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We all know that Milk in direct sunlight is not good. But do you know why? Milk can develop off-flavors and lose nutrients after four minutes of exposure to direct sunlight. But do you know that Refrigerated milk in supermarket cabinets can have the same effect? Most supermarket cabinets have fluorescent display case lighting. After four hours of exposure to fluorescent display case lighting, refrigerated milk will develop
off-flavors and lose nutrients.
Light initiates two chemical reactions which result in off-flavors and the loss of nutrients. Milk in glass or plastic containers are most vulnerable to off-flavors and loss of nutrients. Although milk in paper cartons are less prone to loss of nutrients, it can still develop off-flavors.
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Lipid Oxidation of Milkwhey proteins, composed of amino acids containing sulfur, degrade and break down, producing “sunlight” flavors reminiscent of burnt feathers or burnt hair that can last for two to three days.
In the second reaction, the unsaturated fatty acids in milk lipids (milkfat triglycerides / triacylglycerols) become oxidized, producing malodorous carbonyl compounds that taste metallic or cardboardy and do not dissipate.
Various studies have found that these chemical reactions can occur in milk after four minutes of exposure to direct sunlight, and after four hours of exposure to fluorescent lighting, while the off-flavors were detected by trained tasters after milk was exposed to light for 15 to 30 minutes, and by consumers after milk was exposed to light for one to two hours.
In a Pennsylvania study, light-related off-flavors developed within 36 hours in almost half of 449 samples of milk in translucent plastic jugs. Another study found that off-flavors were present in approximately 80% of the blow-mold plastic milk containers exposed to fluorescent lighting in supermarket dairy display cases.
Milk Nutrient Loss During Lipid Oxidation
Vitamin A (especially added Vitamin A)
Vitamin C (ascorbic acid, present in small quantities in milk)
Vitamin D (especially added Vitamin D)
riboflavin (Vitamin B2) Riboflavin is destroyed by ultraviolet light.
Pyridoxine and Vitamin B12
Flavor Changes of Milk
When milk is exposed to light, two chemical reactions occur that cause the milk to taste like burnt feathers, burnt hair, cardboard, or metal.
Factors that affect the rate of flavor change and nutrient loss in milk caused by exposure to display case lighting are the intensity of the light, the wavelengths present in the light, the distance between the fluorescent light bulb and the milk, the material of the container, the amount of surface area exposed to the light, the temperature of the milk, and the duration of light exposure. The higher the fat content, the more vulnerable the milk or cream is to the off-flavor effects of lipid oxidation.
Milk in light-transmissible containers such as transparent glass or plastic is most commonly affected but milk in paper cartons can also be affected when the cartons are exposed to intense light for long periods of time.
"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." [
safespectrum]
Read more about Dairy Products and the effect of Light in the following websites:
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One study found that 90% of the added Vitamin A and 8% of the riboflavin was lost from milk in polyethylene containers after 24 hours of exposure to fluorescent lights, with increased loss of these nutrients as the fat content of the milk decreased.
