The Mechanism of Frying Oil Degradation
Fats consist of glycerol molecules, in which three fatty acid chains are connected via ester bonds. The frying process typically involves the degradation of fats, that is the break of fatty ester bonds. As a consequence, the so called free fatty acids (FFAs) are released in the frying oil. The FFAs present positive charges favoring binding processes with hydroxyl groups, which are formed in cooking oils due to the ionization of water and other soluble compounds (i.e. proteins). Those processes cause the production of peroxides, which are considered the primary oxidation markers of cooking oils. Peroxides can further oxidize to form aldehydes and ketones, which are measured as anisidine value. The peroxide value of most cooking oil samples will firstly increase, then decrease due to the decomposition of peroxides into aldehydes and ketones. On the contrary, the anisidine value will progressively increase over time.
Quality Control in Industrial Frying Processes
The main goal of industrial frying is to preserve the integrity of cooking oils as much as possible. This is aimed at extending the shelf life of the cooking oil along with the fried/roasted products. The degradation of cooking oils determines the release of FFAs. Food tends to absorb the FFAs, which brings negative effects such as abnormal color, poor taste, and high oil absorption. The FFAs that absorb into the food will also oxidize into peroxides, causing further rancidity.
Therefore, the formation of peroxides can be limited in two different ways:
Reducing the amount of water and amino acids released in the frying oil
Decreasing the concentration of FFAs
The first option is practically impossible, since the frying process requires the immersion of food into cooking oils, with the consequent release of both water and amino acids. Therefore, a decrease in FFA concentration is the only possible way to limit the formation of peroxide and aldehyde and ketone formation (anisidine value), which contribute to the rancid sensory characteristics of the final fried products. For these reasons, it is essential to monitor the FFAs, peroxide value, and anisidine value to properly evaluate quality control of cooking oils. Generally, the FFA content must be kept under a certain threshold level. If the FFA level exceeds the approved range, the cooking oil must be disposed of or treated. However, the estimation of FFAs alone is insufficient. Only the combined analysis of FFAs, peroxides, and anisidine value provides a realistic and reliable overview of the quality of frying oils.
Traditional Methods to Perform Quality Control in Frying Oils
The quality control of frying oils is performed by standard procedures, which are the Official Methods and Recommended Practices of the American Oil Chemists Society (AOCS). FFAs and peroxides are measured by means of titrations. In particular, the evaluation of FFAs is performed by the AOCS Official Method Ca 5a-40, which consists in an acid-base titration (articolo JAOCS, Vol. 75, no. 5 (1998)). The concentration of peroxides is determined by iodometric titrations, according to the AOCS Official Method Cd 8-53. This procedure presents the disadvantage of being highly empirical, its results and accuracy strongly depend on the experimental analytic conditions. For example, the peroxide evaluation by the AOCS Official Method Cd 8-53 can be affected by the iodine absorption at the unsaturated bonds of the fatty material. Also, the presence of oxygen in the solution being titrated determines the liberation of iodine from potassium iodide, consequently causing the so-called oxygen error. All of these factors negatively affect the peroxide analysis, leading to potentially overestimated peroxide values (JAOCS, Vol. 78, no. 12 (2001)).
Anisidine value in frying oils is measured by the AOCS Official Method Cd 18-90. It is a spectrophotometric technique, which consists of reading absorbance values at 350 nm as the result of reactions between aldehydic compounds and p-anisidine in acetic acid solutions (https://chinaoils.cn/uploads/soft/20201216/1608080868770622.pdf). The analysis of anisidine value is very expensive and can cost up to $130 per sample in an outside laboratory in the US. Furthermore, all the previously described AOCS analysis are lab-intensive, requiring the use of bulk reagents, glassware, fume hoods, and trained staff. Therefore, these methods cannot be used directly along the production line and are generally performed by external laboratories. The impossibility to perform real-time quality control is a significant disadvantage in the use of traditional AOCS methods. Industrial producers do not have the possibility to quickly plan corrective actions and solve potential production problems.
Another parameter used to evaluate the quality of cooking oils is Total Polar Compounds (TPC). The TPC measurement detects the total amount of frying degradation products (FFAs, aldehydes, ketones, alcohols, non-volatile products), which are characterized by higher polarity values when compared with triglycerides (Xu XQ, Tran VH, Palmer M, White K, Salisbury P. Chemical and physical analyses and sensory evaluation of six deep-frying oils. J. Am. Oil Chem. Soc. 1999;76:1091–9). The TPC evaluation is performed by the AOCS Cd_20- 91 and ISO 8420. Official AOCS Methods, which consist in chromatographic techniques and must be performed by specialized laboratories and trained staff. Although the TPC content is mentioned by many different regulations (cit 10,11 di https://www.fda.gov.tw/upload/133/Content/2013050913435287631.pdf), this parameter is not useful in providing the whole picture of the quality of cooking oils. Indeed, the TPC content is a cumulative reading of FFAs, peroxides and anisidine values, whereas it is necessary to perform a separate quantification of those compounds to distinguish between initial and advanced degradation states (primary and secondary oxidation, respectively).
Preserving the Quality of Frying Oils with FILSORB®
The release of FFAs in cooking oils affects the quality of fried food and causes a reduction in shelf life. Indeed, the absorption of FFAs in food creates discolored products with poor taste and an enhanced tendency to be rancid. If FFA values are higher than a certain range, companies need to replace the cooking oil with virgin oil. However, doing so is highly expensive. An alternative solution is the use of the innovative oil absorbent, FILSORB®, which consists of a mixture of powdered silicates/silicas with a specific pH value, surface area, particle size, and molar ratio designed to decrease the concentration of FFAs on a large scale. FILSORB® can remove up to 70-80% of all the FFAs present in cooking oils. This enables the reduction of oxidation markers on both oil and final products, thus extending their shelf-life. FILSORB® can work in conjunction with any type of pressure or vacuum filtration system which utilizes paper or fabric membranes. Figure 3 shows the application of FILSORB® XP 20 to reduce the amount of FFAs in a mixture of cottonseed and peanut oil. After treatment with FILSORB® XP 20, the total amount of FFAs in the sample decreases from 0.30% oleic acid to 0.04% oleic acid (-0.26 % oleic acid).
Easy and Fast Analysis of Cooking Oils with the CDR FoodLab®
The traditional AOCS methods for the analysis of cooking oils are lab intensive and must be performed by trained staff. Consequently, those techniques do not allow real-time monitoring over an industrial frying process. A solution to this problem is CDR FoodLab® Analysis System, which is used worldwide to run analysis on fats and oils, directly along the production line. The CDR FoodLab® is an innovative photometric analyzer based on LED technology, with reading and incubation cells thermostated at 37 °C.
By using the CDR FoodLab®, it is possible to perform easy analysis thanks to pre-filled and ready to use reagent kits, with no need of glassware or specialized staff. The quality control of oils in terms of their FFAs, peroxides, and anisidine value can be performed quickly, using very low volumes of sample. Furthermore, CDR FoodLab® is pre-calibrated in correlation to the official methods, with the significant advantage of not requiring further calibration or maintenance over time.
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