Film on Apples and Why Fruits Spoil

So, I was tagged in a users video on Facebook captioned “Look what happens when you pour boiling water over an apple! Is this wax? What do you think it is?”

As the water was poured over the apple, a filmy substance began to whiter away from the skin of the apple.  Some people were genuinely shocked to discover that a wax was overlaid on the surface of the apple (in all actuality it did appear alarmingly gross and disconcerting). This wax is what is referred to in the food industry as edible surface coatings or biopolymers.

Why are fruits and vegetables coated?

To explain why fruits and vegetables are coated, it first needs to be explained why fruits spoil or decay. People often don’t think of it this way, but fruits and vegetables are alive. They are literally living tissue just like you and I, and they undergo respiration. Once fruits and vegetables are picked or harvested, they continue to respire. Apart from the “vine” they no longer have an external source of energy; they cannot eat or drink. Instead they used up their internal supply of stored starches and sugars. As they use up this supply they begin to age and decay. Essentially they die! 

Fruits and vegetables eventually lose nutritional value as they age. They also, lose their juiciness as result of a loss of water, through the process of transpiration; in which they release moisture through their skin and into the air. This is the reason why  you’ll find fruits and vegetables left in plastic bags filled with water.

Surface coatings preserve fresh fruits from rotting. They extend the shelf life, color, and quality of the fruit.

Surface Coatings

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What: Edible surface coatings are natural polymers derived from the proteins obtained from vegetables or animals, and are environmentally safe.

There  are two types: edible coatings  and edible films. Coatings are suspensions or emulsions that are directly applied to the surface of the fruit or vegetable. Later it is transformed into a film. A film is a thick skin, biopolymer solution, that is cast separately, and then applied to the fruit or vegetable.

Coatings are made from the polymerization (creation of polymers from many like subunits) of proteins, lipids (fats/triglycerides/fatty acids), or polysaccharides (sugars). Protein based biopolymers come from wheat gluten, casein, collagen, gelatin, keratin, and corn zein. Carnauba wax is derived from the palm oil of a Brazilian palm tree, and represents a lipid based coating. Other lipid based biopolymers are beeswax, candelilla wax, and monoglycerides. Common sources of polysaccharide based coatings are starches, cellulose derivatives, chitosan , pectin, alginates, and gums.

How: surface coatings or biopolymers create a film that acts a barrier to moisture, lipid, and gas transfer between the internal flesh of fresh produce and the external environment. Temperature, an air quality (humidity, atmospheric pressure, air movement) all impact the quality of and quantity of fresh fruits and vegetables as they transition from the field to the market place, and finally your home.

Microbial Spoilage

Aging aside, there is an even bigger reason why fresh produce are coated in biopolymers. Surface coatings also protect fruits and vegetables from microbial contamination. Bacteria, yeast, and mold are primarily responsible for fruit spoilage. Just like humans, microbes are present on the skin of fruits and vegetables. In their case, the skin is broken down by pecteolytic enzymes released by bacteria and fungi. When they effectively penetrate the skin , the underlying flesh beneath is infected, and the skin presents with dark scars and holes. Flies can contaminate apple wounds with E. coli.

Waxes or biopolymers  are used as a preservation technique to prevent and control microbial contamination and spoilage.

Chitosan, (used in biopolymers) a polysaccharide derived from the shells of crabs and shrimp, has natural antifungal and antibacterial properties.  Its use  however is limited due to poor water vapor impermeability. It is often combined with starch/cellulose or lipid based biopolymers to overcome this limitation.

Contamination by microorganisms is not only a problem for fruits and vegetables, but also a threat to human health. Food-borne illness are spread by the consumption of contaminated fruits and vegetables. Gram-negative bacteria are associated with the microbial spoilage of vegetables, and yeasts and molds are mostly responsible for rotting of fruits. However, any produce can potentially facilitate the transfer of dangerous microorganisms, such as E. coli (Enterohemorrhagic Camphylobacter), S. Aureus (Staphylococcus Aureus), Plague (Yersinia Enterocolitica), Clostridium Botulinim (responsible for botulism poisoning), as well as viruses and parasites.

Summary

Waxes, surface coatings, or biopolymers, whichever you’d like to call them, do more than impart an artificial shine to fruits and vegetables. They are naturally derived films that protect produce from aging and microbial induced rotting. They also serve to protect human health by blocking the colonization of fruits and vegetables with pathogenic microorganisms. Additionally, surface coatings act as carriers for food additives and fungicides, increase flavor production, and retain the supple juicy nature and nutritional content of fresh produce. They are environmentally degradable and edible.

 

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