and their respective energy needs will now be examined more closely as they constitute the most important part of food processing.

There are various ways to kill harmful microbes, parasites and some toxins that are natural contaminants of food. Heating food to 100°C or higher is one of them. Canning follows this procedure and then seals the cooked produce in sterilized containers commonly referred to as 'cans'. So, after the food is boiled to a high temperature, it is canned and completely sealed. Processing temperatures and times depend on the acidity of the foodstuff, its density as well as on the shape and size of the container.

In the commercial canning of vegetables and fruits, the energy required is on average about 2,400 kJ for each kg of food (Pimentel et al, 1996). This energy expenditure is relatively low, so that the cost related to food processing using this method is relatively low. It does not include the energy needed to make the container, which is substantial (discussed later in the section on packaging.) Home-canning is much more energy (and money) consuming than commercial canning.

Freezing allows fresh food to retain its desirable qualities for long periods of time. The low temperatures employed (-18°C or lower) inhibit or at least retard the growth of harmful microorganisms since the water they need to develop is frozen.

Below: frozen meats.

Even though this method is very effective in preserving food, it's energy consumption is high and, consequently, costly. First, prior to freezing, vegetables need to be boiled or steamed for a short time (this process is called blanching). This de-activates the plant enzymes that are responsible for deteriorating the vegetables' natural flavors and colors. Fruits, for their part, can be frozen in a fresh state right away along with added sugar or in syrup.

Freezing vegetables and fruits requires on average 7,600 kJ per kg of food to be frozen. This amount of energy is significantly greater than for canning, as the latter only requires energy for heating. The freezing process needs energy for the brief heating (for vegetables), the cooling and then the actual freezing. And this does not include the energy input for storage of the frozen foods (storage is discussed later).

Sunlight has been used for centuries and is still used today (refer to the drying section at the end of the discussion on energy use) to dry crops such as fruits and vegetables. Drying is an effective preservation method since it reduces the moisture content of meats, grains, legumes and fruits to 13 % or less, giving substantially fewer opportunities for harmful organisms dependent on humidity to develop and propagate. Drying, however, removes the juiciness and tenderness of many items when the water is removed, resulting in a less appealing product in many cases compared to the frozen or canned equivalent.

If drying is carried out using solar energy (hay, raisins), the energy input required is mainly limited to labor, making it a very cheap process. But if heat needs to be provided, drying becomes energy intensive: on average, it takes about 6,360 kJ to remove one liter of water in the food to be dehydrated (Pimentel et al, 1996), which is quite considerable.

Who has never tasted smoked meat or smoked salmon? Smoking originated in early societies and is still used today to preserve mainly fish, meats and grains. It works by drying and dehydrating the food, as well as by neutralizing harmful elements with chemicals such as tars and phenols, that attach themselves to the food and are toxic to microbes and insects. In fact, these chemicals can also become harmful to humans if consumed in too large amounts (they can be carcinogenic, just as the residue from charcoal barbecues.)

In developing countries, it is common to find grain bags hanging from the ceiling. The heat and smoke from the open fire can then preserve the stored grains in a cheap manner. If wood is burned purposely to smoke fish, for example, it takes about 18,835 kJ of energy provided by burning wood chips to smoke 1 kg of thin strips of fish (Pimentel et al, 1996). This process is consumes a lot of energy, and is therefore costly regardless of the material used to produce the heat and smoke. Smoked food often has a unique and robust flavour, and is still widely produced because in many situations is the fastest way to preserve food without the use of specialized equipment that may also need sources of energy that requires cash payment, such as gas or electricity.

The more a food is processed, more expensive it is at the end of the agri-food chain. The amounts of energy and the labor required are larger, and the equipment needed to carry out the processing of the food are all extra expenditures that increase the final cost of the product. Let us take the example of breakfast cereals: before being "ready" to be packed and eventually sold, cereals must be ground, milled, dried and then baked. So, even if one kg of breakfast cereal contains on average about 15,100 kJ of energy for the person eating the product, 65,600 kJ of energy are required to process and prepare this kg of cereal (Pimentel et al, 1996). It is not surprising then, that breakfast cereals are sold at relatively expensive prices in retail stores.

The processing of chocolate and coffee beans are also very expensive since a lot a energy is required for roasting, grinding, wetting and drying of the beans. Just to process 1 kg of chocolate or coffee requires slightly less than 76,000 kJ/kg (Pimentel et al, 1996), which represents a huge energy expenditure.