Effect of Naturally Occurring Tetrapyrroles on Photooxidation in Cow's Milk

Abstract

The objective of this work was to better understand the photosensitizing effect of riboflavin versus naturally occurring tetrapyrroles in cow's milk. This was done by exposure of milk samples to blue light (400-500 nm), which is absorbed by riboflavin and tetrapyrroles, orange light (575-750 nm), which is absorbed by tetrapyrroles but not riboflavin, and white light, which contains the entire visible region. The milk was exposed to about 1.6 W/m(2) in 20 h, and two different light sources were tested: HMI lamp and fluorescent light tubes used for commercial display. Sensory analysis showed that wavelengths longer than 575 nm induced significantly more off-flavors than wavelengths shorter than 500 nm. By fluorescence spectroscopy it was observed that tetrapyrroles, in particular, chlorophyllic compounds, were degraded more by orange light than by blue and that the degree of degradation correlated closely with the formation of sensory off-flavors. The fluorescent agent Singlet Oxygen Sensor Green (SOSG) was used to monitor the formation of singlet oxygen under the different light exposure conditions, and the method verified that singlet oxygen was formed in large proportions in milk exposed to wavelengths longer than 575 nm, presumably with minor or no involvement of riboflavin. The results suggest that cholorophyllic compounds are responsible for a major part of photooxidation in milk. It is also suggested that β-carotene protects against photooxidation under blue light because it absorbs a major portion of the light below 500 nm and thereby reduces reactions with photosensitizers.

Flavonoids protecting food and beverages against light

“…Their presence in dairy products, as influenced by cow feeding patterns, expands the range of harmful wavelengths to the red region and accordingly make the efforts to reduce light sensitivity more complex. Sensory analyses of light‐exposed milk for example showed that wavelengths longer than 575 nm induced significantly more off‐flavours than those shorter than 500 nm, indicating chlorophylls, rather than flavins, are essential mediators of photoactivity …”

Assessment of the action spectrum for photooxidation in full fat bovine milk

“…During the recent years it has been reported that naturally occurring residues of tetrapyrroles in milk play an important role in photooxidation of dairy products. This was first reported for cheese and butter (Wold, Veberg, Nilsen, Iani, Juzenas, & Moan, 2005) and later for milk (Intawiwat et al, 2010;Airado et al, 2011). The exact identification of these tetrapyrroles remains, but protoporphyrin IX (PpIX) is one certain photosensitizer with notable contribution.…”

“…Riboflavin and beta-carotene are the two most prominent light absorbers in milk. They are present in full fat cow milk (typically 3.5 % fat) at the approximate concentrations 141μg/100g and 20μg/100g, respectively (Lindmark-Månsson, Fondén, & Petterson, 2003), and consequently they absorb light at about the same level in the violet and blue region (400-500 nm) of the visible spectrum (Airado-Rodríguez, Intawiwat, Skaret, & Wold, 2011). Of the two absorbers, only riboflavin is a photosensitizer contributing to photochemical reactions leading to photooxidation.…”

“…Of the two absorbers, only riboflavin is a photosensitizer contributing to photochemical reactions leading to photooxidation. Beta-carotene absorbs light in the same region as riboflavin, and it has therefore been suggested to protect against photooxidation since less light then reaches riboflavin (Skibsted, 2000;Airado et al, 2011).…”

“…All tetrapyrroles absorb strongly in the violet region (the Soret band), and then weaker in the blue to red region. Since riboflavin is not photoactive for wavelengths longer than about 500 nm, photooxidation in milk induced by longer wavelengths has so far been ascribed to these tetrapyrroles ( Airado et al, 2011).…”