New study validates light blocking efforts: teens taste light-oxidation in milk and don't like it

Dairy Foods, Sept, 2002 by Kathryn Chapman

Teens influence food purchases...and food bills. They have a great deal of dollar influence and spending power. Although teens spend about $94 billion annually, with more than 10% of this amount directed toward food (mostly soft drinks, snacks, cookies, candy, and fast food), youth exert significant influence on spending across virtually all food categories. This figure does not include the billions of dollars parents spend under the influence of youth. A glitzy image may get youth to buy a product in the first place, but flavor influences repeat performances. If it doesn't taste good, they won't want it again. One mechanism for increasing milk consumption is to provide dairy products that have an appealing taste. According to Dr. Joseph Hotchkiss, packaging specialist from Cornell University, Ithaca N.Y., "if you want to increase milk sales, make sure the milk appeals to children. If they like milk when they are young they are more apt to drink it as an adult." One way to have good-tasting milk is to protect it from light.

Both natural and artificial light can induce quality defects that consumers notice -- and don't like. Light exposure causes chemical reactions in milk that can modify the proteins and fats that are present to produce many negative flavors, ranging from burnt protein (burnt feathers or hair) to cardboard or metallic. The resulting off-flavors are dependent upon various factors such as exposure time, intensity and wavelength of light, and composition of the milk.

To ensure that the highest quality products are on the market, parameters for protecting product quality must be established. To this end, Milk Quality Improvement Program (MQIP) scientists at Cornell University, Ithaca, N.Y. measured the amount of time it took to get noticeable flavor changes. Reduced fat (2%) milk in high-density polyethylene (HDPE) containers were exposed to lighting similar to the retail dairy case (2000 lux), where the average light exposure is 8 hours. The study was conducted from 1999 to 2001. The conclusions? Half of the teen and adult consumers could detect an off-flavor with less than 2 hours of light exposure. Also, the teens were asked how much they liked the milk. They thought that the light-oxidized milk was objectionable. The longer the milk was exposed to light, the less they liked the milk.

Light not only degrades flavor, but also vitamins. Since vitamins are essential nutrients, their loss by photodegradation decreases the nutritional value of food. Vitamins A, C, and B2 (riboflavin) are of particular concern with milk. MQIP found measurable vitamin A losses occurred increasingly at 2,4, and 16 hours for nonfat, reduced fat, and whole milk, respectively. Moderate light-oxidized flavors were detected after 4 hours of light exposure in the whole and reduced fat milk and after 8 hours in nonfat milk. The presence of increased levels of milk fat adversely affects the flavor quality of the products following exposure to light. On the other hand, higher fat levels do appear to provide some protection against vitamin A degradation, the studies have shown. (Journal of Dairy Science 85:351-354).

Anecdotal evidence shows why light can hurt milk sales.

Researchers at Cornell recently presented light-oxidized milk to students in an Introduction to Sensory Analysis Course. One of the students, who is also a mother, said, "That's what the milk tastes like that I serve to my children at home. Now I understand why my children don't want to drink milk."

Preventing light-oxidized flavors in milk involves simply protecting milk from light. Gabletop paperboard containers usually provide sufficient light barrier, but light-oxidation is more common in light transmissible plastic containers, so extra care is needed during transport, storage, and display.

To avoid photodegradation, milk should not be exposed at all to direct sunlight. Since sunlight is even more damaging than artificial light, a few minutes exposure to the sun on a loading dock or during consumer transport has profound results. In storage areas, milk crates should not be stacked in close proximity to lights. In dairy plants and stores, milk handling areas, storage coolers and display cases should be designed with minimum lighting and to facilitate product rotation. When selecting lighting, "cool white" fluorescent lights with wavelengths ranging from 420 to 520 nm, should not be used. If lighting is necessary, "warm white" lights are preferred in the dairy display case. Yellow shielding of the light can be used to reduce the intensity of light. Unnecessary lighting in coolers and display cases should be turned off when milk turn over rate is low.

According to Hotchkiss, three ways that plastics can be modified to protect their products from light-oxidized off-flavors and vitamin degradation are: (1), adding opaque pigment that blocks harmful wavelengths, (2), adding ultraviolet (UV) light blocking agents to clear plastic or (3) modifying the white pigment (titanium dioxide), so that it is translucent, but still blocks the UV light.