Processing of Horticultural Crops (1+2)

Factors affecting action of SO₂

Factors affecting action of SO₂:

• Potassium meta-bi-sulphite (K₂S₂O₅) is commonly used as a stable source of SO₂. It is a crystalline salt which is fairly stable in neutral or alkaline medium but is discomposed by weak acids like citric, malic, tartaric and carbonic acids. When it is added to the fruit juice or squash, the potassium radical react with the acid of the juice forming the corresponding potassium salt and the sulphur dioxide, which is liberated to form sulphurous acid with the water of the juice.
• Potassium metabisulphite + citric acid= Potassium citrate+SO₂ +water
• SO₂ + H₂O = Sulphurous acid.
• The preservative effect of sulphurous acid depends upon the availability of free sulphurous acid and not the total quantity.
• The presence of glucose, aldehydes, ketoses, pectin and breakdown products of pectin like arabinose present in fruit juices generally combine with sulphur dioxide and reduce its effectiveness as a preservative.
• The effectiveness of sulphur dioxide also depends upon the pH of the medium. Generally preservative action of sulphur dioxide increases with decrease in pH. SO₂ is about 100 times more efficient as a preservative in strongly acidic solutions than in neutral solution. The enhanced antimicrobial effect of sulphur dioxide at low pH is due to the ability of undissociated sulphurous acid to penetrate the cell wall more easily.
• The toxicity of SO₂ increases at higher temperatures.
• Sulphur dioxide is more toxic to moulds, mould spores and vinegar bacteria than to yeast, as such its use in conjunction with sodium benzoate, which is toxic to yeast than to vinegar bacteria is desirable for effective preservation of fruit juices.

Limitations on use of SO₂:

• Though, addition of sulphur dioxide in fruit juices and squashes retards oxidation to prevent discolouration and loss of flavour yet, sulphur dioxide is not used in naturally coloured juices like phalsa, jamun, pomegranate juices, strawberry pulp, rhododendron juice etc due to its bleaching action.
• The use of SO₂ recommended in juices which are packaged in tin containers as it causes pin holes in the containers.
• It also forms hydrogen sulphide to give disagreeable smell and results in formation of black compound with the iron of the base plate. Such coloured products are usually preserved by using sodium benzoate.
• Though, sulphur dioxide and sulphites are metabolized to sulphate and are excreted in the urine without any pathological complications, yet it may cause some reactions in asthmatics

Versatility of sulphur dioxide as a preservative
In comparison to sodium benzoate, the use of sulphur dioxide is preferred in most food products due to following reasons:

• SO₂ has a better preserving action than sodium benzoate against bacterial fermentations.
• SO₂ preserves colour of the beverages for a longer time than sodium benzoate.
• Being a gas, it helps in preserving the surface layer of the juice.
• Being highly soluble in juices and squashes, it mixes well and gives better preservation.
• Any excess of sulphur dioxide in the juices or pulps can be removed either by heating the juice to about 71⁰C or by passing through a pulper or subjecting the juice to vacuum.
• Sulphur dioxide inhibits certain enzyme catalyzed reactions like enzymatic browning. Thus, use of sulphite or KMS spray or dips provides effective control of enzymic browning in unpeeled and pre-sliced potatoes, carrots, apples etc.
• Sulphur dioxide is used as effective inhibitor of non enzymatic browning in foods. It is probably involved in bisulphite interactions with active carbonyl groups.

4. Benzoic acid: Benzoic acid is used as a common preservative in many fruits and vegetable products like sauces, tomato ketchup, pickles, coloured squashes, carbonated beverages, sauerkraut etc. The antimicrobial activity is attributed to the presence of undissociated acid which exhibits optimum activity in the pH range of 2.5 to 4. Since sodium salt of benzoic acid is more soluble in water than the acid form, sodium benzoate is commonly used in the foods. One part of sodium benzoate is soluble in 1.8 parts of water while only 0.34 parts of benzoic acid dissolves in 100 parts of water. Pure sodium benzoate is practically tasteless and odourless. It is most effective against yeasts and bacteria and least active against moulds. It does not stop lactic acid and vinegar fermentations. The effectiveness of sodium benzoate depends upon the acidity of the medium in which it is added. At lower pH the effectiveness is increased. At pH 3.5, the benzoic acid is 10 times effective than at neutral medium, while at pH 2 the effectiveness is increased to 100 times. The preservative action of benzoic acid increases in presence of carbon dioxide. For example Bacillus subtilis fails to survive in benzoic acid solution in the presence of carbon dioxide.
Mode of action: Benzoic acid inhibits the activity of decarboxylase enzymes which are essential for TCA cycle. With effect on TCA cycle, microbial growth is hindered. Benzoic acid changes the nature of cell membrane and affects the O₂ uptake. This causes anaerobiosis by checking the exchange of gases and increase of CO_2. Benzoic acid in optimum concentration does not cause any harmful effect as it is readily excreted from the body after conjugation with glycine to form hippuric acid.

5. Parabens (p-hydroxy benzoic acid): Parabens are non dissociating compounds and their anti microbial action is independent of pH as such they are far superior to benzoic acid. The parabens (p-hydroxy benzoic acid) are used as microbial preservative in baked goods, soft drinks, pickles, jams, jellies and syrups. They are effective against moulds and yeast and relatively ineffective against bacteria (gram negative bacteria).
Anti-microbial action of parabens increases with increase in length of alkyl chain. Parabens link to some extent to the proteins on account of their phenolic groups and thus proteins get inactivated. Further, parabens interfere in folic acid metabolism of microorganisms which is a growth factor. Parabens being structural analogue hinders fatty acid metabolism. Parabens are active at pH 7 and higher, due to their ability to remain undissociated at these pH values. The presence of ester linkage in parabens remains stable to hydrolysis even at higher temperatures used for sterilization.

6. Nitrates and Nitrites: K and Na salt of nitrates and nitrites are used in curing of meat to develop and fix colour, to check growth of micro-organisms and to develop characteristic flavour. Nitrite in meat form nitric oxide, which reacts with hemi compounds to form nitroso-myoglobin (pigment responsible for pink colour of cured meat). Nitrite is effective at pH 5-5.5 than at higher pH values. Nitrite is known to react with sulfhydral groups (-SH) to create compounds that are not metabolized by microorganisms under anaerobic conditions.

7. Sorbic acid: Na and K salts of sorbic acid (C-C=C-C=C-COOH) are used to inhibit growth of moulds and yeast in many food products like fruit juices, pickles, cheese etc. The sorbic acid is used directly in foods and as spray or dip coatings on packaging materials. The activity of sorbic acid increases with decrease in pH indicating that undissociated form is more effective up to pH 6.5. They inhibit the growth of yeasts and mould but are less effective against bacteria. Sorbic acid is the safest preservative.

8. Propionic acid: Na and Ca salts of propionic acid (CH₃-CH₂-COOH) are used as effective antimicrobials against mould and few bacteria in cheese and bakery products. It checks ropiness in bread caused by Bacillus mesentericus. The undissociated form of propionic acid is effective up to pH 5. The toxicity to mould and certain bacteria is related to the inability of these micro- organisms to metabolize the 3- carbon skeleton. In human beings, propionic acid is metabolized in a manner similar to other fatty acids and as such does not cause any toxic effect when used within the recommended limits.

9. Acetic acid: Acetic acid is used in a various food products like sauces, ketchups, pickles etc. in form of vinegar to serve as dual functions of inhibiting micro- organisms and contributing to flavour. It is used in various forms such as vinegar (4% acetic acid), glacial acetic acid and sodium, potassium and calcium acetate as well as sodium di-acetate. It is more effective against yeasts and bacteria than against moulds. The salts are used in bread and other baked products to prevent ropiness and growth of moulds without interfering with the activity of yeast. The antimicrobial activity of acetic acid increases with the decrease in pH.

10. Epoxides: Ethylene oxide and propylene oxides are used as chemical sterilants in low moisture foods and to sterilize aseptic packaging materials. They are used in vapour state to achieve intimate contact with micro- organisms and after exposure the residual unreacted epoxide is removed by flushing and evacuation. Epoxides are reactive cyclic ethers that destroy all forms of microbes including spores and even viruses. Ethylene oxide is more reactive than propylene oxide and is more volatile and flammable as such it is supplied as a mixture of 10% ethylene oxide and 90% carbon dioxide.

11. Antibiotics: Antibiotics like nisin, subtilin, tetracycline, oxytetracycline, etc are the metabolites secreted by micro organisms which have selective antimicrobial activity. Nisin - a poly peptide antibiotic is effective against gram-positive organisms, is used to prevent spoilage in dairy products including processed cheese and condensed milk. Nisin is generally non toxic to humans as it is degraded in the intestinal tract without causing any harm.

12. Antioxidants: These are the substances which can delay the onset or slow the rate of oxidation of auto-oxidizable materials. Common antioxidants are BHA (Butylated hydroxy anisole), BHT (Butylated hydroxy toluene), TBHQ (tertiary butyl hydroquinone), PG (Propyle gallate), Tocopherols (Vitamin E) and Ascorbic acid (Vitamin C). The maximum limit of antioxidant used in food is 0.02%. Antioxidants donate proton and act as free radical trap.

R* +O₂=RO₂*
RO₂*+RH= R*+ROOH

13. Colouring agents: Two types of colouring agents (natural and synthetic) are used in the foods to improve the consumer appeal of the product. Natural colourants include carotenoids, chlorophyll, riboflavin, caramel, saffron etc. While, synthetic colourants are coal tar dyes which include tartrazine, sunset yellow, carmosine, amaranth, erythrosine fast red, etc. The maximum limit of use of synthetic colourants in fruit and vegetable products is 0.02% under Prevention of Food Adulteration Act (PFA).

14. Acid alkali control agents: Common acid-alkali agents used in most food products are citric acid, malic acid, lactic acid, tartaric acid, vinegar and sodium bicarbonate etc.

15. Sesquestrants or chelating agents: They are used for food stabilization by reacting with metallic ions and alkaline earth ions to form complexes that alter the properties of the ions and their effects in foods. Common chelating agents used in food industry are polycarboxylic acids (citric, malic, oxalic, succinic acid etc), polyphosphoric acids (adenosine tri phosphate, pyro-phosphate), macro molecules (porphyrin, protein) and salts of ethylene diamine-tetra-acetic acid (EDTA).

16. Emulsifiers: Emulsifiers are used for promoting foaming, whipping as well as stabilizations of foams in certain foods. Common emulsifiers include lecithin (in cake mixes),), stearyl-2-lactate (in confectionary products), mono and di-glycerides (to promote foaming and whipping), etc.

17. Stabilizers and thickeners: These are used for their textural, structural and functional characteristics in foods to stabilize emulsions, suspensions and foams and for thickening properties. Stabilizers and thickeners are hydrophilic and are dispersed in solutions as colloids, and are therefore also called as hydrocolloids. They also help in improving and stabilizing texture, inhibition of sugar and ice crystallization, and encapsulation of flavours. These are used in ice cream, jams, jellies, soups and preparation of foam mat dried products. Examples of stabilizers and thickeners are carboxy-methyl-cellulose (CMC), starch, gelatin, carageenan, agar, pectin, gum arabic and guar gum etc.

18. Firming agents: The substances used to prevent softening of fruit and vegetables tissues caused during heat processing or freezing are called as firming agents. Pectic substances are generally involved in structure stabilization through cross linking of their free carboxylic groups via polyvalent cations. Addition of calcium salts increases the firmness by cross linking with the carboxylic groups to form calcium pectinate and pectate. These stabilized structures support the tissue mass and thus the integrity is maintained even after heat processing. Similarly, trivalent aluminum ions are involved in crisping process through formation of complexes with pectic substances. The firmness in tomatoes, berries, apple slices etc is maintained by adding calcium salts prior to canning or freezing. Commonly used calcium salts are calcium chloride, calcium citrate, calcium sulphate, calcium lactate and mono calcium phosphate. Besides, acidic alum salts, sodium aluminum sulphate, potassium aluminum sulphate, ammonium aluminum sulphate and aluminum sulphate are also used to provide crisp and firm texture in fermented and brined cucumbers and other vegetables.

19. Clarifying agents: Formation of haze, sedimentation and oxidative deterioration due to action of natural phenolics are common problems encountered during storage of fruit wines, beer and many fruit beverages. Clarifying agents are generally used to control the desirable and undesirable effects of polyphenolics. Diatomaceous earth is used as filter aid to remove preformed haze. Bentonite is selective adsorbent used as clarifying/fining agent for wines to preclude precipitation. Clarifying agents having selective affinity for tannins, pro anthocyanidins and other polyphenols include proteins and synthetic resins like polyamides and polyvinyl pyrolidone (PVP). Gelatin and isinglass are proteins used to clarify beverages. The linkage between tannins and protein involves hydrogen bonding between phenolic hydroxyl groups and amide bonds in proteins. The addition of gelatin to apple juice causes aggregation and precipitation of a gelatin-tannin complex, which on settling enmeshes and removes other suspended solids.

20. Miscellaneous additives: Other chemicals used as an aid in food preservation include leavening agents, bleaching agents, anti caking agents, aerating agents and enzymes etc. Chlorine and hypochlorite of calcium and sodium are used for treatment of drinking and process water. Phosphoric acid is used in some soft drinks. Borax is used to wash vegetables and whole fruits like oranges.

a. Bleaching agents: Benzyl peroxide, Cl₂, ClO₂, NaCl, NO₂, KBrO₃, KIO₃, Ca (IO₃)₂, CaO are examples of some bleaching agents.
b. Anti-caking agents: Several conditioning agents are used to maintain free flowing characteristic of granular and powdered foods that are hydroscopic in nature. e.g., calcium silicate, calcium phosphate.
c. Aerating agents: Use of CO₂ in beverage like beer, carbonated fruit juices and aerated waters, N₂, N₂O in baby foods and baby milk are examples of some aerating agents.