Short-term in vivo digestibility of triglyceride polymers, dimers, and monomers of thermoxidized palm olein used in deep-frying

Palm olein used to fry potatoes 40 and 90 times was tested in a short-term in vivo digestibility experiment. Total alteration, expressed as polar content, reached 26.4 g/100 g of olein, and the amount of triglyceride polymers, triglyceride dimers, and oxidized triglycerides increased severalfold, as a result of repeated frying use of the olein. True digestibility of used and unused whole palm olein and that of the nonoxidized triglycerides and different polar compounds was tested after 1 g of palm olein/100 g of body weight was administered to young adult Wistar rats by means of an esophageal probe. Luminal fat was obtained 4 h later, and its polar and nonpolar fractions were separated and quantified using column chromatography. Thermoxidized and hydrolytic compounds were quantified by high-performance size exclusion chromatography and compared to the compounds administered. The true digestibility coefficient of palm olein used in frying 90 times was 30% lower than that of unused palm olein. True digestibility of triglyceride polymers and dimers of unused palm olein was quite high (>50%). After 90 uses, digestibility of dimers was significantly lower (p < 0.01), being similar to 30%. Nonoxidized triglyceride hydrolysis was negatively affected by the presence of large amounts of thermoxidized compounds. The amount of monoglycerides and free fatty acids found in the luminal fat also decreased as a consequence of the arrest of pancreatic lipase activity by thermoxidized compounds formed through repeated frying.