Preservatives are used to improve the colour and keeping qualities of the final product for some fruits and vegetables. Preservatives include items such as sulphur dioxide, ascorbic acid, citric acid, salt and sugar and can either be simple or compound solutions.

The concept of combinations of preservatives and treatments to preserve foods is frequently called the hurdle or barrier concept. Combinations of additives and preservatives systems provide unlimited preservation alternatives for applications in food products to meet consumer demands for healthy and safe foods.

Use of Preservatives

Treatment with preservatives takes place after blanching or, when blanching is not needed, after slicing. In traditional, simple processing the method recommended is:

Put enough preservative solution to cover the cloth bag into a container/pan;

Dip the bag containing the product into the preservative solution for the amount of time specified;

Remove the bag and put it on a clean tray while the liquid drains out. The liquid which drains out must not go back into the preservative solution because it would weaken the solution.

Care must be taken after each dip to refill the container to the original level with fresh preservative solution of correct strength. After five lots of material have been dipped, the remaining solution is thrown away; i.e. a fresh lot of preservative solution is needed for every 5 lots of material.

The composition and strength of the preservative solution vary for different fruit and vegetables. The strength of sulphur dioxide is expressed as “parts per million” (ppm). 1.5 grams of sodium meta-bisulphite in one litre of water gives 1000 ppm of sulphur dioxide.

Sodium bicarbonate is added to the blanching water when okra, green peas and some other green vegetables are blanched. The chemical raises the pH of the blanching water and prevents the fresh green colour of chlorophyll being changed into pheophytin which is unattractive brownish-green.

The preservative solutions in the fruit and vegetable pre-treatment can only be used in enameled, plastic or stainless-steel containers; never use ordinary metal because solutions will corrode this type of container.

As a general rule, preservatives are not used for treating onions, garlic, leeks, chilies and herbs.

Chemical Preservation

Many chemicals will kill micro-organisms or stop their growth but most of these are not permitted in foods. Chemical food preservatives are those substances which are added in very low quantities (up to 0.2%) and which do not alter the organoleptic and physico-chemical properties of the foods at or only very little.

Preservation of food products containing chemical food preservatives is usually based on the combined or synergistic activity of several additives, intrinsic product parameters (e.g. composition, acidity, water activity) and extrinsic factors (e.g. processing temperature, storage atmosphere and temperature).

This approach minimizes undesirable changes in product properties and reduces concentration of additives and extent of processing treatments.

The concept of combination of preservatives and treatments to preserve foods is frequently called the hurdle or barrier concept. Combination of additives and preservatives provide unlimited preservation alternatives for applications in food products to meet consumer demands for healthy and safe foods.

Chemical food preservatives are applied to foods as direct additives during processing, or develop by themselves during processes such as fermentation. Certain preservatives have been used either accidentally or intentionally for centuries, and include sodium chloride (common salt), sugar, acids, alcohols and components of smoke.

In addition to preservation, these compounds contribute to the quality and identity of the products, and are applied through processing procedures such as salting, curing, fermentation and smoking.

Traditional Chemical Food Preservatives

Traditional chemical food preservatives and their use in fruit and vegetable processing technologies could be summarized as follows:

Common salt: brined vegetables;

Sugars (sucrose, glucose, fructose and syrups);

Foods preserved by high sugar concentrations: jellies, preserves, syrups, juice concentrates;

Interaction of sugar with other ingredients or processes such as drying and heating;

Indirect food preservation by sugar in products where fermentation is important (naturally acidified pickles and sauerkraut).

Acidulants and other preservatives formed in or added to fruit and vegetable products are as follows:

Lactic acid: This acid is the main product of many food fermentations; it is formed by microbial degradation of sugars in products such as sauerkraut and pickles.

The acid produced in such fermentations decreases the pH to levels unfavourable for growth of spoilage organisms such as putrefactive anaerobes and butyric-acid-producing bacteria.

Yeasts and molds that can grow at such pH levels can be controlled by the inclusion of other preservatives such as sorbate and benzoate.

Acetic acid: Acetic acid is a general preservative inhibiting many species of bacteria, yeasts and to a lesser extent molds. It is also a product of the lactic-acid fermentation, and its preservative action even at identical pH levels is greater than that of lactic acid.

The main applications of vinegar (acetic acid) includes products such as pickles, sauces and ketchup.

Other Acidulants

Malic and tartaric (tartric) acids is used in some countries mainly to acidify and preserve fruit sugar preserves, jams, jellies, etc.

Citric acid is the main acid found naturally in citrus fruits; it is widely used (in carbonated beverages) and as an acidifying agent of foods because of its unique flavour properties.

It has an unlimited acceptable daily intake and is highly soluble in water. It is a less effective antimicrobial agent than other acids.

Ascorbic acid or vitamin C, its isomer isoascorbic or erythorbic acid and their salts are highly soluble in water and safe to use in foods.

Commonly Used Lipophilic Acid Food Preservatives

Benzoic acid in the form of its sodium salt, constitutes one of the most common chemical food preservative. Sodium benzoate is a common preservative in acid or acidified foods such as fruit juices, syrups, jams and jellies, sauerkraut, pickles, preserves, fruit cocktails, etc. Yeasts are inhibited by benzoate to a greater extent than are moulds and bacteria.

Sorbic acid is generally considered non- toxic and is metabolised; among other common food preservatives the WHO has set the highest acceptable daily intake (25 mg/kg body weight) for sorbic acid.

Sorbic acid and its salts are practically tasteless and odourless in foods, when used at reasonable levels (< 0.3 %) and their antimicrobial activity is generally adequate.

Sorbates are used for mould and yeast inhibition in a variety of foods including fruits and vegetables, fruit juices, pickles, sauerkraut, syrups, jellies, jams, preserves, high moisture dehydrated fruits, etc.

Potassium sorbate, a white, fluffy powder, is very soluble in water (over 50%) and when added to acid foods it is hydrolysed to the acid form. Sodium and calcium sorbates also have preservative activities but their application is limited compared to that for the potassium salt, which is employed because of its stability, general ease of preparation and water solubility.

Gaseous Chemical Food Preservatives

Sulphur dioxide and sulphites: Sulphur dioxide (SO₂) has been used for many centuries as a fumigant and especially as a wine preservative. It is a colourless, suffocating, pungent-smelling, non-flammable gas and is very soluble in cold water (85 g in 100 ml at 25°C).

Sulphur dioxide and its various sulphites dissolve in water, and at low pH levels yield sulphuric acid, bisulphite and sulphite ions. The various sulphite salts contain 50-68% active sulphur dioxide.

A pH dependent equilibrium is formed in water and the proportion of SO₂ ions increases with decreasing pH values. At pH values less than 4.0 the antimicrobial activity reaches its maximum.

Sulphur dioxide is used as a gas or in the form of its sulphite, bisulphite and metabisulphite salts which are powders. The gaseous form is produced either by burning Sulphur or by its release from the compressed liquefied form.

Metabisulphite are more stable to oxidation than bisulphites, which in turn show greater stability than sulphites.

The antimicrobial action of sulphur dioxide against yeasts, moulds and bacteria is selective, with some species being more resistant than others.

Sulphur dioxide and sulphites are used in the preservation of a variety of food products. In addition to wines these include dehydrated/dried fruits and vegetables, fruit juices, acid pickles, syrups, semi-processed fruit products, etc.

In addition to its antimicrobial effects, sulphur dioxide is added to foods for its antioxidant and reducing properties, and to prevent enzymatic and non-enzymatic browning reactions.

Carbon dioxide (CO₂) is a colourless, odourless, non-combustible gas, acidic in odour and flavour. In commercial practice it is sold as a liquid under pressure (58 kg per cm³) or solidified as dry ice.

Carbon dioxide is used as a solid (dry ice) in many countries as a means of low- temperature storage and transportation of food products. Besides keeping the temperature low, as it sublimes, the gaseous CO₂ inhibits growth of psychrotrophic micro-organisms and prevents spoilage of the food (fruits and vegetables, etc.).

Carbon dioxide is used as a direct additive in the storage of fruits and vegetables. In the controlled/ modified environment storage of fruit and vegetables, the correct combination of O₂ and CO₂ delays respiration and ripening as well as retarding mould and yeast growth.

The final result is an extended storage of the products for transportation and for consumption during the off-season. The amount of CO₂ (5-10%) is determined by factors such as nature of product, variety, climate and extent of storage.

Chlorine: The various forms of chlorine constitute the most widely used chemical sanitizer in the food industry. These chlorine forms include chlorine (Cl₂), sodium hypochlorite (NaOCl), calcium hypochlorite (Ca(OCl)₂) and chlorine dioxide gas (ClO₂).

These compounds are used as water adjuncts in processes such as product washing, transport, and cooling of heat-sterilized cans; in sanitizing solutions for equipment surfaces, etc.

Important applications of chlorine and its compounds include disinfection of drinking water and sanitation of food processing equipment.

General Rules for Chemical Preservation

Chemical food preservatives have to be used only at a dosage level which is needed for a normal preservation and not more.

Reconditioning” of chemical preserved food, e.g. a new addition of preservative in order to stop a microbiological deterioration already occurred is not recommended.

The use of chemical preservatives must be strictly limited to those substances which are recognized as being without harmful effects on human beings’ health and are accepted by national and international standards and legislation.

Factors Influencing the Action of Chemical Food Preservatives

Factors related to chemical preservatives are:

Chemical composition;

Concentration.

Factors related to micro-organisms:

Micro-organism species; as a general rule it is possible to take the following facts as a basis:

Sulphur dioxide and its derivatives can be considered as a “universal” preservative; they have an antiseptic action on bacteria as well as on yeasts and moulds;

Benzoic acid and its derivatives have a preservative action which is stronger against bacteria than on yeasts and moulds;

Sorbic acid acts on moulds and certain yeast species; in higher dosage levels it acts also on bacteria, except lactic and acetic ones;

Formic acid is more active against yeasts and moulds and less on bacteria.

The initial number of micro-organisms in the treated product determines the efficiency of the chemical preservative. The efficiency is less if the product has been contaminated because of preliminary careless hygienic treatment or an incipient alteration.

Therefore, with a low initial number of micro-organisms in the product, the preservative dosage level could be reduced.

Specific factors related to the product to be preserved:

Product chemical composition;

Influence of the pH value of the product: the efficiency of the majority of chemical preservatives is higher at lower pH values, i.e. when the medium is more acidic.

Physical presentation and size which the product is sliced to: the chemical preservative’s dispersion in food has an impact on its absorption and diffusion through cell membranes on micro-organisms and this determines the preservation effect.

Therefore, the smaller the slicing of the product, the higher the preservative action. Preservative dispersion is slowed down by viscous foods (concentrated fruit juices, etc.)

Miscellaneous Factors

Temperature: chemical preservative dosage level will be established as a function of product temperature and characteristics of the micro-flora;

Time: at preservative dosage levels in employed in industrial practice, the time period needed in order to obtain a “chemical sterilisation” is a few weeks for benzoic acid and shorter for sulphurous acid.

Table: Chemical Food Preservatives

Agent	Acceptable Daily intake (mg/Kg body weight)	Commonly used levels (%)
Lactic acid	No limit	No limit
Citric acid	No limit	No limit
Acetic acid	No limit	No limit
Sodium Diacetate	15	0.3-0.5
Sodium benzoate	5	0.03-0.2
Sodium propionate	10	0.1-0.3
Potassium sorbate	25	0.05-0.2
Methyl paraben	10	0.05-0.1
Sodium nitrite	0.2	0.01-0.02
Sulphur dioxide	0.7	0.005-0.2

Source: FDA, 1991

Some food products in common usage are summarized as follows:

Citricacid: fruit juices; jams; other sugar preserves;

Aceticacid: vegetable pickles; other vegetable products;

Sodium benzoate: vegetable pickles; preserves; jams; jellies; semi-processed products;

Sodiumpropionate: fruits; vegetables;

Potassiumsorbate: fruits; vegetables; pickled products; jams, jellies;

Methylparaben: fruit products; pickles; preserves;

Sulphur dioxide: fruit juices; dried / dehydrated fruits and vegetables; semi-processed products.

Preservation of Vegetables by Acidification

Food acidification is a means of preventing their deterioration in so far as a non-favorable medium for micro-organisms development is created. This acidification can be obtained by two ways: natural acidification and artificial acidification.

Natural Acidification

This is achieved by a predominant lactic fermentation which assures the preservation based on acidoceno-anabiosys principle which is preservation by lactic fermentation is called also biochemical preservation.

Throughout recorded history food has been preserved by fermentation. In spite of the introduction of modern preservation methods, lactic acid fermented vegetables still have a great popularity, mainly because of their nutritional and gastronomic qualities.

The various preservation methods discussed so far that are based on the application of heat, removal of water, cold and other principles, all have the common objective of decreasing the number of living organisms in foods or at least holding them in check against further multiplication.

Fermentation processes for preservation purposes, in contrast, encourage the multiplication of micro-organisms and their metabolic activities in foods. But the organisms that are encouraged are from a select group and their metabolic activities and end products are highly desirable.

There are some characteristic features in the production of fermented vegetables which will be pointed out below using cucumbers as an example. In the production of lactic acid fermented cucumbers, the raw material is put into a brine without previous heating.

Through the effect of salt and oxygen deficiency the cucumber tissues gradually die. At the same time, the semi-permeability of the cell membranes is lost, whereby soluble cell components diffuse into the brine and serve as food substrate for the micro-organisms.

Under such specific conditions of the brine the lactic acid bacteria succeed in overcoming the accompanying micro-organisms and lactic acid as the main metabolic products is formed.

Under favourable conditions (for example moderate salt in the brine, use of starter cultures) it takes at least 3 days until the critical pH value of 4.1 or less – desired for microbiological reasons – is reached.

Artificial Acidification

This is carried out by adding acetic acid which is the only organic acid harmless for human health and stable in specific working conditions; in this case biological principles of the preservation are acidoanabiosys and, to a lesser extent, acidoabiosys.

Combined acidification is a preservation technology which involves as a preliminary processing step a weak lactic fermentation followed by acidification (vinegar addition).

The two main classes of vegetables preserved by acidification are sauerkraut and pickles;

May be canned by processing sufficiently by heat to assure preservation in hermetically sealed containers; or

May be packaged in sealed containers and preserved with or without the addition of benzoate of soda or any other ingredient permissible under the provisions of Food and Drug Administration (FDA).

Preservation with Sugar

The principle of this technology is to add sugar in a quantity that is necessary to augment the osmotic pressure of the product’s liquid phase at a level which will prevent microorganism development.

From a practical point of view, however, it is usual to partially remove water (by boiling) from the product to be preserved, with the objective of obtaining a higher sugar concentration. In concentrations of 60% in the finished products, the sugar generally assures food preservation.

In the food preservation with sugar, the water activity cannot be reduced below 0.845. This value is sufficient for bacteria and neosmophile yeast inhibition but does not prevent mold attack. For this reason, various means are used to avoid mold development:

Finished product pasteurization (jams, jellies, etc.);

Use of chemical preservatives in order to obtain an antiseptic product surface.

It is very important from a practical point of view to avoid any product contamination after boiling and to assure a hygienic operation of the whole technological process (this will contribute to the prevention of product molding or fermentation).

Storage of the finished products in good conditions can only be achieved by ensuring the above level of water activity

Heat Preservation/Heat Processing

Various Degrees of Preservation

There are various degrees of preservation by heating; a few terms have to be identified and understood.

Sterilisation: By sterilisation we mean complete destruction of micro-organisms. Because of the resistance of certain bacterial spores to heat, this frequently means a treatment of at least 121° C (250° F) of wet heat for 15 minutes or its equivalent.

It also means that every particle of the food must receive this heat treatment. If a can of food is to be sterilised, then immersing it into a 121° C pressure cooker or retort for the 15 minutes will not be sufficient because of relatively slow rate of heat transfer through the food in the can to the most distant point.

“Commercially sterile” This term describes the condition that exists in most of canned or bottled products manufactured under good manufacturing practices procedures and methods. These products generally have a shelf-life of two years or more.

Pasteurization: Means a comparatively low order of heat treatment, generally at a temperature below the boiling point of water. The more general objective of pasteurization is to extend product shelf-life from a microbial and enzymatic point of view.

This is the objective when fruit or vegetable juices and certain other foods are pasteurized. Pasteurization is frequently combined with another means of preservation – concentration, chemical, acidification, etc.

Blanching: This is a type of pasteurization usually applied to vegetables mainly to inactivate natural food enzymes. Depending on its severity, blanching will also destroy some microorganisms.

FoodIrradiation: Food irradiation is one of the food processing technologies available to the food industry to control organisms that cause food-borne diseases and to reduce food losses due to spoilage and deterioration.

Food irradiation technology offers some advantages over conventional processes. Each application should be evaluated on its own merit as to whether irradiation provides a technical and economical solution that is better than traditional processing methods.

Shelf-life extension: Irradiation can extend the shelf-life of foods in a number of ways. By reducing the number of spoilage organisms (bacteria, mold, fungi), irradiation can lengthen the shelf life of fruits and vegetables.

Since ionising radiation interferes with cell division, it can be used as an alternative to chemicals to inhibit sprouting and thereby extend the shelf life of potatoes, onions and garlic.

Exposure of fruits and vegetables to ionising radiation slows their rate of ripening. Strawberries, for example, have been found to be suitable for irradiation. Their shelf-life can be extended three-fold, from 5 to 15 days.

Disinfestation: ionising radiation can also be used as an alternative to chemical fumigants for disinfestation of grains, spices, fruits and vegetables. Many countries prohibit the importation of products suspected of being contaminated with live insects to protect the importing country’s agricultural base.

With the banning of certain chemical fumigants, irradiation has the potential to facilitate the international shipment of food products.

In summary, the choice of method of storage depend the moisture content of the product when it comes from the field and the relative humidity of the outside air during the storage period.

Slight differences in relative humidity in the environment in which the food is kept or in the food package can make great differences in the rate of micro-organism multiplication.

In practice food preservation procedures aim at avoiding microbiological and biochemical deterioration which are the principal forms of deterioration.

Micro-organisms in a healthy growing state may contain in excess of 80% water. They get this water from the food in which they grow. If the water is removed from the food it also will transfer out of the bacterial cell and multiplication will stop.

Partial drying will be less effective than total drying, though for some micro-organisms partial drying may be quite sufficient to arrest bacterial growth and multiplication.

High temperature and high moisture are the most significant factors affecting grain quality in storage. Each can cause rapid decline in germination, malting quality, baking quality, colour, oil composition, and many other quality characteristics.

Insects and moulds impair the quality of grain directly by their feeding and development, and indirectly through generation of heat and moisture. High temperatures and moistures favour development of insects and moulds.

Development of insects is limited by temperatures below 15°C, and by moistures below 9% in cereal grains. Development of moulds is limited by temperatures below 10°C, and by moistures below 13% in cereal grains.

Spraying with insecticides or fumigating minimises insect problems but leaves chemical residues in grain, which break down with time. Presence of residues, and their concentration, affects acceptability of the grain to markets.

Various methods of storage have evolved over time and depending on crops to be stored and the environmental conditions. Food preservation techniques cold storage, freezing, drying/dehydration, sterilization, etc are applied only to one or some categories of foods; others can be used across the board and thus a wider application.

Some guarantee food preservation on their own while others require combination with other procedures, either as principal or as auxiliary processes in order to assure preservation (for example smoking has to be preceded by salting).

Exposure of fruits and vegetables to ionising radiation slows their rate of ripening, to inhibit sprouting and thereby extend the shelf life of potatoes, onions and garlic.