A new insight on oil penetration of breaded fish nuggets during deep-fat frying based on the surfactant effect

The surfactant effect is widely recognized as an important theory on oil penetration of frying foods, but the mechanisms by which oil unsaturation and frying time affect oil oxidation and how oil oxidation influences oil penetration remain unclear based on this theory. The batter-breaded fish nuggets (BBFNs) were fried using four vegetable oils with varying levels of unsaturation for 90, 120, 150 and 180 s, respectively, the decrease in iodine value and increase in total polar compounds content and viscosity of oil indicated that the oil oxidation was aggravated by both increasing the oil unsaturation and frying time. Furthermore, the increased porosity and pore volume, decreased bulk density of fried BBFNs confirmed that oil oxidation promoted the formation of pores, which were verified using scanning electron microscopy. Finally, the decreased interfacial tension and contact angle between the oil and crust as well as surface oil content, the increased penetration oil content and red region by Sudan Red B indicated that oil oxidation facilitated oil penetration. This study clarified the impacts of oil unsaturation and frying time on oil penetration of fried BBFNs based on the surfactant effect theory.