| 1. |
|
|
| 2. |
- Botha, G.E., et al.
(författare)
-
Microwave assisted air drying of osmotically treated pineapple with variable power programmes
- 2012
-
Ingår i: Journal of Food Engineering. - : Elsevier BV. - 0260-8774 .- 1873-5770. ; 108:2, s. 304-311
-
Tidskriftsartikel (refereegranskat)abstract
- Variable power programmes for microwave assisted air drying of pineapple were studied. The pineapple pieces were pre-treated by osmotic dehydration in a 55° Brix sucrose solution at 40 °C for 90 min. Variable power output programmes were designed and ran with different inlet air temperatures between 30 and 70 °C. Results indicated that the use of variable microwave power combined with low air temperatures can result in a fast drying process without significant charring of pineapple pieces. High microwave powers need to be reduced quickly, faster than the decrease in water content would suggest, to minimize charring. In this study an inlet air temperatures of 70 °C was found to be excessive when combined with microwave energy (5 W/g - after compensating for the moisture loss), resulting in fast temperature increase. Microwave power was found to be most effective in the first hour to 1.5 h of processing. It should then be reduced to 0.1 W/g (initial product weight) in the final stages of drying to avoid charring of the fruit pieces. The best microwave programme tested lead to 20% water content with just 1% losses due to charring, but the results allow to conclude that charring could be completely reduced by switching off microwave energy altogether after 1.5 h and then finish off drying with higher air temperatures. The use of low air temperatures (30-50 °C) is advantageous with microwave energy in the first stages of drying as it limits the peaks of specific energy absorption, but it slows down drying towards the end probably because of a too low point of equilibrium (saturation humidity of air). Microwave energy did not significantly influence the drying process towards the end, although drying rates showed a "memory effect", that is, drying rates in processes with the same conditions after a given time depended on the conditions up to that point. © 2011 Elsevier Ltd. All rights reserved.
|
|
| 3. |
- Botha, G.E., et al.
(författare)
-
Quality optimisation of combined osmotic dehydration and microwave assisted air drying of pineapple using constant power emission
- 2012
-
Ingår i: Food and Bioproducts Processing. - : Elsevier BV. - 0960-3085 .- 1744-3571. ; 90:2, s. 171-179
-
Tidskriftsartikel (refereegranskat)abstract
- Combination of osmotic dehydration with microwave assisted air drying offers increased flexibility for process control and product quality. Osmotic dehydration (55°Brix solution at 40 °C for 90 min) combined with microwave assisted air drying (MWAD) was tested on smooth cayenne pineapples. The influence of the four most relevant processing parameters (osmotic treatment time, microwave power, air temperature and air velocity) was studied using a 2 4 circumscribed central composite experimental design. The product quality was evaluated in terms of charred appearance at the surface, moisture content, soluble solids content, water activity, firmness, colour and volume. Microwave power and air temperature were the two most important processing parameters that influenced the quality of the dehydrated pineapple, with the parameters most affected by the operating conditions being water content and percentage of charred pieces. Only in the latter was a significant quadratic effect found, all others were approximately linear. There was also a significant interactive effect between microwave power and air temperature affecting the percentage of charred pieces. Model predictions using a quadratic surface for water content and % charred pieces were validated with an additional experiment. Quadratic models were used to indicate optimum drying conditions for various targets. © 2011 The Institution of Chemical Engineers.
|
|
