Edible coatings revolutionising the ready-to-eat fruit and vegetable sector

Minimally processed fruits and vegetables currently account for about 10% of all produce sales, with sales exceeding $10 billion annually according to the International Fresh-cut Produce Association. The large, and growing, market share of ready-to-eat fruits and vegetables has been made possible by the development of invisible, colourless, odourless, tasteless, edible coatings.

These formulations can extend the shelf life anywhere between 7 and 21 days without compromising the fresh quality. They can:

• inhibit enzymatic browning,
• maintain the product’s natural texture,
• prevent sliced product from dehydrating,
• retain natural liquids in produce.

Now you may have thought that the skin on fruits and vegetables already provides natural protection against drying out, discoloration and other forms of spoilage. But consumer demands for convenience mean they want their produce to be ready to eat. They want to purchase it already peeled and sliced. Unfortunately, the acts of cutting and peeling remove the natural protection, allowing deterioration and spoilage to begin - everyone is familiar with the browning of apples within minutes of having their skins breached.

The use of edible films to protect produce is not new - at least as early as the 1100s, merchants in citrus-growing regions of southern China used wax to preserve oranges shipped by caravan to the emperor’s table in the North. In Europe, fresh fruit has been preserved by ‘larding’ - coated with melted fat from hogs - for centuries. In both of these cases the coatings sealed off the fruit, preventing the exchange of gases with the air, which is essential for sustaining good quality.

A thin layer of carnauba wax, obtained from the leaves of palm trees, has long been applied to apples to replace the natural wax coating on the fruit that is lost in post-harvest washing. The same wax is used to make many chocolate confections shiny.

Other common edible coatings include starch, alginate, carrageenan, gluten, whey and beeswax.

Attila E Pavlath, PhD, is the scientist who turned fresh-cut apple slices into a today’s convenience food, available ready to eat in grocery stores, school cafeterias and fast-food restaurants. At a recent meeting of the American Chemical Society, Pavlath presented an address about his research into edible coatings, where he pointed out that the use of edible films has grown dramatically since the mid-1980s. Then only 10 companies were in the business, whereas today more than 1000 companies with annual sales exceeding $100 million are involved. And, the use of edible films will probably continue to expand dramatically in the future - especially for fruits and vegetables - as health-conscious consumers look for more foods that require minimal preparation, like cut fruit and premixed salads.

Pavlath, a scientist at the US Department of Agriculture’s Western Regional Research Center, began working on edible films in the 1980s. The first formulation was created specifically to maintain the taste, texture and colour of sliced apples and pears. Pavlath and his Agricultural Research Service (ARS) team has continued to work on shelf-life extension technology and continues to add new formulas and technologies to its broad portfolio.

In 1986, the key discovery by the ARS researchers was that certain calcium salts protected apple slices from colour, taste or texture changes. “When you cut an apple, many physiological changes occur to the fruit tissue, including browning and the breakdown of cells. But this new treatment slows the process for at least two weeks. This time is crucial to allow for packaging, shipping, and marketing,” Pavlath explained back then.

The technology his group pioneered enabled refrigerated, packaged apple slices to last 2-3 weeks without turning brown or losing crispness and also without leaving a detectable residue.

Today, edible film technology has progressed even further - the films allow exchange of gases and have other features that maintain freshness, flavour, aroma, texture and nutritional value. They generally provide the same protection against bacteria as the natural skin if the foods are handled under sterile conditions when they are cut in the factory. Workers either spray on the films or immerse the foods in the liquid coating after cutting. The finished fruits and vegetables then go to consumers in sealed containers.

Challenges still remain

In the US, more bananas are consumed each year than apples and oranges combined but a coating that makes fresh-cut sliced banana a commercial reality is still elusive. Likewise, coatings for avocados, which are notorious for discolouring quickly after peeling, are still under development.

Why do we need edible films?

While some products can be eaten in their original form, in most cases they need to be pared, cored, sliced and diced for immediate household use. If this is done in a factory before reaching the consumer, products start to dehydrate, deteriorate and lose appearance, flavour and nutritional values. Without special protection the product becomes immediately more perishable. The damage can occur within hours or days, even if this damage is not immediately visible. This can be prevented if after the processing the natural skin is replaced with an edible film. The right edible films can prevent moisture losses while selectively allowing the controlled exchange of important gases, such as oxygen, carbon dioxide and ethylene, involved in respiration processes.

Proteins, carbohydrates and fats can fulfil requirements for edible films. The general rule is that fats reduce water transmission, polysaccharide films control oxygen and other gas transmission and protein films provide mechanical stability. These materials can be added separately or mixed, provided they do not change flavour. The major deciding factor is whether the protecting films have the necessary physical chemical properties to maintain the transmission of the various gases and liquids at the same rate as the natural protection does. Each of these components has different properties, which are efficient in controlling one type of transmission while sometimes have detrimental effect on others.