Virginia Polytechnic Institute and State University

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Changes in Aromatic Chemistry and Sensory Quality of Milk Due to Light Wavelength

Janet Webster, doctoral student in VA Tech's Department of Food Science, recently defended her dissertation and received her Ph.D. The following abstract is a summary of her research.

Many foods and beverages are susceptible to photooxidation. These foods are often packaged in transparent and/or translucent materials and displayed under high intensity fluorescent light, causing odor and flavor changes in the food. Even though photooxidation can be detrimental to the flavor and nutrition of food, the practice of displaying food in “see through” material continues because consumers prefer foods and beverages packaged in clear containers where they can see the product. This research was performed to 1). determine specific light wavelengths that are most damaging to milk quality so that packaging materials might be designed that block those wavelengths 2). determine the efficacy of unique multilayer polymers in inhibiting light oxidation in milk; and 3). develop a method for monitoring the chemical changes occurring in milk during photo-oxidation in real time using fourier transform infra-red (FTIR) spectroscopy.

The effect that specific light wavelengths had on the quality of milk was elucidated using gas chromatography (GC) and gas chromatography olfactometry (GCO). Generally, exposure to UV wavelengths (200-400 and 395 nm) and full light produced the highest amounts of volatile compounds (compounds produced during photooxidation that are responsible for off-flavors and odors). However, pentanal and an unidentified volatile compound were produced in high concentrations when exposed to 610 nm light. It is suspected that photo-oxidation in milk exposed to 200-400 and 395 nm wavelengths and to full light was through riboflavin sensitization, while the increase in compounds when exposed to 610 nm light was due to some other sensitizer.

The efficacy of film over-wraps, made from single and multi-layers of iridescent film, to reduce the production of light oxidation in milk was tested. Packaging over-wraps were designed to block either a single visible riboflavin excitation wavelength or all visible riboflavin excitation wavelengths. GC and GCO were used to assess the production of volatile and odor-active compounds. Packaging over-wraps reduced production of volatile and odor-active compounds in milk compared to milk with no over-wrap, but not to the extent that the complete light block (foil wrap) did. Hexanal, pentanal, 1-octene-3-ol and an unidentified compound increased over time in all treatments except the complete light block treatment. In general, no film treatment was more efficient at reducing oxidation than any other treatment. However, blocking the 400 nm wavelength appeared to be slightly better at reducing hexanal production than blocking 446 or 570 nm or all three wavelengths. There were more odor-active compounds in milk wrapped with single layer over-wraps than multi-layer over-wraps, and specific odor-active compounds detected in the single layer over-wrap treatments were not seen in the multi-layer over-wrap treatments. Both single layer and multi-layer over-wrap treatments had similar or higher numbers of odor-active compounds than the light-exposed treatment.

Film over-wraps were also evaluated for effectiveness in controlling light oxidized flavor in milk. A balanced incomplete block multi-sample difference test using a ranking system and a trained panel was used for evaluation of light oxidation flavor intensity. Packaging over-wraps limited the production of light oxidation flavor in milk over time but not to the same degree as the complete light block. Blocking all visible riboflavin excitation wavelengths was better at reducing light oxidation flavor than blocking only a single visible excitation wavelength. However, blocking transmission of all riboflavin excitation wavelengths at the levels suggested by the International Dairy Federation (IDF) was not sufficient to completely protect against the production of light oxidation flavor, suggesting the presence of a photosensitizer other than riboflavin in the milk.

A method for observing photooxidation in a model dairy system was developed using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Oleic acid with added riboflavin (1.75 ppm) was exposed to light (50W, halogen bulb) for up to 48 hrs. Preliminary data indicated that this method had the sensitivity to measure small changes associated with primary and secondary photooxidation in oil. Spectral changes associated with hydroxyl, carbonyl and double bond functional groups were monitored and correlated with the rise in peroxides and saturated and unsaturated aldehydes and ketones, respectively. Spectral bands associated with the C-H functional group decreased in intensity and were associated with peroxidation and loss of the cis double bond functional group.

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