Light’s Effect On Milk

It is inevitable that pasteurized milk turns sour and thickens after a defi nite period of time under commercial conditions of storage, due to the growth of microorganisms. These microorganisms of bacterial origin enter the milk prior to heat treatment and/or more frequently via post-pasteurization contamination and have to be regarded as the predominant shelf-life-limiting factor for pasteurized milk. However, packaging may also be a shelf-life-relevant factor depending on its capability to protect the product from the infl uence of light.

Spoilage factor
Light causes both nutritional changes (ie the destruction of certain vitamins) and sensory changes (ie the development of off-fl avors) in milk. The latter changes appear under commercially-relevant storage conditions within the fi rst few days, even in packages with reduced light transmittance in the critical wavelength range below 500 nanometers. However, as the use of transparent milk packages is common practice in some countries, people are accustomed to the resultant light-induced fl avor changes and may not classify such milk as off-fl avor.

Preserving the nutritional qualities of pasteurized milk and extending its shelf-life has been the subject of much research and development by many different organizations.

Consequently, Alexander Saffert and Gabriele Pieper from Tetra Pak’s research laboratories in Germany, in collaboration with Jan Jetten from the TNO Quality of Life Institute in the Netherlands, undertook a study to assess the effect of light on pasteurized milk during its shelf-life (published in Packaging Technology and Science 2006; 19: 211-218).

Research objectives
The goal of Tetra Pak’s study was to accurately evaluate the degradation of vitamins A, B2 and B12 in pasteurized whole milk under commercially-relevant storage conditions.

Four different types of one-liter PET bottles were used in the study— one being transparent, and the remaining three having varying levels of light transmittance. These were designed to represent packaging types commonly used in a number of European countries. The fi rst type of bottle was chosen as being representative of completely clear packages on the market, such as natural high-density polyethylene (HDPE), or clear glass. The second and third types were white pigmented PET bottles with varying light-transmittance levels, representing the white bottles that can be found on the European market. The fourth type was pigmented with white and yellow and represented coated paperboard cartons, with light transmittance of fi ve percent or less. All the samples were exposed to a light intensity of 1,700 lux for 13 hours each day over a 10-day period.

Process
Homogenized, pasteurized whole milk with a fat content of three percent was manually fi lled under laminar fl ow into the gamma-irradiated bottles under cleanroom conditions. The remaining head space in the capped bottles was around 70 milliliters. The bottles were then stored at eight degrees Celsius for a period of 10 days with 13 hours of light exposure each day. Several of the most highly-pigmented bottles were stored at the reference temperature in total darkness, to act as a control sample.

Milk samples were collected from sealed bottles at sampling times of one, two, four, seven and ten days after initial packing for vitamin A and B2 analysis. Samples for vitamin B12 analysis were taken after four and ten days, and microbiological testing was carried out after seven and ten days. Duplicate analysis was carried out on two bottles per variant. All vitamin levels were determined according to the respective in-house method of TNO Quality of Life Institute (Zeist, The Netherlands).

Findings
The pigmented variants of PET signifi cantly reduced light transmittance at 450 nanometers from nearly 100 percent in the clear bottles to between fi ve to 10 percent in the most heavily-pigmented version, clearly showing the UV-A blocking effect of the added pigment. Vitamin-A losses over the ten-day period were most pronounced in the clear bottles, falling by 22 percent. No signifi cant difference in vitamin-A levels could be found in the other three bottle variants, with maximum losses reaching six percent—the same as observed in the control samples.

The only samples to show no loss of vitamin B2 were the control samples. All three variants of pigmented bottle showed losses between 11 and 20 percent, and the clear bottles, a loss of 33 percent. The content of vitamin B12, on the other hand, was unaffected by light transmittance of the packaging.

Summary
In conclusion, clear PET bottles offer no protection against light in the critical range for vitamin degradation, while bottles made of pigmented PET protect pasteurized milk against vitamin-A and -B2 degradation more effectively.

This study clearly demonstrates the need of pigmentation in PET bottles to protect the nutritional quality of milk. The use of highly-pigmented PET bottles to reduce the light transmittance to around 10 to 15 percent, or to an optimal level below 10 percent appears to protect vitamin B2 suffi ciently within the common shelf-life period for pasteurized milk. It should be pointed out, however, that light-induced sensory changes in pasteurized milk cannot be eliminated under commercially-relevant storage conditions in packages with light transmission rates of 10 percent or less.

So it becomes apparent that maximum nutritional value and minimum food degradation occurs when light cannot permeate the product, as in the case of using opaque paper-based packaging materials.