The food available in the various stores can be fresh or preserved. The former are not subjected to any treatment aimed at extending their shelf life or, at most, are treated minimally; the latter, on the other hand, are subjected to multiple interventions which, while on the one hand increase, even enormously, their shelf life in edible condition, on the other hand can sometimes worsen their nutritional and organoleptic (i.e. taste-related) characteristics. This appendix will examine the main food preservation methods and their influence on food

Classifications

There is both a classification of food storage methods and a classification of food according to the way it is stored.

Classification of food preservation methods

Food preservation methods can be classified into physical methods, chemical methods and biological methods. Physical methods include all those that exploit low temperatures (refrigeration, freezing and deep-freezing), those that exploit high temperatures (pasteurisation and sterilisation), those that exploit the dehydration of food (concentration and drying), those based on the production of a modified atmosphere (storage in a controlled atmosphere, vacuum and modified atmosphere proper) and radiation treatment. Chemical methods include the use of preservative additives. These can be natural or artificial: natural preservatives are sodium chloride, ethyl alcohol, sucrose, oil and vinegar, while for artificial preservatives I refer the reader to the appendix on additives. Then there is a method that can be classified as physical-chemical, i.e. smoking, and finally bacterial fermentation, a method that is organic. The following table summarises what has been said:

Physical methods		Chemical methods		Chemical-physical methods	Biological methods
Low temperatures	Refrigeration	Natural preservatives	Salting	Smoking	Fermentation
	Freezing and deep-freezing		Sucrose
			Ethyl alcohol
High temperatures	Pasteurisation		Under oil
	Sterilisation		Under vinegar
Dehydration	Concentration	Artificial preservatives	Antioxidants
	Drying
Modified atmospheres	Storage in a controlled atmosphere
	Storage in a protective atmosphere		Antimicrobials
	Vacuum storage
Radiation treatment

Classification of foods according to their preservation

Foods can be classified according to the process they have undergone to increase their shelf life into:

range I: this range covers all fresh products, i.e. those that have not undergone any process aimed at increasing their shelf life, with the exception of so-called fresh milk, which has undergone some of these processes;
range II: all pasteurised or sterilised products and canned products (i.e. those products that can be stored for a long time at room temperature) belong to this range;
range III: the third range encompasses all foods subjected to temperatures below 0°C;
range IV: these are fresh, packaged and ready-to-eat products;
V range: this range covers all fresh, raw or cooked, vacuum-preserved products.

Conservation methods

Let us therefore look in detail at the most common methods of food preservation.

Refrigeration

Refrigeration is a process that tends to decrease the temperature of the environment in which the food is located without the water it contains solidifying, so the temperatures to which it is subjected must not be too low. This treatment decreases the reproductive and enzymatic activity of bacteria and destroys parasites. Refrigeration is the process we all normally carry out when storing our food in the refrigerator. Food can only be preserved in this way without its nutritional and organoleptic characteristics being compromised to the limit of edibility for a relatively short period of time, which varies from food to food. Moreover, although a product preserved in this way retains its edibility properties for a certain period of time, the organoleptic and nutritional characteristics are nevertheless altered during its stay in the refrigerator.

Freezing and deep-freezing

These processes aim at exposing the food to very low temperatures until the water it contains freezes. When all the water contained in the food is solidified, it could ideally be preserved forever, since under these circumstances no enzymatic reactions (i.e. not even degradation reactions) can take place, with the exception of certain lipases contained in the flesh of some fish. However, the complete freezing of a foodstuff is impossible to achieve, whatever means one possesses, so degradation reactions will still take place, albeit over a much longer period of time. The reason why complete freezing of a food cannot be achieved lies in the intrinsic properties of the water it contains: it is present in two different forms, namely the free form and the bound form (with electrostatic bonds to proteins, carbohydrates, cellulose and other parts of the food). Now, while the free form has a freezing point, as is well known, at 0°C, the water in bound form has a much lower one and, although during freezing much of the bound water passes to the free form, the little that remains in bound form concentrates and further lowers its freezing point, which could drop to over -40°C (Tibaldi, 2004). The freezing process takes place in two different phases: a first phase, known as nucleation, involves the formation of ice crystals as soon as the freezing point is passed, and a second phase, known as growth, involves the enlargement of the previously formed ice crystals until the complete solidification of the food. Now, freezing can be classified into quick freezing (or deep-freezing) and slow freezing. The former is the one that is practised industrially by the food manufacturers themselves. It involves subjecting the food, through various procedures, to a temperature of thirty or fifty degrees below zero. In this case, the nucleation phase will take the longest time, resulting in the formation of many small ice crystals that will form the solidified skeleton of the food. In the case of slow freezing, on the other hand, the food is subjected to temperatures no lower than -20°C, in which case the time required to complete the process will be much longer. In this second case, the growth phase will be the predominant one during the process and the result will therefore be a food with few but large ice crystals. These, as they grow larger, usually affect the texture of the food, which then presents varied organoleptic and nutritional characteristics. This is what happens when you freeze food with common freezers in your home. Let us now turn to the effect this method of preservation has on nutrients. Indeed, many nutrients are altered as a result of the freezing treatment: protides undergo partial denaturation, which ironically leads to a greater perishability of the food, as it becomes more easily attacked by proteases (i.e. the enzymes responsible for protein digestion), but also to an increase in their digestibility. And while the biological value of proteins is not altered, these nutrients can undergo substantial structural mutations that can become irreversible even after thawing. The polysaccharides contained in a frozen food generally undergo a very slow breakdown process to produce simple sugars. As far as lipids are concerned, these are the nutrients most affected by the deleterious influence of the freezing process: they can undergo hydrolysis and thus go rancid. In addition, oxidation processes can result in the formation of peroxides with consequent damage to fat-soluble vitamins. Water-soluble vitamins and mineral salts may also be damaged and lost. In particular, it should be noted that vitamin B1 undergoes greater alteration when it is found in frozen products of vegetable origin. They may also be lost as a result of the thawing process. In order to minimise the damage of these nutrients, the thawing of food should be carried out according to precise rules: with regard to frozen food, it does not need to be thawed, but can be cooked immediately; on the other hand, with regard to food that has been subjected to slow freezing, it is first necessary to avoid too rapid a thawing process, such as that which occurs when the frozen food is left to stand at room temperature or when the food is subjected to cold water jets or, worse, immersed in hot water. In the first case, bacterial activity increases enormously, while in the other cases many nutrients solubilise in the water and are lost. A defrosted food cannot under any circumstances be refrozen (whether frozen or deep-frozen in the beginning), since when bacterial activity resumes, following thawing, it proceeds at almost double speed. Moreover, the ice crystals that will form during the second thawing process will be particularly large and consequently the damage to the food tissue will be even greater, which will necessarily have a negative effect on the nutrients.

Pasteurisation

The pasteurisation process consists of subjecting food to high temperatures in order to achieve disinfection. It can be classified according to the intensity of the heat to which the product is subjected and the duration of the treatment. Generally, a higher temperature corresponds to a shorter exposure time. In more detail, there are (in ascending order of heat and descending order of exposure time):

low pasteurisation: used for wine, beer and milk for cheese production:
High pasteurisation: this method was once used for milk, but has now mostly been replaced by HTST, mentioned below;
Rapid pasteurisation or HTST (High Temperature Short Time) pasteurisation: this variant of pasteurisation is practised on liquid foodstuffs flowing in a thin layer between two heated metal walls;
very high pasteurisation: this type of pasteurisation is used for milk intended for yoghurt production and for the cream used to make butter.

Usually, in order to limit the development and proliferation of pathogens that survive this process, the food is subjected to rather rapid cooling. Pasteurisation implies changes in the organoleptic and nutritional characteristics of the food: as far as proteins are concerned, they undergo denaturation, which implies an increase in their digestibility, and, as far as proteins containing sulphur are concerned, the release of hydrogen sulphide. The release of these molecules can result in unpleasant odours and ‘burnt’ flavours; as for starch, it is hydrolysed to maltose and dextrins, while lipids undergo no alteration. The lipases responsible for lipid metabolism are in fact inactivated by high temperatures, with the exception of treatments in the presence of air and light, during which rancidity due to oxidation may occur. Fat-soluble vitamins are not damaged or altered in any way, given their thermostability, except when pasteurisation takes place in the presence of air. In the latter circumstance, oxidative degradation phenomena may occur. Water-soluble vitamins are very thermolabile, so the higher the temperature they are subjected to (thus depending on the type of pasteurisation chosen by the manufacturer), the more of these nutrients will be lost. A further decrease in the nutritional properties of the food subjected to pasteurisation is due to the so-called Maillard reactions that occur when the product contains both sugars and proteins. The pasteurisation process will also lead to a change in the colour of the food.

Sterilisation

Like pasteurisation, sterilisation also involves subjecting the food to high temperatures, but in the case of the latter treatment the heat to which the product is exposed is much higher than in pasteurisation. Also here, as a rule, a higher temperature corresponds to a shorter exposure time. It is therefore possible to classify sterilisation processes into:

classical sterilisation or appertization: the temperature is relatively low and the duration of treatment relatively long;
UHT (Ultra High Temperature): the temperature is relatively high and the duration of treatment relatively short.

UHT sterilisation can in turn be classified into indirect UHT and direct UHT (also called uperization). In the first case, the bulk food is immersed in water or in an autoclave, where it undergoes heat treatment; in the second case, on the other hand, superheated steam is injected into the bulk product. The choice of temperature to which to subject the product is a function of its pH (the reader is quickly reminded that pH is the concentration of free protons): foods with a relatively low pH are generally subjected to relatively low temperatures, while those with a higher hydrogenionic concentration require to be subjected to higher temperatures. An acid environment is not an ideal condition for the proliferation of pathogens. The fact that a food is subjected to sterilisation does not imply that it can be stored for an unlimited time. This is evidence that degradation processes, however slow, will continue to take place even during storage of the sterilised food. Organoleptic and nutritional changes in food that undergoes sterilisation are the same as in food that undergoes pasteurisation.

The concentration of liquid foods

A large proportion of pathogenic micro-organisms have a tendency to proliferate in water-richenvironments, so dehydrating a food can prevent its contamination and, consequently, positively influence its shelf life (as well as its transportability, due to the decrease in weight that follows the process). Well, the purpose of the concentration practice is precisely to decrease the water content of the food. Concentration can be achieved by following several routes:

concentration by evaporation: consists of evaporating the water present in the food. It has major drawbacks, as it appreciably alters the nutritional and organoleptic characteristics of thermolabile foods and causes a considerable loss of volatile components;
cryoconcentration: this consists of lowering the temperature at which the food is below 0°C, so that pure water separates from the rest of the solution in the form of ice, which is then removed. The nutritional and organoleptic characteristics remain unaltered, even in thermolabile food;
osmosis: the solution to be dehydrated is placed in a first compartment separated from a second by a thin semi-permeable membrane. In the second compartment, a dehydrating solution based on sucrose, other sugars and starch is placed. The water, due to the osmotic phenomenon, moves from the first compartment, in which it is more concentrated, to the second compartment, in which it is less concentrated;
reverse osmosis: as in the previous method, only that solutions are used which have no dehydrating power and therefore the water flows to the second compartment due to the fact that in the first it is under greater pressure;
ultrafiltration: as in the osmosis method, with the difference that the membrane consists, this time, of a series of layers made up of pores whose size progressively decreases from the compartment in which the solution to be dehydrated is located to that in which the dehydrating solution is located. This method assumes that not only water, but also salts, sugars, and all other substances of a very small size are transferred to the second compartment.

Drying

Drying, like concentration, is also a fooddehydration practice. The difference between drying and concentrating lies in the fact that during the drying process, far more water is removed from the food than during the concentrating process. The most common method of drying is freeze-drying: this involves freezing the food, placing it in a vacuum and evaporating the frozen water in it. A freeze-dried food retains not only its original form, but also (for the most part) the original organoleptic and nutritional characteristics (only the vitamins are slightly altered), but it is more fragile and ‘spongy’. All other drying methods, on the other hand, lead to considerable changes in the nutrients they contain, as well as in their organoleptic qualities. As for nutritional characteristics:

proteins may become harder and lose the ability to absorb back the water they have lost, but they also undergo denaturation that will increase their digestibility;
carbohydrates can undergo a caramelisation process and the Maillard reaction;
lipids can go rancid due to oxidation phenomena;
many water-soluble vitamins are totally destroyed during drying processes involving high temperatures;
A large proportion of fat-soluble vitamins are destroyed due to oxidation phenomena occurring during treatment.

With regard to organoleptic characteristics, what is altered is above all the colour (the food darkens due to Maillard reactions, caramelisation of sugars and enzymatic activity at the expense of pigments such as chlorophyll and carotenes), as well as the aroma, which becomes less intense due to the loss of certain volatile components. Precipitation of some poorly soluble substances, colour variation and the formation of surface areas of high solute concentration also take place.

Storage in a controlled atmosphere

This mechanism for increasing the shelf life of food consists of keeping the oxygen content of the environment in which it is located below the respiratory requirements of the product. It involves replacing oxygen with nitrogen and carbon dioxide (CO₂ ) and the composition of the atmosphere is kept constant through the use of automatic control systems. By eliminating oxygen, those oxidation reactions responsible for the deterioration of the food can no longer take place, but since oxygen is never completely removed, as the time during which the product is stored passes, its nutritional and organoleptic qualities will diminish as well.

Storage in a protective or modified atmosphere

The principle on which this method is based is the same as the previous one, i.e. decreasing the oxygen content of the environment in which the food is located. The difference is that in this case it is the respiration of the food itself that changes the atmosphere and decreases the amount of oxygen in the air by increasing the amount of carbon dioxide.

Vacuum storage

Vacuum preservation, also known as ‘cryovac’, is another method of achieving environmental oxygen depletion. The industrial method of vacuumpreservation consists of placing the food in polyvinyl bags, removing the air in it and exposing it to a temperature of around 90°C for a few seconds. Polyvinyl is a shrinkable material, so it has the tendency to adhere perfectly to the food during the process. It does, however, have the disadvantage of being thermolabile beyond certain temperatures, so it cannot be subjected to temperatures above 90°C and in any case not for more than a few seconds. In some circumstances, it may be that the manufacturer opts for actual vacuum cooking of the food, subjecting it to higher temperatures than the standard 90°C or for a longer than standard time. In this circumstance, however, alternative coating materials to polyvinyl are used, which are highly thermostable. Vacuum packaging can result in a loss of food quality, as the moisture in the food, as well as certain gases and volatile substances can be extracted from the food along with the surrounding air, and although this phenomenon is limited by the airtightness of the packaging and the small volume available, it does occur.

Radiation treatment

The radiations that are used to increase the shelf life of food are gamma rays and microwaves. The former, in particular, although they are not radioactive, are an example of ionising radiation and any food that has undergone this procedure (the packaging must clearly state this and must bear an internationally approved symbol) must under no circumstances be irradiated again. Radiation kills many microorganisms in the food and does not alter its organoleptic and nutritional characteristics.

Salting

Salting consists of adding sodium chloride (NaCl) to food. It is mainly used for meat and various sausages. The principle on which the salting mechanism is based is that when the amount of sodium chloride increases in a foodstuff, it becomes more concentrated and thus, should pathogenic microorganisms be present in it, they will tend to lose water until, when dehydration becomes too great, they are killed. In addition, the addition of salt also leads to a decrease in the amount of free water present in the food, which, as we have already seen when discussing dehydration methods, is another factor that benefits long shelf life. Finally, both sodium and the chloride ion are toxic to micro-organisms as they bind to certain anions (i.e. negative ions, atoms with one more electron than protons) that make up the cellular structure of pathogens, interfering with their normal metabolic mechanisms. The percentage of salt required to kill the well-known bacterium clostridium botulinum is 10%. Salting can be done in two different ways:

dry salting: the product is placed in contact with coarse salt grains for a period of between fifteen days and eight weeks;
wet salting or brine: the food is immersed in water with added sodium chloride. It is a faster method than dry salting but less resistant and usually requires to be combined with other preservation methods (such as adding chemical preservatives).

Salting has several contraindications for good nutrition. The most important variations in meat are:

colour change: haemoglobin turns into a grey pigment;
increased acidity;
decrease in nutritional value due to the loss of minerals, some vitamins and some amino acids;
acceleration of oxidative processes in lipids.

There are also deleterious effects on health that a diet rich in salt can cause. Finally, it is noteworthy that this method causes the denaturation of muscle proteins.

Storage with sucrose

The addition of sucrose to food is based on the same principle as salting, i.e. increasing the concentration of the solution, the difference being that in this case much higher percentages of the preservative are required (65% to 70%). The use of lower sucrose percentages is only permissible if other preservation methods are also used at the same time or for foods with a very acidic pH (perhaps with acidifying additives). This method forces the consumer to consume an enormous amount of sucrose in parallel with the food, thus incurring all the consequences this has on blood sugar.

Preservation with ethyl alcohol

When added to food in percentages of between 50 and 70 per cent, ethyl alcohol (i.e. neutral ethyl alcohol of agricultural origin, 95 per cent pure) is lethal to all types of pathogenic microorganisms (with the exception of the very resistant spores), as it is able to denature the proteins of which it is composed and damage their cell membranes. The germ that is exposed to alcohol therefore dies of dehydration. Due to the large percentage of alcohol, food preserved by this method should be avoided for all athletes.

Preservation in oil

The addition of olive or seed oil to a food can isolate the air by preventing the productfrom coming into contact with it. This causes the food to cease being a suitable substrate for the proliferation of aerobic pathogenic microorganisms, but not for that of anaerobic ones (such as clostridium botulinum), which is why preservation in oil should always be combined with other preservation methods. Food preserved in this way is often very fatty, and this is rarely good for athletes, not to mention the fact that the oils used for this purpose seldom possess the genuine characteristics that edible oils should have.

Pickling

Vinegar, being very acidic, favours lowering the pH of the food when it is added to it. It therefore becomes an unsuitable substrate for the proliferation of most pathogenic microorganisms. Furthermore, apart from the action that vinegar has on the pH of the environment, its acetic acid content (if this, as is usually the case, is above 6%) has a disinfectant action against many pathogens.

The use of artificial preservatives

Regarding these, I refer the reader to the specific section in the appendix on food additives.

Smoking

This method involves exposing the food to the smoke produced by the incomplete combustion of certain woods (e.g. beech, oak or chestnut), during which substances such as formic aldehyde, acetic acid, ethyl and methyl alcohol, which have a bactericidal action, are released. The food is also exposed to the heat resulting from this combustion. Smoking can be classified into two different types:

cold smoking: the heat source has a temperature of 30-35°C and the process takes several weeks;
Hot smoking: the heat source has a temperature of 80-100°C and the food is placed very close to it. However, these temperatures must not be reached quickly, since if this were to happen, the proteins on the surface of the food would coagulate, preventing the fumes from reaching deep into the product.

Smoking makes it possible for the food to have a longer shelf life because of:

increase in temperature, which kills all thermolabile pathogens,
of dehydration,
the consequent decrease of oxygen in the environment surrounding the food;
the presence of certain antibacterial substances found in it, in particular formic aldehyde.

Smoking has some notable side effects: the smoke to which the food is exposed, in addition to containing numerous volatile substances (which are responsible for the preservation and aroma of the product), also contains a solid part composed of polycyclic aromatic hydrocarbons (first and foremost benzopyrene) which has a recognised carcinogenic effect. The presence of these substances in smoked food depends on the way in which the process is carried out, which we will not dwell on since it would not be helpful for greater food awareness (the producer is not obliged to specify how he smoked the food).

Fermentation

Fermentation by bacteria is a biological method to increase the shelflife of foodstuffs: it is well known that foods subjected to this procedure (such as bread, yoghurt or certain cheeses) have superior shelf life characteristics. This method is particularly suitable for those foods that possess food enzymes, i.e. those enzymes necessary for the digestion of the food that contains them. If the food is fermented raw, it retains its quantity of food enzymes (the food must then be eaten raw, because the food enzymes are destroyed by cooking).

Conclusion on food preservation methods

Having analysed the most common methods of increasing the shelf life of a food, I can only recommend my reader to prefer fresh food to preserved food whenever possible, since whatever method the manufacturer or consumer uses to preserve the food, it will never be such as to completely eliminate the deterioration of nutrients. Where it is not possible to consume fresh foods, it is best to prefer those that are preserved by mechanisms that do not immediately altertheir nutritional characteristics, such as refrigeration. Below is a summary table, with judgements, on all the preservation methods analysed in this appendix.

Preservation method	Judgement	Motivation
Refrigeration	Excellent	Apart from the short shelf life, it has no other side effects
Quick freezing	Excellent	No side effects
Slow freezing	Medium	The food presents alterations in nutritional characteristics, although these are not alarming in scope
Pasteurisation	Excellent	Apart from the short shelf life, it has no other side effects
Sterilisation	Excellent	No side effects
Concentration (by evaporation)	Medium	It appreciably alters the nutritional and organoleptic characteristics of thermolabile foods and causes the loss of valuable volatile components
Concentration (cryoconcentration)	Excellent	No side effects
Drying	Medium	It appreciably alters the nutritional and organoleptic characteristics of food. Freeze-drying makes the food more fragile and spongy
Storage in a controlled atmosphere	Excellent	No side effects
Storage in a protective atmosphere	Excellent	No side effects
Vacuum storage	Medium	Causes the loss of valuable volatile food components
Salting	Medium	The high sodium chloride content in food subjected to this procedure does not present any health characteristics. The food thus treated also presents alterations in nutritional characteristics
Use of sucrose	To be avoided	The high sucrose content in foods subjected to this procedure is unhealthy; glycaemia and insulinemia can vary appreciably in a relatively short time, adversely affecting training performance and recovery
Use of ethyl alcohol	To be avoided	The high content of ethyl alcohol in foods subjected to this procedure does not present health characteristics
Use of oil	To be avoided	The high oil content in foods subjected to this procedure is not healthy
Use of vinegar	Medium	The high vinegar content in foods subjected to this procedure is such that their pH is very acidic
Use of artificial preservatives	To be avoided	Artificial preservatives can be toxic, especially if taken in large quantities, and some of them can have carcinogenic effects
Smoking	To be avoided	Foods treated by this method assume carcinogenic characteristics
Bacterial fermentation	Excellent	Apart from the short shelf life, it has no other side effects