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Title:Formation and transport of undesirable compounds during repeated deep frying
Author(s):Shen, Ru
Director of Research:Engeseth, Nicki
Doctoral Committee Chair(s):Helferich, William
Doctoral Committee Member(s):Takhar, Pawan; Andrade, Juan
Department / Program:Food Science & Human Nutrition
Discipline:Food Science & Human Nutrition
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Undesirable compounds
Repeated deep frying
Food Types
Formation and transport mechanism
Abstract:Deep frying promotes desirable flavors, colors and textures in food; yet, it also results in development of undesirable compounds especially when oil is repeatedly reutilized. Once these reutilized oils reach a stage where they show visible signs of deterioration due to repeat frying cycles, they can be considered as thermally abused Frying oil (TAFO). Consumption of foods fried with TAFO may have various adverse health outcomes due to the accumulation of undesirable compounds. In response to consumers’ demands of healthy food, efforts are made to optimize frying parameters aiming at reducing undesirable frying byproducts without affecting the desirable food characteristics. There is no systematic analysis to track the quality and safety of repeated frying; how undesirable compounds are generated and migrate between food and oil during frying remains unclear to date. These facts become a major scientific hurdle in the optimization of industrial food deep frying and the production of healthy deep-fried foods. Therefore, the overall objective of this dissertation research was to investigate the formation and transport mechanisms of undesirable, sometimes unhealthy, compounds during repeated deep frying. In the present dissertation, the frying of three typical foods (potato strips, chicken strips and fish nuggets) was performed using soybean oil. Frying experiments to generate TAFO were conducted intermittently at 180 ± 5°C with oil being reutilized for up to 100 cycles. Unheated soybean oil and oil heated separately at 180 ± 5°C without food were used as controls. Oil and food samples were collected from different cycles periodically, analyzed for their physicochemical characteristics, fatty acid (FA) profile and specific groups of undesirable compounds, i.e. trans fatty acids (TFA) and monoepoxy fatty acids (MEFA). The heat and mass transfer behavior during repeated deep frying were also investigated and the diffusion parameters of polar compounds were experimentally determined. The major findings for this dissertation were: (1) Introduction of food to oil overweighed the high-temperature frying as a more influential factor contributing to oil deterioration and accumulation of degradative products in TAFO. The progress of oil deterioration showed high dependence on the type of food being fried: meat products (chicken and fish) led to greater deterioration than carbohydrate-rich foods (potato). Total polar compound (TPC) measured spectrophotometrically was demonstrated to be a reliable, simple and rapid analysis-adaptable parameter for tracking the thermal history of frying oil. (2) Prolonged heating, frying oil composition, food introduction and food type impacted the accumulation and partitioning of undesirable compounds in food and oil during frying. Compared with the heated control, deep frying of foods hindered the formation of TFA and promoted the formation of MEFA in TAFO. Endogenous (existing in non-fried food) and exogeneous (accumulating in TAFO) TFA and MEFA may migrate bi-directionally during frying. However, the partitioning of MEFA and TFA were different at the end. TFA underwent complete exchange and tended to be equal partitioning between food and oil, whereas the oil-to-food transport of MEFA was inhibited for chicken and fish. (3) Heat and mass transfer behaviors as affected by repeated utilization of frying oil during the frying of potato were investigated. As oil degraded, heat and mass transfer behaviors during the frying of potato were altered. Effective diffusion coefficient of polar compounds was determined experimentally for the first time. With the increase of reutilized cycles of oil, the effective diffusion coefficient of polar compounds during the frying of potato decreased from 3.577 × 10-7 (cycle 5) to 0.248 × 10-7 (cycle 100) m2/s. In conclusion, this work demonstrated that food interactively affected oil deterioration especially in accumulation of undesirable compounds. The thermal degradation history of oil during frying can be tracked accurately and efficiently in real-time by spectrophotometrically measuring TPC. TFA and MEFA that accumulated in TAFO and endogenously existing in non-fried food migrated bi-directionally during frying. Their accumulation and partitioning were influenced by food matrix, oil composition and nature of undesirable compounds. Experiments demonstrated that heat and mass transfer behavior were impacted by the degradation of frying oil. Effective diffusion coefficient of polar compounds was determined for the first time and decreased as oil degraded.
Issue Date:2019-12-06
Rights Information:Copyright 2019 Ru Shen
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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