Uncontrolled variables in frying studies: differences in repeatability between thermoxidation and frying experiments

Palm oil, soybean oil and partially hydrogenated soybean oil were used to define the repeatability of oil degradation under thermoxidation in the absence of food and during the frying of potatoes. Total polar compounds and their constituent polymeric triglycerides, oxidized monomeric triglycerides and diglycerides were analyzed. The oils were thermoxidized under standard conditions in a Rancimat apparatus at 180 °C for 25 hours. Discontinuous frying experiments were carried out in triplicate in three apparently identical domestic fryers. Five batches of potatoes were fried per day in each fryer. Oils were heated at 180 °C in periods of 5 h per day during 5 consecutive days so that the total heating period was 25 h. Repeatability was high in the thermoxidation assays and rather low in the frying experiments. Significant differences between the oils were found in the thermoxidation experiments, while no significant differences were found in the frying assays due to large standard deviations. The participation of variables related to the fryer performance which are difficult to control, was deduced. Among them, differences in the heating/cooling cycles at high temperature might be important. The contribution of fryer performance to the decreased repeatability was confirmed in further frying experiments with palm oil. Results showed that repeatability for the same fryer was higher than that obtained when different fryers were considered.