Preservation of fruits and vegetables by wax coating
T
he magnitude of post harvest losses in fresh fruits and vegetables is estimated to be 25 – 80 % depending upon the commodity and the technological level of post harvest operations. This reflects the
lack of knowledge by post harvest handlers of the biological and environmental factors involved in the deterioration or the absence of adequate post harvest technologies required preserving fresh quality. In recent years, much attention has been paid to explore the potential of surface coating to maintain the quality of harvested fresh produce and to reduce the volume of disposable non-biodegradable packaging materials. Many fruits develop a waxy coat on their epidermis as they mature on the plant (apples, bananas, mangoes, and grapes) they develop a waxy coating on the epidermis, which is either felt or seen as a powdery bloom. Wax usually develops when the fruit has attained 2 / 3 of its growth. But much extraneous matter and spray residues.
The natural waxy coat is not adequate to offer protection against water loss and high respiration rate. Thus the approach towards prolonging the life of fruit / vegetable involves restricting the rate of respiration and preventing moisture loss. So as to maintain the vital food elements in as near the same quality as in the freshly picked fruit or vegetable.
The growing importance of fruits and vegetables in commerce has led to development in various protective wax coatings with and without fungicides, bactericides, growth regulators, etc., wax coating of fresh fruit and vegetable is quite old and it has great potential in the storage and transportation of fresh produce. Surface coatings have been used extensively on bulky organs to modify internal atmosphere composition and thereby delay ripening, reduce the water loss, and it improves the finish of the skin.
Post harvest losses can be reduced to some extent by, selection of varieties, careful handling, cleaning and sorting, control of ripening process, control of spoilage, packaging, use edible films, use of fungicide, use of ethylene absorbent, use of sprout inhibitors, use of growth regulators, irradiation, packaging home operations, dumping, washing and waxing.
Waxing Waxing of fruits and vegetables is done according to the recommendations. Food grade waxes are used to replace some of the natural; waxes removed in washing and cleaning operations, and this helps to reduce the water loss during handling and marketing. If produce is waxed, the wax coating is allowed to dry thoroughly before further handling. Waxing of the produce is an old age art, which was started in the beginning of the 19
th
century. Waxing was used as a preservation
technique for fruits since 1900. Wax has been used since pre historic times. In 4200 BC, the ancient Egyptians kept bees and used the wax from honeycombs to make models and to preserve their dead and in encaustic painting. The Romans used in pottery making textile printing, as early as 12
th
century, the Chinese were using
waxing processes on fruits, but it was not until 1922 that a waxing process was introduces for wide spread use on fruits and vegetables.
Wax Wax is an ester a long chain aliphatic acid chain aliphatic alcohol. Waxes are the esters formed from a fatty acid and a high molecular weight alcohol. Principle involved during storage: The produce continues respiring and uses up all the O2 with in the fruit which is not being replaced as quickly as before waxing because of the coating and produces CO2, which accumulates within the fruit because it cannot escape as
easily through the coating. Eventually the fruit will switch to anaerobic respiration that does not require O2. An aerobic respiration produces much more CO2, acetaldehyde and ethanol. The acetaldehyde and ethanol give off – flavour to the product, which are detrimental to the perceived quality. The natural barrier of the fruit and vegetable, the type of wax and amount if was applied will influence the extent to which the internal atmosphere (O2 and CO2) are modified and the level of reduction in weight loss. All fruits and vegetable are living organisms and as such require intake of O2 and release CO2 for respiration (aerobic). They also release vapour top the atmosphere
through
diffusion. Both these processes
create
a
loss in weight of the produce. All fruit and vegetables
are
covered naturally in a cuticle, which is barrier to gas exchange. The cuticle
is
impermeable
very to
O2,
slightly more permeable to CO2 and several magnitudes more permeable to water vapour. Unwaxed fruit or vegetables have pores in their surface through which most the diffusion of O2 and CO2 offers. Water vapour can move through the pores, cuticle and micro–cracks in the cuticle. During the process of waxing a tightly adhering thin film of the coating substance is applied to the surface f the fruit. The surface coating is often relatively impermeable to O2 and CO2 and water. Protective coating on fresh produce: Human became more health conscious. This gave new awareness of fresh fruits and vegetables, and resulted in rapidly increasing demand. Consumption of fresh produce, to help the consumers to obtain possible product, Fruits and vegetables are living organisms, which contain 80 - 90 % water by weight. If they are left without cuticle, the water quickly begins to evaporate, resulting in poor product shelf life to increase freshness and appearance. The fruit and vegetable growers should concern about consumer safety. There are few active ingredients which when combined with water, wetting and drying agents and form the compounds used a protective coating. The coating may contain one or more of the following. Carnauba wax, shellac wax, paraffin wax, candelilla wax, bees wax.
Different types of waxes ANIMAL WAXES: 1. Bees wax. 2. Spermaceti wax 3. Shellac wax 4. Chinese insect wax
VEGETABLE WAX 1. Carnauba wax 2. Candelilla wax 3. Sugarcane wax 4. Palm wax 5. Esparto wax 6. Japan wax 7. Oricury wax 8. Waxol 0.12, is the vegetable wax manufactured by Central Food Technological research Institute (CFTRI), Mysore.
MINERAL AND SYNTHETIC WAX 1. Ozocerite 2. Montan wax 3. Synthetic wax
Foodstuffs that are waxed are, Fruits: Apple
Avocadoes
Banana
Melons
Peaches
Pine apples
Bell pepper
Lemon
Oranges
Lime
Sweet potato
Melon
Grapes Passion fruit
Vegetables: Cucumber
Tomato
Methods of Waxing: Four principle methods of waxing of fruits and vegetables are there,
1. Liquid Paraffin wax method: In this method fruits and vegetables are dipped in hot paraffin. Some times resins are added. The main disadvantage of this method is too much of coating material is used. 2. Slab wax method: In this case the wax is pressed against rapidly revolving brushes. But the efficiency is very less. 3. Spray method: Spraying of melted wax on the fruit, which is subsequently brushed mechanically until a film of desired thickness is obtained. The wax is dissolved in a suitable solvent. This depends on, •
The pressure employed
•
Volume of wax used
•
Wax temperature
•
Distance of fruit from the spray
•
Number of spray nozzles.
4. Dipping or cold wax method: Fruits and vegetables are washed and then without being dried are dipped into a wax emulsion of proper concentration. They are dried before packing. Purified wax is odour less, tasteless and nontoxic and it can be heat- sealed.
PROPERTIES OF WAX •
Kneadable at 20° C.
•
Easily emulsifiable.
•
Should not impart undesirable odour.
•
Should be economical.
•
Efficient drying performance.
•
Non-sticky or tacky.
•
Should never interfere with the quality of fresh fruit / vegetable.
•
Melts above 40° C without decomposition.
•
Has relatively low viscosity.
•
Capable of being polished by slight pressure.
•
Translucent to opaque form but not like glass.
The properties of wax depend primarily on molecular structure rather than molecular size and chemical constitution.
Purpose of waxing •
To reduce the weight loss.
•
To replace the natural wax.
•
To increase the appearance
•
To increase the freshness
•
To decrease the rate of transpiration
•
To reduce the shrinkage losses.
•
To inhibit mold growth.
•
To prevent other physical damage and disease.
Necessary steps involved in waxing •
There should not be any production of off-flavours.
•
Rapid drying of the product after waxing on the fruit.
•
Manufactured from good grade materials.
•
Competitive price.
During harvesting, transport, washing, grading, it can undergo some scratching or abrasion, which removes natural layer or was and also scars the protective layer of skin.
Lipids and Resins Lipids and resin are added to coating formulations to reduce gas exchange, but mainly to impart hydrophobicity (to reduce water loss) and gloss appearance. Fruits coated with resins will develop whitening of the skin due to condensation that develops when they are brought from cold storage to ambient conditions.
Composite and Bilayer Several composite and bilayer films have been investigated with goal of combining the desirable properties of different material to improve permeability characteristics, gloss, strength, flexibility, nutritional value, and general performance or coating formulations. Plasticizers have been incorporated into edible coatings as a processing aid to facilitate coating application and to increase merchantability. The most commonly used plasticizers are polyols (sorbitol and glycerol, mono, di or oligo saccharides, lipids0. the plasticizing effect is due to their ability to reduce internal hydrogen bonding while increase intermolecular spacing. This will reduce the brittleness of the film but increases the permeability of water.
Several wax formulations have been developed and tasted experimentally. Suitable emulsifiers are used to produce a wax emulsion. Synthetic or natural resins are added to the wax emulsions to give more gloss to the treated produce. Waterwax emulsions are safer to use than solvent waxes, which are highly flammable. Water -wax emulsion can be used without drying the fruit prior to the treatment, therefore washing and wax treatment can be combined. Carnauba wax, sugarcane wax, thermoplastic terpene resins and shellac are common substances, which are used commercially; however waxes of plant origin are preferred. Development in wax emulsions were not limited to the formulation of new types of wax, but also included the addition of chemical fungicide, which gave waxing treatment an additional function. For example, biphenyl amine added to the wax emulsion not only gave additional shine and reduced weight loss and respiration rate of apples and pears, but also reduced the occurrence of scald. Addition of post harvest fungicides to the wax, emulsion to control Penicillium rot in stored citrus fruits also proved helpful. Waxes may be applied by dipping, spraying, foaming or brushing. Foaming is a better method of application since it leaves a very thin layer of wax on the fruit. If a thicker layer of wax is applied to the fruit surface, it becomes an undesirable barrier between the external and internal atmosphere and restricts exchange of respiratory gases (CO2 and O2). This may result in anaerobic respiration, resulting in fermentation and development of an off flavor. It is therefore necessary to adjust the thickness of the wax coat according to the variety and storage and marketing temperatures. Research on the Effect of Wax Coatings The effect of the wax emulsions Fruitex, Britex-561 and SB65 on oranges, Kinnow, lemons and grape fruits was studied during refrigerated and non-refrigerated storage (Farooqi et al 1988). Wax Coating of fruit improved its external appearance by giving additional shine to the fruit surface. It reduced weight loss and kept the fruit firmer, thus marinating its fresh look, while it reduced rates of respiration and ethylene production, thus delaying senescence. No significant change in constituents like Vitamin C, acidity, and sugar contents (In citrus fruit especially) were recorded due to treatment. However higher concentration of wax resulted in the development of off flavor in citrus fruits particularly during holding at 20° C, making this treatment
unsuitable for this fruit. It was, however found necessary to tailor the thickness of the wax coat according to storage temperature to avoid development of off flavor. FDA regulates these waxes, or coatings, as food additives approved or "generally recognized as safe" for human consumption. However, some consumers have concerns about their use. Vegetarians and others who avoid animal products may worry that fruits and vegetables contain animal-based waxes, such as oleic acid. Some people fear that the wax traps pesticides, making the fruit or vegetable unsafe to eat--even though FDA's pesticide monitoring program indicates that pesticide residues on fruits and vegetables are consistently within acceptable safe limits. If consumers want to avoid waxed fruits
and
vegetables, FDA regulations that took
effect
in
1994 may help them identify the appropriate products
for
them. These regulations require produce packers or grocers to provide point-of-sale information about the presence of waxes on fresh fruits and vegetables. This information can appear on labels of individual products, packing cartons (if they are used at the point of sale), or on counter cards or signs. The information will say that the product is: •
Coated with food-grade animal-based wax to maintain freshness, or
•
Coated with food-grade vegetable-, petroleum-, beeswax-, and/or shellacbased wax or resin to maintain freshness. If only one of these types of waxes is applied, the label can simply identify the type, such as "vegetable-based."
FDA also will allow the statement "No wax or resin coating" on fresh fruits and vegetables that do not contain wax.
Besides reading labeling information, consumers can reduce their concerns about waxes by rinsing fruits and vegetables with warm water and, when appropriate, scrubbing with a brush. This will eliminate much of the wax. New Developments in coating
Scientists are hard at work developing new methods of coating food to help it keep its freshness and visual appeal even longer--so a refrigerated slice of apple, for instance, could retain its fresh-cut look for up to three weeks, instead of turning brown in short order. Researchers at Food Science Australia, for example, are looking at creating more coatings made from naturally occurring compounds found in the foods themselves. Called modified atmosphere coatings, or MACs, these new coatings would allow only certain gases to pass through, preventing the food's deterioration without the need for plastic packaging. At the University of Georgia, food science and technology professor Manjeet Chinnan is studying both existing and new coatings and "edible films"--wrapping material that is made from plants instead of plastic. Edible films--based on the plantbased wrappings people in Asia have been using for centuries for various foods, such as sticky candies--being either eaten or at the very least biodegradable, could greatly reduce waste and the use of natural resources, since they're often made from food-processing byproducts. They have found limited use so far, but further research could lead to their gradually supplanting the ubiquitous plastic film almost everything is wrapped in today. Chinnan has discovered coatings that can increase the shelf life of foods like roasted peanuts by up to 60 percent and plant-based coatings that reduce the fat absorption of fried chicken strips by up to a third (and the chicken remained moister, to boot). Other coatings he's studied prevent the fats and acids in food being fried (vinegar and lemon juice in a marinade, for instance) from migrating into the frying oil, which keeps the frying oil fresher and allows it to be used longer. In future, Chinnan's team hopes to engineer films that could transfer flavors to foods, add vitamins, or even contain anti-microbial agents that could reduce the risk of microbial contamination, and hence the risk of food poisoning. For something food coatings accomplish a lot.
Conclusion Coating improves product appearance, colour, crispness, flavour, nutritive value, juiciness, texture, etc. From fruits/vegetable to meat products are can be coated including chocolates, various nuts, wafers, French fries, chicken, fish, corn chips, apple, lemon.
Though coating is an effective method of value addition and preservation for the food product, it is having some problems. Continuous Mechanization is not available for various coating operations. Till now, many of the coating operations are done by batch processing.
Safety of food product should be maintained in the coating operations. For example reuse of batter in battering operation will induce the chance of contamination over the food product.
Many of the coating materials are high in cost and some of the coating operations are also higher cost. This leads to increase the cost of operation of coating. So, we have to reduce the cost of operation by finding out the alternative coating martial and method instead of costly coating material and method.
# V.Thirupathi,
S.Sasikala and Z.John Kennedy,
Department of Food & Agricultural Process Engineering, Agricultural Engineering College & Research Institute, Tamil Nadu Agricultural University, Coimbatore - 641 003.
