The Antioxidation Mechanism of Polydimethylsiloxane in Oil

Polydimethylsiloxane Shows Strong Protective Effects in Continuous Deep-Frying Operations

Polydimethylsiloxane (PDMS) was previously reported to show no protective effect in continuous deep-frying. In this study, we used canola oil with/without added PDMS to deep-fry shredded potato at 180°C either continuously or with 10-, 20-, or 30-min intervals between frying sessions for 6 h. In continuous deep-frying in canola oil not containing PDMS, far more oil vapor was generated from the oil and the water in the potato compared to frying with 20- and 30-min intervals between sessions and the oil in the fryer accordingly had a lower polar compound content (PC). The longer the oil was used to deep-fry potato, the more steam was generated from potato. Thus, polar compounds evaporated into the air in the steam, resulting in a low PC value of oil in the fryer. In contrast, both thermal deterioration and oil vaporization were remarkably inhibited in canola oil containing PDMS regardless of the frying pattern, and the PC value of the oil in the fryer increased in proportion to the amount of potato deep-fried. Canola oil with/without added PDMS was heated at 180°C for 6 h to confirm the effect of water released from potato on the oxidation of oil. A large increase in PC was observed in canola oil not containing PDMS when heated without water but this increase was inhibited to some extent when water was supplied continuously. On the other hand, the PC of canola oil containing PDMS was far lower than that of oil not containing PDMS, but the addition of water promoted an increase in PC. In conclusion, we observed superior protective effects of PDMS regardless of the deep-frying pattern employed, but the PC value nonetheless increased as the amount of food deep-fried increased. In addition, we confirmed that water in potato strongly correlates to PC increase of oil in the fryer.

Active Cooling of Oil after Deep-frying

Oil used for deep-frying is often left to stand after cooking. A major concern is oxidation during standing that might be avoidable, especially in the case of oil used repeatedly for commercial deep-frying as this involves large volumes that are difficult to cool in a conventional fryer. This paper describes a method to minimize oil oxidation. French fries were deep-fried and the oil temperature decreased in a manner typical for a commercial deep-fryer. The concentration of polar compounds generated from thermally oxidized oil remarkably increased at temperature higher than 100°C but little oxidation occurred below 60°C. Heating the oil showed that the peroxide and polar compound content did not increase when the oil was actively cooled using a running water-cooled Graham-type condenser system to cool the oil from 180°C to room temperature within 30 min. When French fries were fried and the oil was then immediately cooled using the condenser, the polar compound content during cooling did not increase. Our results demonstrate that active cooling of heated oil is simple and quite effective for inhibiting oxidation.

Polydimethylsiloxane Droplets Exhibit Extraordinarily High Antioxidative Effects in Deep-Frying

The addition of more than about 1 ppm polydimethylsiloxane (PDMS) into oil results in PDMS forming both a layer at the oil-air interface and droplets suspended in the oil. It is widely accepted that the extraordinarily strong and stable antioxidative effects of PDMS are due to the PDMS layer. However, the PDMS layer showed no antioxidative effects when canola oil did not contain droplets but rather was covered with a layer of PDMS, then subjected to heating under high agitation to mimic deep-frying. Furthermore, no antioxidative effect was exhibited by oil-soluble methylphenylsiloxane (PMPS) in canola oil or by PDMS in PDMS-soluble canola oil fatty acid ester during heating, suggesting that PDMS must be insoluble and droplets in oil in order for PDMS to exhibit an antioxidative effect during deep-frying. The zeta potential of PDMS droplets suspended in canola oil was very high and thus the negatively charged PDMS droplets should attract nearby low molecular weight compounds. It was suggested that this attraction disturbed the motion of oxygen molecules and prevented their attack against unsaturated fatty acid moiety. This would be the reason in the deep-frying why PDMS suppressed the oxidation reaction of oil. PDMS droplets also attracted volatile compounds (molecular weight below 125 Da) generated by heating canola oil. Thus, adding PDMS to oil after heating the oil resulted in the heated oil smelling less than heated oil without PDMS.

Inhibition of Frying Oil Oxidation by Carbon Dioxide Blanketing

The oxidation of oil starts, in general, from the penetration of atmospheric oxygen into oil. Inhibition of the vigorous oxidation of oil at deep-frying temperature under carbon dioxide flow, by disrupting the contact between oil and air, was first demonstrated using oil in a round bottom flask. Next, the minimum carbon dioxide flow rate necessary to blanket 4 L of frying oil in an electric fryer (surface area 690 cm(2)) installed with nonwoven fabric cover, was found to be 40 L/h. Then deep-frying of potato was done accordingly; immediately after deep-frying, an aluminum cover was placed on top of the nonwoven fabric cover to prevent the loss of carbon dioxide and the carbon dioxide flow was shut off. In conclusion, the oxidation of oil both at deep-frying temperature and during standing was remarkably inhibited by carbon dioxide blanketing at a practical flow rate and volume. Under the deep-frying conditions employed in this study, the increase in polar compound content was reduced to half of that of the control.