Critical Analysis of Oxidative Instability in Repeatedly Heated Palm and Soybean Oils: The Role of Fish Proteins, Metals, and Stainless Steel in Deep-Frying Keropok Lekor

By: Brian S. MH, MD (Alt. Med.)

Critical Review of Kamisah et al.'s Study

This critical review is based on a study titled, 'Deep-fried keropok lekors Increase Oxidative Instability in Cooking Oils', conducted by Kamisah et al (2012, Malaysian Journal of Medical Sciences).

Kamisah et al.'s study, which examines the oxidative instability of palm and soybean oils during repeated frying of keropok lekors and potato chips, provides essential insights but lacks consideration of certain key factors that could significantly impact peroxide formation. Here, we critically examine three overlooked aspects: the Maillard reaction, trace metal contamination from fish proteins, and potential iron leaching from the stainless steel wok. Each factor may contribute to the elevated peroxide values observed in the oils used to fry keropok lekors, as compared to those used for frying potato chips.

1. The Maillard Reaction in Keropok Lekors and Its Role in Oxidative Instability

The study attributes the higher peroxide values in oils used to fry keropok lekors to the susceptibility of fish oil to oxidation due to its high polyunsaturated fatty acid (PUFA) content. While this is a plausible cause, the Maillard reaction—a reaction between amino acids from fish proteins and reducing sugars at high temperatures—should have been considered as an additional mechanism of oxidative stress. Studies indicate that the Maillard reaction not only produces browning but also generates reactive oxygen species (ROS), contributing to oxidative degradation in both the fried food and the cooking oil (Liu et al., 2018). 

This reaction has been shown to accelerate the formation of hydroperoxides, particularly when fish proteins are heated with sugars, as would occur in keropok lekor frying (Vistoli et al., 2013). By not accounting for the Maillard reaction, the study potentially underestimates a critical pathway for free radical and peroxide production, impacting both the keropok lekors and the cooking oils.

2. Trace Metals in Fish Proteins and the Fenton Reaction

Fish naturally contain trace metals like iron and copper, which are known to accelerate lipid oxidation through their catalytic role in the Fenton reaction (Akl & Azzazy, 2013). In the presence of PUFAs, these metals can produce hydroxyl radicals, a highly reactive type of ROS that promotes oxidative degradation (Halliwell & Gutteridge, 2015). The presence of iron and copper in the keropok lekors could therefore have contributed to peroxide formation in both the fried food and the oils, amplifying oxidative instability. Without quantifying metal levels, the study fails to address the pro-oxidative effects of these trace metals, particularly when heating occurs repeatedly, allowing for cumulative oxidation. Addressing this aspect would have given a clearer picture of the oxidative pathways in play during the frying process.

3. Potential Iron Leaching from the Stainless Steel Wok

Although stainless steel cookware is generally heat-stable and resistant to leaching, prolonged exposure to high temperatures (such as the 180 °C used here) may still cause trace iron to leach into the oils (Frohne et al., 2018). This trace iron could catalyze the oxidation of PUFAs in both palm and soybean oils, contributing to peroxide formation. While stainless steel is indeed more resistant than other metal cookware, it is not entirely inert. Kamisah et al. could have included trace metal analysis of the oils to evaluate the extent of iron leaching from the stainless steel wok during repeated heating cycles. Considering the potential for even minimal iron leaching to impact oxidative stability, this omission is a limitation in the study’s methodology.

Recommendations for Future Studies

Based on the above critique, we propose several considerations for future studies on oxidative stability during deep-frying:

1. Investigate the Role of the Maillard Reaction:

Future research should measure the impact of the Maillard reaction on peroxide formation when frying fish-based products like keropok lekors. This could involve analyzing the ROS and hydroperoxides produced during the reaction, particularly focusing on how fish proteins combined with sugars influence oxidative degradation. Studies such as those by Vistoli et al. (2013) have underscored the Maillard reaction’s impact on lipid oxidation, making it a crucial factor to include.

2. Quantify Trace Metals in Fish-Based Foods and Oils:

Trace metal analysis of both the food product and the oils should be performed before and after frying to assess how metals like iron and copper from fish proteins influence peroxide levels. Evaluating their concentration would provide more insight into the extent of metal-catalyzed oxidation via the Fenton reaction and could help differentiate the oxidative roles of various trace metals in fish-based versus plant-based fried foods.

3. Analyze Iron Leaching from Stainless Steel Cookware:

A more thorough analysis of trace metal leaching from cookware is recommended, especially when repeated heating cycles are involved. Iron levels in cooking oils could be monitored before and after frying to confirm any potential influence of iron leaching on PUFA oxidation. While stainless steel is generally stable, it may not be entirely inert at high temperatures, as some research suggests (Khurana et al., 2019). Including trace iron measurements in cooking oils would provide a comprehensive evaluation of cookware choice on oxidation rates.

Conclusion

Kamisah et al.'s study offers valuable insights into oxidative stability in palm and soybean oils during the frying of keropok lekors. However, the study could benefit from a more comprehensive analysis that considers the Maillard reaction, trace metals from fish proteins, and the potential for iron leaching from stainless steel. Including these elements would provide a more nuanced understanding of oxidative instability and peroxide formation during frying.

References

Akl, A., & Azzazy, H. (2013). Oxidation and metal ions in foods. Journal of Food Science, 78(1), 39-45.

Frohne, T., Dung, L. M., & Riederer, M. (2018). Leaching behavior of metals from stainless steel utensils in food simulants at elevated temperatures. Journal of Food Engineering, 238, 103-111.

Halliwell, B., & Gutteridge, J. M. C. (2015). Free radicals in biology and medicine. Oxford University Press.

Khurana, V., Seth, R., & Kumar, V. (2019). Metal leaching from cookware: Assessing the health risks. Journal of Hazardous Materials, 374, 324-329.

Liu, Y., Yao, L., & Xu, Z. (2018). Advanced glycation end products: Implications in oxidative stress and human health. European Journal of Clinical Nutrition, 72(3), 400-411.

Vistoli, G., De Vita, D., & Galli, F. (2013). Maillard reaction and its impact on food quality and human health. Food Chemistry, 138(2), 1062-1073.

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