Chemistry and microscopy of crust formation in French fried potatoes: Contribution of lipid polar products in frying oils to molecular organization and texture

Effects of polar lipid components in frying oils on physical, sensory, and microscopic characteristics of crusts on French fried Russet potatoes were investigated in two different scales of frying.;In laboratory scale frying, 0.375 inch shoestring strips were fried in fresh oil, degraded oil, and a 1:1 mixture of fresh and degraded oil with total polar contents of 1.2, 14.7, and 29.3%, respectively. The oil polar content significantly influenced textural properties, moisture content, and oil uptake of the fries. Moisture contents and compression coefficients were highest in mixed oil fries; and low oil uptakes and high shear coefficients were found in degraded oil fries.;In foodservice scale frying, 500 lb frozen 0.25 inch shoestring par-fry strips were fried in oil that progressively increased to a 5.9% total polar content and a 1.0% polymer content over an eight day period of frying. Oil uptake and crust thickness increased with oil degradation, while moisture content decreased. Although a semi-trained sensory panel was unable to distinguish textural differences in fries prepared in oils with different levels of polar materials, instrumental texture analysis showed a decrease in the compression coefficient and a slow increase in the shear coefficient as oil polar contents increased. Scanning electron microscopy revealed that polar materials in frying oil increased damage to cell wall structures, leading to higher frequencies of cell separation and rupture, with a corresponding leaching-out of starch paste onto the potato surface. An oil-starch matrix was subsequently formed, which was seen as patches on the surfaces potato cells. Sorbed oils were located primarily on the surface and within intercellular regions, and to a lesser extent within the starch gels inside cells. Microscopy showed lipid bound to the surface and tightly associated with potato cell components in fries prepared in high-polar oils. Enzymatic analysis of lipid-polysaccharide complexes released starch-associated lipid as well as lipid physically occluded in cell components.;Results demonstrate the critical integral role of polar lipid products in mediating molecular interactions involved in the crust formation in French fried potatoes and also suggest a mechanism for predicting fried potato quality from the determination of the polar content of the frying oil.