Volume 4 | Issue 1 | Year 2008

It’s clear. Clear packaging provides greater visibility of the product within and the reassurance to customers that the product looks good. However, packaging that let’s the light in may not be the best choice for all concerned. For many beverages, including beer, wine and milk, clear packaging is not the best choice.

Many foods and beverages are sensitive to light and this light sensitivity is exacerbated by the practice of displaying food, packaged in transparent and/or translucent materials, under high intensity fluorescent light. Transparent and translucent packaging allows light to reach the food, making it susceptible to photo-oxidation. Packaging material can have a protective effect on beverage quality through blocking or reducing the transmission of certain light wavelengths. This protection is based on material thickness, processing conditions and material coloration. For many foods, sensory (taste, odor, color) quality can decline considerably even when there is only a small amount of oxidation.

Sensitive reactions

Sensitivity to light is caused when a reactive molecule, one that is sensitive to light, responds to the light energy. This energy causes a chemical excitation of the molecule, resulting in an exchange of energy with other molecules. The transfer of energy can initiate a cascade of reactions with fats, proteins, and other reactive molecules within the food system. The result is an increase in small molecules that frequently have odor or flavor contributions, causing a degradation of sensory quality.

Examples of reactive molecules include flavonoids, such as the vitamin riboflavin, and chlorophyll. Heme and vitamin K compounds also may behave as reactive molecules. Riboflavin is one of the most studied sensitizers. Riboflavin is found in high concentrations in the whey fraction of milk and increases the susceptibility of milk to photo-oxidation. Riboflavin is highly sensitive to light in the ultraviolet wavelengths but also at the low-end visible light. This means that blocking only the ultraviolet wavelengths from reaching the milk does not protect riboflavin completely.

Research by Virginia Tech food scientists has provided significant evidence that visible wavelengths of light cause taste and odor changes of food. Materials research for protecting food from light damage focuses on UV light in the range of 200 to 400 nanometers, which is the range that can damage skin. These are the same wave lengths that cause nutritional and sensory damage in food. For example, visible light degrades riboflavin in milk, interacts with flavor and odor molecules, and causes pigment damage in food.

Ultraviolet wavelengths are not the only ones that cause damage, but they are important from the perspective of the food processors, who want beverages to look appealing. Packaging has moved away from paper board to polymers such as polyethylene, so the consumer can see the product. Then they started to have color and flavor problems.

Adding UV absorbers to the packaging helped and still allowed the consumer to see the product, but didn’t totally resolve the problem. The only way to completely protect the product is to use a totally opaque container. Generally, consumers like to see a product, particularly milk, to make sure it isn’t curdled, or juice to make sure there is no sedimentation. But we also want a product to have a long shelf life.

Packaging Protects Nutritional Value

If riboflavin functions as a photosensitizer over a prolonged time period, the vitamin molecule will breakdown and no longer be available to function as a vitamin. The rate of riboflavin destruction is proportional to the amount of light coming through the container, the wavelength of light and the presence of riboflavin. The destruction rate is greater when riboflavin is in its free form and unassociated with proteins or fat in milk. Riboflavin destruction is also inversely related to fat content. Lower fat content, as in nonfat or lowfat milk, allows more light to penetrate into the milk. Unflavored milk can lose up to 75 percent of the riboflavin after only one hour in sunlight. Chocolate milk will have less loss of riboflavin than will unflavored milk because the chocolate powder will absorb some of the light energy.

A secondary nutritional effect of light-induced degradation is the creation of flavor and odor molecules that affect milk flavor. Flavor and odor changes affect the satisfaction received by the consumer when drinking milk and may cause a decision to drink less milk. This, then, translates to decreased milk consumption and the loss of the nutrient-rich cocktail or high-valued proteins, vitamins and minerals that are biologically available for the human body to battle osteoporosis, dental health, and other possible benefits related to chronic diseases (cardiovascular health, hypertension, colon cancer are possible links).

It is important, then, to develop packaging materials that will enable a consumer to “see” the product yet block the most damaging wavelengths to milk or bever-age quality.

Enlightened Packaging

Milk is commonly packaged in either high density polyethylene (HDPE) or polyethylene terephthalate (PET). HDPE is a translucent polymer that transmits up to 62 percent of light wavelengths between 300-700 nm. Polyethylene terephthalate (PET), a packaging material utilized for single-serve milk products, is a clear polymer that transmits up to 75-85 percent of visible light. PET with an ultraviolet light barrier does help reduce light oxidation in milk but is not as effective as an opaque container or an amber PET bottle, based on evidence from the Duncan research laboratory.

The Virginia Tech researchers have tested a number of new materials (not developed at Virginia Tech) that are not being used for food packaging. New polymer materials, with unique optical properties, such as iridescent and pearlescent films that have a shimmer or shine, have recently come on the market. These materials may have a protective effect when used as package over-wraps; satisfying the demand for a “see through” appearance while protecting against sensory deterioration by photo-oxidation.

Iridescent films block wavelengths through light interference rather than the use of dyes and pigments. They are translucent and can be engineered to block specific light wavelengths, including those that are most damaging to milk quality. They can be laminated to complex shaped containers and could easily be used as film over-wraps for single-serve milk containers. They also provide an upscale look that appeals to consumers and could potentially increase market share: In an independent survey, >50 percent of respondents said that iridescent film packaging made the product more distinctive and unique than existing packaging.

Graduate students tested these films for possible benefit in protecting milk flavor and we found evidence of improvement, but still not as good as opaque. It is important, then, to develop packaging materials that will enable a consumer to “see” the product yet block the most damaging wavelengths to milk and beverage quality. Material scientists working with food scientists can develop better materials for protecting foods from light damage and providing the desired product visibility.

Susan Duncan, Ph.D., R.D., is professor of food science and technology and director of the Macromolecular Interfaces with Life Sciences program at Virginia Tech. She co-authored this paper with Janet B. Webster, Ph.D. and Susan Trulove, also at Virginia Tech.

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