A simple method for quality assessment of heated oils and fats is described. The proposed method involves precise determination of molecular weight averages (MWA} viz., the weight average molecular weight (Mw}, the number average molecular weight (Mn} and the Z-average molecular weight (Mz} by high performance size exclusion chromatography (HPSEC} and their quantitative correlation to percent polar material obtained by column chromatography (CC}. Change in MWA on heat treatment of fourteen different edible oils and fats at 180 ° +---2°C for eight 8-hr days is studied. Relative standard deviations and regression coefficients of correlation between MWA and their ratios (Mw, Mn, Mz, Mz/Mn, Mw/Mn and Mz/Mw} vs percent polar material have been reported. Probable discard time was predicted for all the oils based on the above-mentioned correlations and also from the percent of high molecular weight {H. Mwt} species formed. It was observed that the oils which are generally recommended for coronary patients deteriorated faster. A possibility of extending this methodology to the frying oils is suggested.
KEY WORDS: Column chromatography, discarding time, heated oils, high molecular weight species, high performance size exclusion chromatography, molecular weight averages, quality assessment.Deep-fat frying is one of the most commonly used procedures for the preparation of food (1). On repeated frying, oil is prone to undergo several oxidative and thermal reactions which ultimately change the physical, chemical, physico-chemical, physiological, nutritional and sensory properties of the oil which therefore is considered as heat abused (1,2}. The measurement of heat abuse in an oil and its discard time is important.Different analytical methodologies and criteria (3) for quantitative evaluation of heated oils are available. The quality assessment of heat abused oils is generally done based on the level of petroleum ether insoluble oxidized fatty acids (4) and percent polar compounds (5). The level of percent polar material is determined by silica gel column chromatography (CC) proposed by Guhr and Waibel (3,4) and is a standard official procedure since 1984 {6). Subsequently, high performance liquid chromatographic (HPLC} methods using silica gel (3,7-9} and size exclusion column packings (10-13} for determination of total polar artifacts in deep-fried oils are reported and the results are also correlated with those of CC method and petroleum ether insoluble oxidized fatty acids for studying quality deterioration in heated oils.The composition of heated oils is complex in nature. In this connection we are of the opini_on that their molecular_ weight averages (MWA) viz., Mw, Mn and Mz and distribution patterns viz., Mz/Mn, Mz/Mw and Mw/Mn may be more useful in understanding the nature of composition particularly in terms of quality deterioration. High performance size exclusion chromatography (HPSEC) is a powerful tool providing precise MWAs and their distribution patterns. Recently, we have reported the use of HPSEC technique ...