9 Food Additives in Fruit Processing 155
O
O C
O C C
H H
C
HO HO
OH
OH C
H H
H
OH OH
OH H C C C
CH2OH C H
Hydrolysis Dehydration
C OH H
CH2OH Glucono-deltalactone Gluconic acid Figure 9.2.Hydrolysis of glucono-delta-latone.
products apart from the juices it is widely used in jellies and structured fruits. The relatively higher cost of GDL is a drawback for its extensive use in lieu of acidulants such as citric and malic acids (Montinez et al., 1997).
Benzoates
Benzoates have a typical aromatic ring structure.
Benzoic acid and sodium benzoate are widely used in a number of fruit products with an effective functional pH range of 2.5–4.0. These compounds are used pri- marily as antimycotic agents, and most yeasts and fungi are inhibited by 0.05–0.1% of the undissoci- ated acid. Food-poisoning and spore-forming bacte- ria are generally inhibited by 0.01–0.02% of undis- sociated acid, but many spoilage bacteria are much more resistant. Therefore, benzoic acid cannot be re- lied upon completely for effective preservation of foods capable of supporting bacterial growth (Baird- Parker, 1980). The antimicrobial properties have been attributed to undissociated benzoic acid according to the results of a study involving the uptake of ben- zoates bySaccharomyces cereviceae(Macris, 1975).
Benzoates have an advantage of low cost compared to other antimicrobial additives. The lower pH makes benzoates suitable for use in maraschino cherries, fruit pie fillings, fruit-based carbonated and noncar- bonated beverages, pickles, sauces, fruit preserves, and minimally processed acidified vegetables (Raju et al., 2000). The benzoate compounds are most effec- tive in the low-pH acid foods such as apple cider, soft drinks, tomato sauce/ketchup, and the like and not as effective in low-acid vegetables such as peas, beans, lettuce, etc. The pKa value of benzoate is 4.2 making a pH range of 4.0–5.0 as the functional pH, and most of the fruit products fall within this range. At a pH of 6.0, which is normal for many vegetables, only 1.5% of the benzoate is undissociated (Jay, 1986).
Care should be taken with the addition of benzoates to acid foods because they can deliver a “peppery” or a burning taste sensation at levels of about 0.1%.
As far as regulatory status is concerned, benzoates are considered as GRAS substances with a max- imum limit of 0.1% set apart (21 CFR 184.1021 and 21 CFR 189.1733). In most of the countries, the maximum permissible-use concentration is 0.15–
0.25%. Sodium benzoate is used as an antimicro- bial in carbonated and still beverages (0.03–0.05%);
syrups (0.1%); cider (0.05–0.1%); jams, jellies, and preserves (0.1%); and pie pastry fillings and salads (0.1%).
Parabens
Alkyl esters of p-hydroxy benzoic acid are collec- tively known as parabens and are permitted in the United States with reference to methyl, ethyl, and
heptyl parabens. Parabens are most effective against molds and yeasts and as such less effective against bacteria especially the Gram-negative bacteria. The pKa of these compounds is around pH 8.47. Their an- timicrobial activity tends to increase with the length of the alkyl chain and extends up to pH 7.0 (Dziezak, 1986). It is also known that the parabens are effective inhibitors of growth and toxin production ofClostrid- ium botulinum(Robach and Pierson, 1978).
Parabens are used in fruit juices, salads, and arti- ficially sweetened jams and jellies. The methyl and propyl esters of parabens are considered GRAS with a maximum total content of 0.1% (21 CFR 184.1490 and 21 CFR 184.1670).
Sorbates
Sorbic acid is a monocarboxylic fatty acid that is used to preserve foods. Sorbic acid is slightly soluble in water, whereas the potassium salt is highly water soluble up to 58.2 g/100 ml at 20◦C (Chichester and Tanner, 1975). The optimum pH range for effectiveness extends up to 6.5, higher than the upper range of benzoates and propionates but below that of parabens. A number of reports ex- ist regarding the antimicrobial activity of sorbates.
Spoilage-causing and heat-resistant fungi such as Neosortorya fischerialong with the ascospores were subjected to studies on thermal death rate, and the sorbate was found to control the organism without impeding the sensory value of preserved fruit juices such as grape and mango (Rajashekhara et al., 1998).
Effective inhibition of fungi was also observed in low-sugar preserves through the synergistic effects of sorbates and water activity regulation. Sorbates are currently used to preserve dehydrated prunes, figs, and many beverages such as orange juice, lemonade, and apple juice (Robach, 1980).
Sorbates are considered GRAS substances (21 CFR 182.3089), and they have been used in more than 90 food products having standards of identity. Sorbic acid is considered as one of least harmful antimi- crobial preservatives, even at levels exceeding those normally used in foods (Sofos and Busta, 1981).
Acidulants
The preservative role of acidulants is discussed ear- lier under the category of acidulants. The undis- sociated moiety is responsible for the bactericidal property of acidulants. They act to reduce the pH,
9 Food Additives in Fruit Processing 157 minimizing microbial growth and often enhancing
the effect of weak acid preservatives. The mecha- nism of action leading to preservative function may be attributed to lowering of pH as well as metal ion chelation. Citric acid has been primarily used in many fruit-based products, representing more than 60% of all food acids used. Malic acid and GDL are rela- tively newer and emerging acidulant preservatives, with the potential to impart excellent sensory prop- erty to the fruit products. The GRAS status of food acidulants along with the cost effectiveness favor the widespread use in a variety of fruit products includ- ing fruit-flavored carbonated and noncarbonated bev- erages.
Sulfites
Elemental sulfur and sulfur compounds are known to show antimicrobial activity and sulfur obtained from volcanic lava as well as hot spring water containing sulfur are used extensively for various dermal infec- tions and wounds. The pKa values for sulfur dioxide are 1.76 and 7.2, indicating a rather weak dibasic acid. It is useful to have sulfur dioxide in a salt form.
The dry salts are easier to store and less of a problem to handle than the gaseous or liquid forms (Ough, 1983). In water solutions, sulfur dioxide shows the following reaction equilibriums:
SO2+H2O←→[H2SO3] [H2SO3]←→HSO3−+H+
HSO3− ←
→SO3−2+H+
The growth inhibiting or lethal effects of sulfurous acid are most intense when the acid is in the unionized form (Hailer, 1911). It was also noted that bacteria were much more sensitive to sulfur dioxide than were yeasts and molds. It is also known that the bisulfites had lower activity than sulfur dioxide against yeasts and the sulfites had none. The three main groups of microbes of interest in the high acidic beverages and fruits are (1) acetic acid-producing and malolactic bacteria, (2) fermentation and spoilage yeasts, and (3) fruit molds.
As far as food applications are concerned, sulfites are used in a number of fruit products, i.e., dried fruits, frozen fruits, fruit-based beverages, glazed fruits, jams, and jellies within the purview of GMP and GRAS. However, any product with sulfur diox- ide levels above the detectable limits needs to specify
on the labels the nature of sulfitation and the residue levels (Taylor et al., 1986).
The FDA considers sulfur dioxide and several sul- fite salts as GRAS (21 CFR 182). However, sulfites cannot be used in fruits and vegetables intended to be served, presented, or sold raw/fresh to the con- sumers. They are allowed in fruit juices and concen- trates, dehydrated fruits, vegetables and wine. The maximum level of sulfur dioxide allowed in wine has been set at 350 mg/l by the regulating body for the U.S. alcoholic beverage industry. Sulfur elicits al- lergenic responses in certain individuals, especially steroid dependent and therefore, the usage levels in ready-to-eat fruit and vegetable products have been under stringent scrutiny, leading ultimately to its ban in such products (Anon, 1990).
Biopreservatives
The use of biopreservatives is gaining an increasing popularity. These are basically of biological origin and therefore can easily be considered GRAS as com- pared to the chemical additives. The biopreservative
“nisin” is the foremost among them as the use of nisin is gaining momentum for a range of food ap- plications. The compound is a peptide produced by the lactic bacteriaLactococcus lactissp.lactis. The structure and amino acid content of nisin was deter- mined by Gross and Morell (1971). The solubility is 56 mg/ml at pH 2.2, while at pH 5.0 the solubility is 3 mg/ml.
Nisin by itself has a narrow spectrum affecting only Gram-positive bacteria includingBacillus,Clostrid- ium, Enterococcus, Lactobacillus, Listeria, Pedio- coccus, and Staphylococcus. The spectrum of ac- tivity of nisin can be expanded to include Gram- negative bacteria by combining it with chelating agents, such as ethylene diamine tetra acetic acid (EDTA) (Carneiro et al., 1998).
Nisin as a food additive has been approved in many countries. The FDA has approved nisin as a prepa- ration (21 CFR 184.1538) with a content of not less than 900 IU/mg. It is approved to inhibit the growth ofC. botulinumspores in pasteurized cheese spreads with fruits and vegetables. It has been accorded a GRAS status with a maximum use level of 250 ppm.
As such it is used to reduce the thermal process levels in different canned products and the future for nisin appears bright as a need is being felt to decrease the thermal processing levels to protect finished product flavor (Davidson et al., 2002).
Antibrowning Agents
Antibrowning agents are of special significance in fruit products as a majority of them are susceptible to browning reaction during processing and storage.
The fruit products are highly susceptible toward the browning reactions of both the nonenzymatic and en- zymatic nature. The major reasons for higher rate of browning in fruit products can be cited as, abundance of sugars with particular reference to reducing sugars.
Nonenzymatic browning is a maillard reaction between carbonyl and amino groups with a host of intermediates, finally resulting in the formation of nitrogenous polymers and copolymers known as melanoidins. Nonenzymatic browning reactions can also result in the loss of vital nutrients such as ascor- bic acid, which gets oxidized to dehydroascorbic acid, which further undergoes aldol condensation or reaction with amino groups to form brown pigments (Loescher et al., 1991).
Browning due to ascrobic acid is very important in processed fruit juices enriched with vitamin C.
Nonenzymatic browning can also take place due to sugar degradation, iron complexing of polyphe- nols (Smith, 1987), and oxidation of polyphenols by hypochlorites (Choi and Sapers, 1994).
Nonenzymatic browning reaction in fruits and veg- etables depends on a number of factors such as (1) product composition, (2) moisture content of the product, and (3) storage temperature and exposure to oxygen. The compositional factors include maillard precursors or ascorbic acid (Kennedy et al., 1990).
Nonenzymatic browning in fruits and vegetables can be inhibited by refrigeration and through the control of water activity in dehydrated foods (Labuza and Saltmarch, 1981). Other methods of control include use of glucose oxidase for reduction of glucose levels, reduction of amino nitrogen content in juices by ion exchange, packaging with oxygen scavengers, and use of sulfites (Bolin and Steele, 1987). Sulfhydryl- containing amino acids have been found to be nearly as effective as bisulfite in inhibiting nonenzymatic browning in a model system (Friedman and Molnar, 1990). However, cysteine treatment was ineffective in dried apple (Bolin and Steele, 1987).
Additives to control nonenzymatic browning are used in a number of fruit products such as dehydrated fruits, glazed fruits, beverages, fruit bars, texturized fruit products, and fruit candies. The sulfite treat- ment levels vary in foods widely depending on the application. Residual levels usually do not exceed several hundred parts per million but may approach
1000 ppm in certain fruit and vegetable products (Taylor et al., 1986). FDA has proposed that max- imum residual sulfur dioxide levels of 300, 500, and 2000 ppm be permitted in fruit juices, dehydrated potatoes, and dried fruits, respectively (FDA, 1988).