Processing of Horticultural Crops (1+2)

Factors affecting action of preservatives

Factors affecting action of preservatives

1. Molecular structure: Molecular structure affects the effectiveness of preservative. Out of parabens (p-hydroxy benzoic acid), m-hydroxy benzoic acid and benzoic acid; benzoic acid is more effective than parabens. Esterification of parabens leads to formation of much effective preservatives. So depending upon the requirement, the type of the preservatives can be used.

2. Position of Double Bond: Double bond in a compound in its cis form is more effective than in trans form. For example sorbic acid (CH₃CH=CH-CH=CHCOOH) a 2, 4- hexadienoic acid is a strong anti microbial agent since position of double bond is in cis form. With change in position of double bond to trans form the sorbic acid becomes less effective than cis form. Sorbic acid is the safest preservative as it undergoes ß- oxidation and changes to pyruvic acid.

3. Presence or absence of double bond: With the increase in degree of unsaturation, the effectiveness of preservative is increased.

4. Presence or absence of side chain: Straight chain molecules are more effective preservatives than the branched chain molecules.

5. pH of the medium: Acidic pH increases the effectiveness of preservative. For example, benzoic acid at pH 3.5 is 10 times effective than at pH 7. At low pH of 2, the effectiveness is 100 times because at low pH, the un-dissociated molecules of the acid are more and it is the unassociated molecules with create the effect.

6. Solubility: Some of the preservatives are insoluble in their natural form but when converted to their sodium salts, becomes more effective. For example sodium benzoate in place of benzoic acid.
The food additives having significance in fruit and vegetable preservation and their mode of action are discussed as under:

Mode of action of different preservatives

1. Salt: Salt is mostly used as preservative in pickles, raw mango slices, vegetables slices etc. It is used in brines and curing solution or applied directly to food. The minimum concentration of salt to act as preservative is 20%. Salt functions as a preservative through several mechanisms like i) creation of high osmotic pressure, ii) dehydration of food by drawing out water molecules iii) due to harmful effect of chloride ion on organisms iv) reduction in oxygen solubility in moisture v) interference with proteolytic enzymes action.

Mode of action: Salt (NaCl) acts as preservative in food by acting in following ways:

• Dehydration: Since sodium stays in its ionic form of Na+ Cl-, it attracts H+ OH- resulting in a desiccating effect leading to decrease in water availability. Under conditions of non availability of water many of the microbes fail to grow.
• Effect on cell membranes: NaCl (salt) increases the permeability of the cell wall and thus cause changes in the permeability. Only the haloduric microorganisms can tolerate these effects while others are unable to survive.
• Bactericidal effect: Cl- (Chlorine) being good oxidizing agent is toxic to micro-organisms.
• Effect on solubility of oxygen: Salt affects the solubility of oxygen and thus aerobic micro-organisms cannot grow.
• Inhibitor of protease enzyme: Salt (NaCl) acts as inhibitor for protease enzyme system and affects the growth of micro-organisms.

2. Sugar: Sugar is added to many foods for purpose of preservation e.g. jams, jelly, preserves, candies etc. Generally the concentration of sugar in the final products should be above 66% at room temperature to act as preservative.

Mode of action: Sugar act as preservative by increasing the osmotic pressure and binding of water molecules. Due to non availability of water the microbes fail to grow.

3. Sulphur dioxide (SO2): Sulphur dioxide is the most common class II preservative used in fruit and vegetable products in the form of sulphur dioxide gas and sodium or potassium salts of sulphide, bisulphite or metabisulphite. Dipping of fruit or vegetables in the solution of potassium metabisulphite is known as sulphiting while, using SO2 fumes by burning sulphur in an enclosed chamber is called as sulphuring. In aqueous solution sulphur dioxide and sulphite salts form sulphurous acid and ions of bisulphite and sulphite.

SO₂ + H₂O= H₂SO₃
2H₂SO₃= H+, HSO₃- + 2H+, SO₃²-

Mode of action:
1. Sulphitolysis: SO₂ acts on enzymes to cause sulphitolysis of S-S bonds in the proteins. When S-S bond is broken, the configuration is changed which results in loss of activity of the microbial protein.
SO₂ adds with the aldehyde group: When SO2 adds with the aldehyde group of the carbohydrate, the free CHO group is made limited and spoilage such as browning is stopped.
2. Reaction with thiamine: SO₂ reacts with thiamine and if vitamin B1 is short in foods, the micro-organism requiring vitamin B1 fails to grow.
3. SO₂ reduces oxygen tension resulting in anaerobiosis in the food system.
4. SO₂ reacts with NAD and inhibits many important enzymes and the whole energy processes are reduced.