Engineering Chemistry

Lesson 25 Food Preservators

25.2   Requirements of a good preservative

The requirements of a good preservative are:

• Simple and economical compounds and easy availability

• Have good inhibitory action against a wide range of microorganisms

• Have low order of toxicity - safe and no carcinogenic

• Have high stability in the environment of food (in terms of pH, temperature, redox potential, moisture etc) in which it is added

• Do not alter the identity and quality of the product

• Should be practical and compatible with product processing

• Should not adversely affect the nutritive value of the product

• Should have sufficient solubility in the medium prevailing in the food

25.3   Classification of Preservatives

Preservatives are classified into two classes class I and class II.

25.3.1 Class I preservatives: They include most of the natural substances and their addition in the foods is generally not restricted. Examples include:

• Common salt

• Sugar

• Dextrose

• Glucose

• Spices

• Vinegar or acetic acid

• Honey

• Edible vegetable oil

25.3.2 Class II preservatives: They include some natural substances as well as synthetic compounds. Their use is restricted and are added only to specified products and at permitted concentrations. Examples include:

• Benzoic acid including salts theirof

• Sulphurous acid including salts theirof

• Nitrates and nitrites of sodium or potassium

• Sorbic acid including its sodium, potassium and calcium salts

• Nicin

• Propionic acid and its salts

• Methyl or propyl para-hydroxy benzoate

• Sodium diacetate

• Sodium, potassium and calcium salts of lactic acid

25.4   Mode of action of preservatives

The chemical preservatives interfere with cell membrane of microorganisms, their enzymes or their genetic mechanisms. The interference can be caused in many ways, some of which are : changing the properties of preoteins and nucleic acids (e.g., formaldehyde), inactivation of enzyme mechanisms (e.g. formaldehyde), release of active Oxygen which reacts with amino acids, aldehydes and ketones (e.g., H₂O₂), raising redox potential to such a level which is not suitable for microbial growth (e.g., H₂O₂, nitrates), changing pH of medium to such a level which is not suitable for microbial growth (e.g., acetic acid), neutralize the acid produced by microorganisms (e.g., carbonates and bicarbonates), changing water activity so that microorganisms cannot use water for their growth (e.g., salt, sugar), inhibiting synthesis of DNA & RNA (esters of p-hydroxy benzoic acid), etc.

25.5   Commonly used Preservatives

Following are the commonly used preservatives, their mode of action and products in which they are used.

25.5.1 Sodium Chloride: Salt is used to control microorganisms in foods such as butter, cheese, olives, meat, fish, etc. It causes dehydration by drawing out water from the tissue cells. It also interferes with action of proteolytic enzymes and stops action of microorganisms.

25.5.2 Sugar: Sugar is used as preservative in foods like syrups and confectionery products, jams, jellies, marmalades, etc. Sugar has high osmotic pressure which is unfavourable for growth and reproduction of most species of bacteria.

25.5.3  Sulphur dioxide: It is used in treatment of fruits and vegetables. It is used to extend shelf life of fresh grapes, to kill undesirable microorganisms during wine making, in manufacture of fruit juices and also to prevent browning reactions. Sulphur dioxide is used in the form of gas (SO₂), Sodium or potassium bisulphites (NaHSO₃, KHSO₃), sulphites (Na₂SO₃, K₂SO₃) and metabisulphite (Na₂S₂O₅, K₂S₂O₅).

25.5.4 Acetic acid: It is used in the form of vinegar to preserve pickled vegetables. Acetates of sodium, potassium and calcium are used in bread and other baked foods. The acid is also used in ketchups, mayonnaise, etc. The antimicrobial activity of acetic acid is due to the low pH it creates.

25.5.5 Propionic acid: It is used in the form of sodium and potassium salts in bakery products. The salts are active against molds and some bacteria.

25.5.6 Benzoic Acid: Benzoic acid and its Sodium salt are widely used as preservatives. The acid form exhibits antimicrobial activity. It is active against yeast and bacteria. It is used in acid foods like fruit juices, carbonated breverages, pickles etc.

25.5.7 Parabens: These are esters of p-hydroxy benzoic acid. They are effective against molds and yeasts.

25.5.8 Epoxides: Epoxides like ethylene oxides and propylene oxide are used in preserving dry items like nuts and spices. They are cyclic ethers and used in gaseous form. The epoxides destroy all microorganisms including spores and viruses.

25.5.9 Sorbic Acid: Sorbic acid is a naturally occurring, unsaturated fatty acid. It is sparingly soluble in water and hence Sodium or Potassium salts of Sorbic acid are used. Sorbic acid and its salts are effective against yeasts and molds as well as bacteria. Sorbates are widely used as preservatives in preserving dairy products, fruits and vegetables, bakery products, confectioneries, meat, fish, etc.

25.5.10 Nisin: Nisin is an antibiotic produced by bacterium Lactococcus lactis. It is active against only limited microorganisms, particularly spore producing gram positive bacteria. It is used in canned foods and other food products.

25.5.11 Lactic acid and its salts: It is formed during fermentation of lactose by lactic acid bacteria. Lactic acid is a mild antiseptic and is used along with other preservatives. It is not commonly used and used in some types of pickles (with acetic acid) and biscuits.

25.5.12 Sugar: It is a disaccharide and is made of glucose and fructose units. It is effective against bacteria and molds. It is used in fruit products like jams, jellies, squashes etc, dairy products like condensed milk, sweets, etc.