Propionic acid is inhibits mould and rope bacteria growth but negligible effect on yeast. Propionic acid and its salts, sodium and calcium propionates, are approved by USFDA as GRAS (Generally Recognized As Safe) substances for food use and also by PFA act in India. Propionates are effective up to maximum limit of 5.5 pH. They are used in preserving cheese, non-alcoholic beverages, jams and jellies. Typical usage level of propionic acid and propionates is 0.2 per cent.
The parabens are alkyl esters of para-hydroxy benzoic acid. The two most common esters are methyl and propyl parabens approved by USFDA as GRAS. PFA act has mentioned it among the class II preservatives. The maximum concentration allowed is 0.1 per cent. They are most active against yeasts and moulds (0.5-0.1 per cent) but ineffective against bacteria, especially gram negative bacteria.
Solubility of parabens increases with increase in temperature of water. Methyl paraben is more soluble in water but less effective against moulds than propyl paraben. Therefore, a mixture of methyl paraben (2 to 3 parts) with propyl paraben (1 part) is normally used to negate the difference in their solubility.
Parabens are effective at higher pH values from 3 to 8 and also stable at low and high temperatures, even up to steam sterilization. But they are not as widely used due to high cost and objectionable flavour. They are used in beverages, jams, jellies, preserves, smoked fish and pickles.
Lactic acid is formed by microbial fermentation of sugars in preserved food products such as sauerkraut and pickles. The acid produced decreases the pH to levels unfavourable for growth of spoilage organisms such as putrefactive anaerobes and butyric-acid-producing bacteria. It does not control yeasts and mould growth, which can grow at such pH levels. Inclusion of other preservatives such as sorbate and benzoate may be used in that case.
It is also known as vinegar. Acetic acid is a general preservative inhibiting many species of bacteria, yeasts and to a lesser extent moulds. It is also a product of the lactic-acid fermentation. It is more effective in preservative action than lactic acid at same pH levels. It is mainly used in products such as pickles, sauces and ketchup.
Sulphur dioxide and sulphites
Sulphur dioxide and its derivatives have been widely used in foods as a food preservative. It serves both as an antioxidant and reducing agent and prevents enzymatic and non-enzymatic reactions, leading to microbial stability. The common used forms are sulphur dioxide gas and sodium, potassium and calcium salts of sulphite, bisulphite or metabisulphite, which are powders. Various sulphite forms dissolve in water and yield 50 to 68 per cent sulphur dioxide gas. It has bactericidal and bacteriostatic properties and is more effective against bacteria especially gram negative bacteria than moulds and yeasts.
Sulphur dioxide gas (SO2) is one of the oldest known fumigant and a wine preservative. The gaseous form is produced either by burning sulphur or by its release from the compressed liquefied form. It is a colourless, suffocating, pungent-smelling, non-flammable gas and highly soluble in cold water.
Sulphites are effective in producing more SO2 ions at pH values less than 4.0. Metabisulphite are more stable to oxidation than bisulphites and the latter show greater stability than sulphites.
Sulphites inhibit microbial growth by reacting with the energy rich compound like adenosine triphosphate; inhibiting some metabolic pathways; and blocking cellular transport systems. Sulphur dioxide also inhibits browning, both enzymatic and nonenzymatic, reactions in fruits and prevents darkening of colour and alterations of flavour. Therefore, sulphites are used to prevent or reduce discolouration of light-coloured fruits and vegetables, such as dried apples and dehydrated potatoes. These are added to sun-dried tomatoes, dried apricots, dried potatoes and lemon juice. These are also commonly used to lengthen the life of fruit juices.
They are also used in wine-making because they inhibit only bacterial growth but do not interfere with the desired development of yeast. Potassium metabisulphite (KMS) is generally used in non-coloured products whereas in coloured products containing anthocyanin pigment, sodium benzoate is used to prevent discolouration.
USFDA prohibits the use of sulphites in foods as it destroys thiamin (vitamin B1) and also causes severe allergic reactions, especially in asthmatics though, for the majority of the population, they are safe.
Nitrites and Nitrates
Nitrites have been used in meat curing for many centuries. It is used along with a mixture of salt, sugar, spices, and ascorbate for curing meats. Nitrite contributes to the development of the characteristic colour, flavour and texture improvement in addition to preservative effects. Sodium nitrite is quite soluble in water and is more effective below neutral pH (below 7.0). Along with salt, nitrite exhibits stronger antimicrobial action.
Nitrates break down in the body to nitrites and this stops the growth of bacteria, especially Clostridium botulinum, the bacteria that cause botulism poisoning. This is the reason nitrites and nitrates are used mainly among the packaged meats.
Nitrites also stabilize the red colour in cured meat and stop it from turning grey. Nitrates get readily converted into nitrites, which then react with the protein myoglobin to form nitric oxide myoglobin. During cooking, this is converted to nitrosohemochrome, a stable pink pigment, which impart a pink, fresh hue to cured meat. This chemical stabilizes the red colour of the meat and gives an appearance of fresh meat. That is why nitrites are a preferred preservative of meat processors even though its excess use is restricted in many countries.
Nitrite salts should be used with precaution because they can react with certain amines in food at acidic pH to produce nitrosamines, which are known to cause cancer by giving rise to compounds like nitrodimethyl-amine. Addition of sodium ascorbate inhibits nitrosamine formation and reduces the problem of nitrosamines. Nitrites and nitrates are permitted as preservatives in cured meat and meat products including poultry at levels below 200 ppm by USFDA and FSSA in India.
Antibiotics are antimicrobial substances produced by microorganisms have been allowed for food use only in recent years. But they are not widely used in food preservation due to the risk of ill effects on consumer and possibility of appearance of resistant strains. However, nisin and have been permitted in some foods.
Nisin is a polypeptide produced by Steptrococcus lactis (now called Lactococcus lactis). Its solubility depends on the pH of the medium and it is more soluble in acidic pH. Its antimicrobial action increases as the pH decreases. Nisin has a narrow spectrum affecting only gram-positive bacteria, including lactic acid bacteria, streptococci, bacilli, and clostridia. It does not inhibit gram-negative bacteria, yeasts or moulds. Nisin has been permitted in processed cheese up to 12.5 ppm under FSSA.
Natamycin is produced by the bacterium Streptomyces natalensis. The compound has a large lactone ring which is substituted with one or more sugar residues. Natamycin is primarily effective against yeast and moulds and is ineffective against bacteria. It has been permitted for surface treatment of hard cheese under FSSA with maximum level of application not to exceed 2mg/dm3
Ethylenediamine tetra acetic acid (EDTA)