Abstract:
Pak-Wanpa (Melientha suavis Pierre.) is a traditional vegetable in Saraburi province of Thailand and Southeast Asia. It has nutritional values and contains antioxidants. Vegetable perishes rapidly after harvest and cannot be preserved for more than a few days under ambient conditions. Drying is an ancient process used to preserve foods. Drying methods lead to modifications that can cause quality degradation. In this work, the effects of microwave vacuum drying (960-1680 W), hot air drying (600C) and freeze drying (-600C) were investigated on drying times, rehydration, color, structural, antioxidant capacity (FRAP, DPPH assay) and total phenolic content of Pak-Wanpa. Pak-Wanpa dried by microwave vacuum drying had higher drying rates and shorter drying time compare to hot air drying and freeze drying (p<0.05). Freeze dried Pak-Wanpa had higher rehydration ratio at both temperature of 30oC and 100oC compare with hot air dried and microwave vacuum dried Pak-Wanpa (p<0.05). Brown color (L*, a*, b*) were occurred by hot air drying (p<0.05), which contributed to the discoloration of Pak-Wanpa during this process. Microwave vacuum dried Pak-Wanpa had higher hardness and crispiness compare to hot air dried and freeze dried Pak-Wanpa. The total phenolic content decreased with drying methods. Microwave vacuum dried Pak-Wanpa had the highesttotal polyphenol (593.36 mg GAE/g dry weight). The radical scavenging on DPPH assay of freeze dried Pak-Wanpa had higher antioxidant activity when compare to hot air dried and microwave vacuum dried Pak-Wanpa. The FRAP assay of microwave vacuum dried Pak-Wanpa had the highest antioxidant activity (867.23 ± 56.98 µM trolox/ g dry weight) compare to hot air dried and freeze dried Pak-Wanpa. Microwave vacuum dried Pak-Wanpa (1680W) had lower total phenolic content and antioxidant activity when compared to hot air and freeze dried Pak-Wanpa during first 15 days of storage (p<0.05). However, microwave vacuum dried Pak-Wanpa had the highest amount of total phenolic compounds and antioxidant activity when compared to hot air and freeze dried Pak-Wanpa after storage for 90 days. These is because of its high initial values of total phenolic compound and antioxidant activity, Therefore, ,microwave vacuum drying is an alternative drying method for preserving Pak-Wanpa which can preserve physical and chemical properties of product