Red pitaya (Hylocereus polyrhizus) peel is a rich source of fibre, antioxidant and betacyanin; however, it is discarded during processing, so it is able to cause environmental problems. In order to convert the fruit waste to potential by-product ingredients, drum drying is used as pre-treatment to create an ingredient which is shelf-stable. In this study, the effects of rotation speed and steam pressure of drums on the physico-chemical properties of pitaya peel that had undergone drum drying are investigated. Pitaya peel was dried in a laboratory scale double drum dryer at rotation speed of 1, 2, and 3 rpm at steam pressure of 1, 2, and 3 bar. The drum dried pitaya peel was then further analyzed based on percentage yield, moisture content, water activity, betacyanin retention and color change. Interaction of steam pressure and rotation speed gives significant effect (p < 0.05) on percentage yield, moisture content, water activity, betacyanin retention and Hunter L value, whereas it has no significant on Hunter a and b values. The best combination parameters (1 rpm and 2 bar) yield the highest betacyanin retention (80.21 mg/g of dry solid), acceptable moisture content (10.66% wet basis), water activity (aw = 0.42) and with 7.61% of yield.
Traditional drying methods involve high temperatures that degrade heat-sensitive compounds.
Dehumidified-air drying, an alternative to traditional drying methods, is suitable for heatsensitive
compounds; however, it consumes a large amount of energy and is comparatively
expensive. In this study, a multi-chamber dehumidified-air dryer was designed to dry Hydrocotyle
bonariensis, and the retention of the polyphenol content of Hydrocotyle bonariensis under
various drying conditions was examined. Multi-chamber dehumidified-air drying involves two
chambers; each chamber was operated at temperatures of 30, 40, and 50o
C with air volumetric
flow rates of 30 and 50 L/min. The results indicated that the highest retention of total phenolic
content and total flavonoid content, 24.67 mg of GAE/g dry weight (DW) and 2.204 mg of
catechin/g DW, respectively, was obtained at 50o
C with a 50 L/min air flow rate in the first
drying chamber. Multi-stage dehumidified-air dryers have the potential to dry heat-sensitive
products with reduced energy consumption.