Study On Radio Frequency Cooking Characteristics Of Purple-fleshed Potato

Potato is one of the most important principal foods,which is the fourth staple in the worldwide(after wheat,rice and maize).The potato has several strengths over other food:large harvest amount,good source of potassium,energy and high-quality starch.Also,purple-fleshed potato contains a lot of antioxidants such as polyphenols,anthocyanins and flavonoids.Conventional,potatoes are cooked in different ways including boiling,frying and roasting.In this study,radio frequency(RF 27 MHz)treatment was employed forpurple-fleshed potato cooking.In this pragramme,the characteristics of RF heated purple-fleshed potato including physico-chemical characteristics,dielectric properties,thermal properties,antioxidant activity,temperature distribution,penetration depth and heating dynamic and related mechanisms were ascertained.Besides,traditional cooking methods(boiling,roasting,microwaving(MW 2450 MHz)and infrared radiation(IR))were used as the comparison to highlight the characteristics and superiorities of RF cooking.The main conclusions listed as follows:(1)RF was observed to retain the highest amounts of vitamin C(loss rate:24.47%)and carotenoid(loss rate:52.70%)because of shorter heating time and a lower surface temperature during RF heating compared with boiling and roasting.Also,RF heating had better temperature distribution uniformity and less prone to partial scorching than MW,RF heated potato can retained the highest amount of functional components of purple-fleshed potato.Besides,RF heated potato had preferable color compared with MW,boiling and roasting.After four treatments,purple-fleshed potato generated alcohols,aldehydes,esters and alkane.And the RF treated potato had the highest amount of esters and alkane.The hardness value of the purple-fleshed potato was larger than other cooking methods after MW and roastingbecause of serious water loss of the purple-fleshed potato,which reduced the expansion pressure.In addition,the purple-fleshed potato after MW heating and roasting was more brittle,which due to the local overheating of the MW caused by faster heating rate and large loss of moisture.Through the four treatments,the granule morphology of purple-fleshed potato occurred in certain change.The micro-structure of RF treated potato was more orderly.Under the condition of MW and RF,the dielectric constant increased when the temperature rasing from 20 to 60 ~oC.It can be explicated that the mobility of ions and higher electric conductivity due to the elevation of temperature.After,the purple-fleshed potato starch begins to gelatinize between 60 to 80 ~oC,which reduced the mobility of the water and the dielectric constant begin to decrease.Under the condition of 27 MHz and 2450 MHz,the dielectric loss of purple-fleshed potato showed a tendency to increase firstly and then decrease with the increase of temperature.(2)The gelatinization degree(DG)curves and thermodynamic properties showed the MW(3 min,23.3 ~oC/min)and RF(7 min,10.0 ~oC/min)had shorter totally gelatinization time and faster heating rate than IR(15 min,6.9 ~oC/min)due to the dielectric heating has the higher electromagnetic energy conversion efficiency and endogenous heating.During the cooking process,it can be observed that the interior temperature surmounted the surface temperature during MW and RF treatments.Whereas,the interior temperature was lower than surface when IR cooking methods employed.And RF would offer more preferable penetration and better heating uniformity than MW.(3)The crystallinity degree of purple-fleshed potato powder decreased after RF,MW and IR treatments.And the FT-IR indicated that the cooking processing could significantly affect the purple-fleshed potato starch molecular configurations.The heated purple-fleshed potato still had a higher content of antioxidants,the total polyphenols content(TPC)total anthocyanin content(TAC)and total flavonoid content(TFC)loss rate were 4.28%,3.71%and 35.05%after RF cooking,at the same time the purple-fleshed potato after RF treatment still maintained higher antioxidant activity than MW and IR through DPPH,ABTS and FRAP tests.Compared with other cooking methods,RF treated potatoes were observed to retain the best of appearance,structural characteristics,nutritional quality and antioxidant capacity.Moreover,compared with the conventional cooking method,the RF has better heating uniformity and faster heating rate.Besides,RF heated potatoes has better controllability than MW heated ones due to its lower frequency.