Light induced oxidation of nut oils (Short communication)

Abstract

The nuts are invariably sun-dried in Pakistan, Iran, Afghanistan and adjoining areas of China and USSR. Their oils are generally kept in the unrefined form, being exposed to light during subsequent handling or marketing and consequently undergo changes in colour, flavour and chemical changes. Light induced reactions have been found to damage the quality and stability of edible oils, fats and fat containing food products [l, 21. Oil characteristics of walnuts [3], and oxidation of almond and walnuts oils during heating have been studied [4]. Influence of clear flexible pouches and amber-colour bottles on storage behaviour of groundnut oils was reported by MAHADEVAIAH et al. [5] and NASIRULLAH et al.[6]. Dry nut oils are generally sold in clear glass bottles in Pakistan and several other countries but their stability to fluorescent light in relation to different packages has not been tested, which is the object of these studies. Materials and methodsThe almond, peanut, pinenut and walnut oils were extracted by the Soxhlet using petroleum ether (b. p. 40-60 "C). The solvent was initially removed with the help of a rotary evaporator at 40 "C and then in an air oven at 40-50 "C. Lighting consisted of two 20 watt Philips cool white fluorescent tubes. The oils in 1 cm thick layers were placed in clear and amber coloured glass containers at a light intensity of 540 lux which was measured by means of a General Electric type-214 light meter. Control samples were kept for corresponding time-temperature conditions in the dark. The spectral transmission (200-1 100 nm) measurements of glass containers were made with a UV-visible recording Shimadzu spectrophotometer UV-160. Oxidation rates of oils were followed by determining the peroxide value [7]. The fatty acids were determined by employing the trans-esterification method of SHEHATA et al. [8]. The resulting methyl esters were analyzed with a Perkin Elmer model-3920 gas chromatograph (conditions: column, SE-30 Scot. 50 feet x 0.5 mm; carrier gas, nitrogen 30 ml/min, air 50 Psi (3.4 . Id Pa); H, 20 Psi (1.4 . lo5 Pa); detector (FID) temperature 250 "C; column temperature 200 "C and injector temperature 300 "C). Results and discussionOn the basis of 7 weeks exposure, light induced oxidation rates (POV/day) of these oils in relation to different packages were calculated as shown in Table 1. Relative peroxidation of nut oils is shown in Fig. 1.The results revealed significant (P < 0.05) differences in the peroxidation of different oils. The rate of peroxidation was profoundly decreased by the amber glass or complete elimination of light (unexposed control). Determination of CV revealed striking differences in the peroxide values. Previously amber glass was found to be better in protecting the quality of liquid milk than clear glass bottles and Tetrapak package 191.

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“…Riboflavin may lose their activity and nutritive value decreases through light exposure. It may also lead to color changes and degradation of proteins (Sattar, 1989;Anonymous, 1993 …”

Oxidative rancidity in nuts

“…This effect is different from photo-oxidation which takes place in the presence of photosensitizer. The oxidation process is likely to be accelerated during retail storage as the kernels are often held at ambient temperature and relative humidity and are also exposed to light that catalyses oxidation (Sattar et al , 1989). Here the multifactorial effect on the oxidative stability of the nut is clearly evident.…”