Milk and light do not mix. Milk exposed to light tastes bad and is nutritionally inferior. We’ve known that since we started putting milk in clear glass bottles. The influence of light on milk flavor was first reported in Europe as early as 1890. In 1920 researchers in the US, investigated packaging milk in brown colored bottles to offer some protections (see photo 1); today we still package milk in relatively clear containers. The influence of light on milk quality came further into “light” in the 1930’s when UV irradiation of milk was used to increase vitamin D3 content of milk. The benefits did not outweigh the potential for off-flavors and loss of light sensitive vitamins; the process was replaced by fortification. More recently, UV irradiation was investigated as an alternative to milk pasteurization. When a representative of a research group speaking on the feasibility of the process at the 2011 National Conference on Interstate Milk Shipments was asked about the potential for light-induced off flavors, the speaker stated that “that’s what consumer’s think milk is supposed to taste like.” That is not what milk is supposed to taste like.
The flavor of milk is defined as “pleasantly sweet with no foretaste or aftertaste other than the natural richness due to milk fat or other solids.” Any other “flavor” is likely a defect. Milk exposed to light is thought to have two distinct flavor profiles; one referred to as “light-induced” or “activated,” the other similar to lipid oxidation. Activated flavors have been described as plastic or chemical-like, medicinal, scorched, mushroom, or burnt hair/feathers. It is a unique flavor that once you learn to recognize it, you will know the cause. While the activated off-flavor develops relatively quickly after light exposure, the lipid oxidation component generally appears later during storage and is described as wet cardboard, stale, old oil or tallow, painty, and when intense, fishy. Methional, which is thought to be derived from the degradation of amino acids (i.e., methionine), has historically been associated with the initial activated phase, although it’s role in flavor is unclear. While riboflavin has been implicated as the primary photosensitizing agent in milk that initiates the free radical driven process, other photosensitizing compounds have been identified including chlorophyllic and heme compounds. Aroma compounds shown to increase with light exposed milk include acetaldehyde, pentanal, hexanal, heptanal, 2,3, butainedione, dimethyl disulfide and others.
Preventing light induced off-flavors is simple; keep milk in the dark, totally, from start to finish (packaging, cooler, load-outs, delivery docks, retail storage, retail display, consumer transport and consumption). Factors that influence the development of light induced off flavors in milk and dairy foods include:
- light transmittance of the packaging
- emission spectrum of the light source
- intensity / distance from the light
- time of light exposure
- light reflection, transmission and absorption of the food
- photosensitizer and oxygen levels of the food
Ideally milk should be packaged in containers that provide 100% light blocking. The reality is that most milk is still packaged in clear plastic, such as unpigmented high density polyethylene (HDPE), which allows estimated 60% light transmission (300 – 700 nm). Milk in HDPE can develop detectable off flavors in less than an hour when exposed to 2000 lx of fluorescent light, a level that is commonly found in retail displays (see photo 2). Exposure to sunlight would take a matter of minutes. Even paperboard allows some transmission. Addition of light block materials (e.g., opaque white or yellow) or use of overwraps (see photo 3) can be effective in reducing light induced off-flavors, but most have not proven to be 100%. Recent research suggested that 4.3% titanium dioxide (TiO2) added to HDPE provided protection of ESL milk similar to foil wrapped controls when stored in a lighted case (2,400 lx) up to 22 days. Lower TiO2 level (1.3%) did not provide consistent protection. Multilayer bottles that claim 100% blocking properties have been developed (e.g., Fonterra’s Light Proof™) but have seen limited use. While most research focuses on fluid milk, other products such as sour cream and cottage cheese are susceptible to light induced defects and likely subject to longer exposure times, thus light transmission of packing for these products should be considered.
Regardless of light blocking properties, light intensity and exposure times should be minimized and lights should be selected that have spectra with less damaging wavelengths (e.g. based on absorption spectra of photosensitizing agents), especially in retail displays where lights are used to “show off” products. Past studies on light exposure of milk commonly were based on fluorescent lighting normally used; “warm white” bulbs that were considered less likely to activate riboflavin were recommended. While critical wavelengths for riboflavin are in the 400-500 nm range, other identified photosensitizing agents have activations at wavelengths above this, making selection of lighting based on wavelength spectra challenging. Recently, light emitting diodes (LED) have become more common for lighting retail display cases. Two recent studies evaluated consumer liking of milk exposed to LED lighting. One study that compared fluorescent (2,200 lx) to LED (4,000 lx) lighting, found that consumers were able to detect off-flavors after 12 and 48 hours, respectively, suggesting that LED was less damaging. The other study found consumer liking was reduced for milks exposed to 1,200 lx of 3,500 K LED light for only 4 hours compared to unexposed controls. Additional information and research on the best available LED lighting based on intensity, wavelength spectra and retail appeal is still needed.
While perhaps considered a challenge, light induced off flavors can be minimized and are preventable. The sales of fluid milk have declined and the possibility that light oxidized off flavors in milk playing has played a role in this decline is often raised. This is not the way that milk is supposed to taste. We’ve known light and milk don’t mix for over a hundred years; it’s time to turn off the light.
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