Determination of protein levels in soy and peanut oils by colorimetric assay and ELISA.

Analytical methods are needed for measuring the levels of protein from allergenic food transferred into cooking oil. A simple method for determination of total protein in cooking oils was developed. Oil was extracted with phosphate-buffered saline with 0.05% Tween (PBST) and the extracts were partitioned with hexane to remove residual oil. Total protein in the PBST extracts was assayed with bicinchoninic acid (BCA), micro-BCA, reducing-agent compatible BCA and CB-XT kits. These methods were used to measure recovery of protein from peanut butter spikes of soy and peanut oil in the range of 50-1000 ppm. Recoveries were generally above 70%. However, the BCA and micro-BCA assays were subject to interference and enhanced color formation which were probably due to co-extracted antioxidants present in oil. The reducing agent-compatible BCA and CB-X protein assays reduced interference and gave lower protein values in crude, cold-pressed, and refined peanut oils. Heating oil to 180 degrees C before extraction also reduced interference-induced color enhancement. A commercial ELISA test kit was also used to measure peanut protein in oil spiked with peanut butter. Recovery of peanut residues measured by ELISA was significantly decreased when the peanut butter-spiked oil was heated to 180 degrees C compared to unheated oil. Recovery of spiked peanut butter protein measured by the buffer extraction-colorimetric method was not decreased in heated oil. The method developed here could be used to determine protein levels in crude and refined oil, and to assess the potential for allergen cross-contact from reused cooking oil.