When fat surface is covered with a certain hood and the steam vaporizes from the fat surface, the fat surface is protected from contact with air, resulting that the thermal oxidation of fat is prevented according to the lessened degree of air contact.
In the present study, as shown in Fig.-3, fat surface was covered with a funnel shape hood, and water was sprayed through the small side opening. The sprayed water was vaporized from the fat surface into the hood, and discharged out through the outlet hole on the top of hood into atmosphere. Relation between the flow rate of steam at the outlet hole of hood and the thermal oxidative deterioration was examined (Fig.-5, 6). As a result, no thermal oxidative deterioration was observed when the flow rate of steam was over 10 g/cm²/hr. This indicates that the leak of the air into the hood space can be completely prevented. However, if the contact of the edge of hood bottom with fat surface was not complete, such effect was not observed (Fig.-7, 8).
Under such steam protection by means of hood, the rate of increase in acid value was less as compared with the case of metal float protection, although slightly increased than the case without protection.
This hood system is considered suitable as a practical mean to protect frying oil from thermal oxidative deterioration during deep fat frying because of its simplicity and reliable effect.