D EHYDRATION AND DEHYDRATION EQUIPMENT

The principle of preservation by dehydration process is to remove the moisture content of a material to a level where micro-organism may not be able to grow and spoil it. In this process, the dehydrated product has better flavor, color, aroma, rehydration, acceptability, etc. in comparison to sundried dehydrated products.

P ROCESS OF DEHYDRATION The fresh vegetables are washed thoroughly and prepared into suitable size. These prepared vegetable pieces are then blanched or treated with permitted chemicals/preservatives. The treated vegetables are then dehydrated in two stages. In first stage, the moisture content is reduced upto 20_+5%. In the second stage moisture content is reduced to 8-+2% depending upon type of vegetables.

The temperature of drier in first stage drying is normally kept at 5O-60 C. and in second stage, the temperature is maintained at C. The dehydrated vegetables are then cooled and packed in suitable polythene bags and stored at cool and dry place.

A food dehydrator refers to a device that removes moisture from food to aid in its preservation. A food dehydrator uses a heat source and air flow to reduce the water content of foods. The water content of food is usually very high, typically 80% to 95% for various fruits and vegetables and 50% to 75% for various meats.

Removing moisture from food restrains various bacteria from growing and spoiling food. Further, removing moisture from food dramatically reduces the weight of the food. Thus, food dehydrators are used to preserve and extend the shelf life of various foods. Devices require heat using energy sources such as solar or electric power or biofuel and vary in form from large-scale dehydration projects to commercially sold appliances for domestic use.

W ORKING PRINCIPAL OF AN COMMERCIAL DEHYDRATOR A commercial food dehydrator's basic parts usually consist of a heating element, a fan, air vents allowing for air circulation and food trays to lay food upon. A dehydrator's heating element, fans and vents simultaneously work to remove moisture from food. A dehydrator's heating element warms the food causing its moisture to be released from its interior. The appliance's fan then blows the warm, moist air out of the appliance via the air vents.

This process continues for hours until the food is dried to a substantially lower water content, usually fifteen to twenty percent or less. The key to successful food dehydration is the application of a constant temperature and adequate air flow. Too high temperature can cause hardened foods: food that is hard and dry on the outside but moist, and therefore vulnerable to spoiling, on the inside.

D EHYDRATOR F EATURES Double wall construction of metal or high grade plastic, not wood Enclosed heating element Enclosed thermostat from 85ºF-160ºF Fan or blower 4-10 open mesh trays, plastic, sturdy, washable Dial for regulating temp A timer, auto- shut off

T YPES OF D EHYDRATORS Horizontal Air Flow – Heating element and fan are located on side Major advantages – reduces flavor mixing so different foods can be dried at once, all trays get equal heat, juices don’t drip into heating element Vertical Air Flow – Heating element and fan located at base Major disadvantage – if different foods dried, flavors can mix and liquids can drip into heating element

There are two basic dehydrator types 1) Stackable Tray Dehydrators and 2) Shelf Tray Dehydrators

S TACKABLE T RAY D EHYDRATORS Stacking dehydrators have trays that stack snugly on top of one another. They have a fan mounted either in their base, or, in a power unit that sits atop the stacked trays. The trays themselves have a hole in the center that forms a core channel through which air is routed. Heated air is forced up along the outside walls and then across each tray to the center channel where it exits the machine. This design allows air to move evenly across all trays, and generally makes it unnecessary to rotate trays while drying.

A key benefit of stacking dehydrators is their flexible and efficient capacity. Because you can add or remove trays to accommodate both small and large batches, the volume inside a stacking dehydrator is matched to the amount of food you are dehydrating. In addition, the larger stacking models can handle a lot of trays. The NESCO 1018P for example, can use up to 30 trays, for a total maximum drying area of 30 sq. ft. Stacking tray models do require you to stack and un-stack trays as you check progress and move trays around. You can’t just slide out the second tray from the bottom to see how it’s doing – you’ll need to un-stack it.

S HELF T RAY D EHYDRATORS The other main kind of dehydrator is a “shelf” dehydrator. With shelf-type dehydrators, the trays of the dehydrator are shelves that slide in and out of a box. These dehydrators have a different airflow system. Usually the fan and heating elements are mounted in the rear of the unit, and blow air horizontally across the shelves toward the front. This design promotes even drying, so that the trays do not need to be rotated.

Because the box shape of a shelf dehydrator limits the number of trays that can be used, the capacity of a shelf model isn't as flexible as a stacking tray machine. However, shelf units have their own advantages. For example, like an oven, you can remove some trays when you need to dry bulky items, and you don't have to un-stack trays to access one particular tray. Also, unlike stacking dehydrators, shelf dehydrators don’t have a hole in the middle of each tray. As a result, it’s easier use the entire tray, especially when making things like fruit leathers or raw crackers.

S OLAR F OOD D EHYDRATOR

Inside, the box holds diagonal layers of black metal screen. The vertical drying chamber on top has a back door to access food-drying trays inside. The sun’s radiation passes through the plastic top of the collector box to the absorber screens, which retain heat. Air entering through the intake is warmed as it passes over the absorber screens, and then flows into the drying chamber. There, the heated air draws out the food’s moisture before exiting through vents just below the roof.

The rising warm air creates negative pressure at the bottom of the collector box, which draws in more outside air to replace the air that left through the top vents. Air will continue to heat and rise, passing through the collector box and into the drying chamber, as long as the sun is shining or the dryer has access to another source of heat.

The drying chamber of this dehydrator supports 11 trays to hold up to 10 pounds of thinly sliced food — about 35 to 40 medium- sized apples, for reference. It can dry this amount of food in two sunny days, or about half this amount of food in one sunny day because of better ventilation and reduced food mass. The temperature inside the chamber can easily soar to more than 140 degrees Fahrenheit.

V ACUUM FREEZE DEHYDRATOR