In Pacific Rim countries, mango and pineapple are a very popular fruits. These fruits generally are not dried below a water activity of 0.4 and even when air dried below 0.70 water activity, both these fruits (pineapple and mango) become tough with an undesirable texture, color and a less than desirable cooked flavor. As a result dried mangos and pineapples have not become popular dried fruits
U.S. Pat. No. 4,341,803, Koshida, issued Jul. 27, 1982 discloses a method for producing dried fruit chips in which starting fruit chips having an adjusted water soluble sugar concentration (6 to 25%) are freeze-dried to reduce the moisture content to a predetermined moisture content range (between 15 to 60%). The freeze dried fruit chips are vacuum microwave-dried to further reduce the moisture content to a second predetermined range (between 10 and 40%); then the vacuum microwave-dried fruit chips are vacuum dried to further reduce the moisture content (below 5%) to a range suitable for ready eating as a snack. The patent stresses that all three steps are essential to produce a suitable product. The porous snack, so produced has a high density since it is infused with 6% to 25% sugar and the final product has 29% to 63% sugar content.
U.S. Pat. No. 5,020,237 Gross, issued Jun. 4, 1991 and U.S. Pat. No. 5,135,122 Gross, Aug. 4, 1992 each describe a method and apparatus for vacuum dehydrating fruit and vegetables to a moisture content below 15% but preferably below about 5%, using the combined infrared energy and microwave energy simultaneously to produce a dried product that retains its natural flavor, a puffed appearance and crisp texture (about 3% moisture). The process applies microwave energy to drive the internal moisture out and infrared energy to dry the surface which when applied simultaneously speed up drying. This technique subjects the fruit to drying temperatures in the order of about 95.degree. C. and the final product takes on "cooked" flavors.
U.S. Pat. No. 4,948,609, Nafisi-Movaghar issued Aug. 14, 1990 discloses a vacuum oven drying process to produce a crispy, tender, puffed dried fruit or vegetable product with no discoloration. The fruits and/or vegetables are first frozen and thawed to tenderize, infused with various additives to prevent browning (about 25 to 45% sugar along with an acid and chelating agent) then dried at atmospheric pressure and at a temperature between 35 and 75.degree. C. to a moisture level of between 10 and 45%. The intermediate product so produced is then vacuum dried at temperatures between 50 and 125.degree. C. (66.degree. C. used with fruits) on heated trays to less than 5% moisture. The fruit changes from a "fresh" flavor to a "cooked" flavor when held at these temperatures for the period of time required to complete the drying.
The publication "New drying technology makes dried fruits taste like fresh" Author unknown Food Eng. V. 60, n. 7, 1988 pages 81, 82 and 84 describes a process for drying grapes by submersion of the grapes in heated paraffin wax at a very low pressure until 50 to 65% of the grape moisture is removed then subjecting the grapes to a three step vacuum microwave drying process to produce a puffed, dried grape product. In the first vacuum microwave drying step, the grapes are dried under a 20 Torr vacuum, 65 to 80.degree. C. temperature for 30 to 40 minutes at 2.5 to 3. KW power. The second step is at less than 20 Torr vacuum, 65 to 80.degree. C. for 20 to 30 minutes at 1 to 2 KW power. The final step is at less than 20 Torr vacuum, temperature not reported for 30 to 40 minutes at 0.25 to 0.5 KW. The grapes are held another 15 to 20 minutes under vacuum with no microwave power and allowed to cool and set. The resulting grape has a hard, dense exterior and a hollow center and have a cooked flavor.
The publication "Principles of Food Chemistry" second edition, John M. deMan published by Van Nostrand Reinhold pages 97-99 discusses denaturation of fruits when subjected to drying temperatures.
Drouzas and Schubert, in an article entitled "Microwave application in drying fresh fruits" Journal of Food Eng. V.28, n.2 (1996) 1996, pages 203-209 describe a microwave vacuum dehydration process for drying bananas. The process involves drying banana slices in a domestic microwave oven (Sharp 5V12W) at 70.degree. C. at 25 mbar vacuum at 0.15 KW power over a 30 minute period to a moisture content of 5 to 8%. The power was pulsed on 10 seconds and off 20 seconds. The resulting banana rehydrated in half the time compared to an air dried banana and absorbed twice as much moisture. The dried product was recommended for baking.