1K.Shunmugapriya, 2T.R.Thirumuruga Ponbhagavathi and 2T.Aruna
1&2Ph.D Scholars, Department of Food Science and Nutrition, TNAU, Madurai – 625104
Fresh fruits and vegetables are highly perishable, shelf life is so short. If not handled properly, fruits and vegetables that have been harvested will undergo physiological, physical, chemical, and microbiological changes that become damaged or rotten. Meanwhile, the tubers usually have a longer shelf life, although some will soon rot in storage, such as cassava. One effort to maintain quality and shelf life of fruit and have a pretty good market is processed into chips. Chips are more durable than the stored fresh fruits or vegetables (incl. tubers) because of the low water contents. Deep-fat (ordinary, atmospheric) frying is one of the oldest food preparation processes and is widely used in the food industry. Frying temperature can range from 130-190°C, but the most common temperatures are 170-190°C. The high temperature of the frying fat typically leads to the appreciated surface color and mechanical characteristics of fried foods; and besides that, heating the reducing sugar affects a complex group of reactions, called mailard reaction and producing acrylamide compound which is genotoxic.
Vacuum frying is a promising technology that could be an option for the production of novel snacks such as fruit and vegetable crisps that present the desired quality attributes and respond to new health trends. The operating pressure used is usually lower than 7 kPa which produces a good reduction in the boiling point of water and allows the frying temperature to be lower than 90oC. Some of the vacuum fried fruits and vegetables are apple, apricot, banana, jackfruit, green and gold kiwifruits, carrot, mushroom, potato, shallot, sweet potato and purple yam.
Types of frying
Fried products can be processed at atmospheric, high, and low (vacuum) pressures.
Deep fat frying: In atmospheric frying, the product is fried at temperatures between 165-190oC to produce a product with the right color, texture, and oil content. This technique is used to fry potato chips, French fries, tortilla/corn chips, doughnuts, tempura, and fish.
High-pressure frying: It is carried out in the pressure range of ~184 kPa. Mainly it is used to fry chicken in the bone. The high pressure allows for the product to have the correct crispness while keeping the meat moist and cooked. This is accomplished by keeping the moist inside at high saturation temperature (> 100oC), which allows for the water to remain inside the product for longer time, and completely cooking the product.
Low-pressure or vacuum frying: Vacuum frying (< 6.65 kPa) is a technique used to fry delicate products such as fruits and vegetables without destroy their structure. In this process, fruit and vegetables can be fried at temperature as low as 90oC and still have the same crispness and oil content of those produced at atmospheric pressure.
Vacuum frying Vs Deep frying
The main difference between vacuum frying and atmospheric frying is the lower boiling point of water at lower pressures that enables to fry at lower temperatures. For that reason, vacuum frying has many advantages over atmospheric frying in relation to product quality attributes.
The oil content of vacuum-fried samples were lower compared to atmospheric fried one. This difference was explained by the lower temperatures during vacuum frying due to the lower vapor pressure of water. This low temperature will reduce temperature-induced tissue matrix degradation that increases the oil absorption. Atmospheric fried samples had a larger portion of small pores and absorbed more oil by capillary suction compared to vacuum- fried samples.
Vacuum frying equipment design
A vacuum fryer consists of four components: (1) the frying vessel, (2) the vacuum pump, (3) the de-oiling mechanisms, and (4) the condenser.
The vessel consists of a heating element, a thermostat, a basket where the product is placed, and a lid where the vacuum gauge is located. The frying basket is raised and lowered into the heated oil by a lift rod. The lift rod is usually connected to a spinner motor that is used for centrifuging the product after frying to get rid of surface oil. The method of heating the oil in the frying chamber can be gas, steam or electricity.
The vacuum pump used to generate desired vacuum level and it can be a liquid ring vacuum pump or an oil-sealed vacuum pump.
De-oiling mechanism is a centrifuge, where a motor connected to the basket’s rod spins the product at a constant rpm (~750 rpm) for a period of time.
The condenser (either water or refrigeration is used) is a spiral-type heat exchanger, which condenses any vapor coming from the fryer before it reaches the pump. This is necessary to protect the pump from the water vapor, which would damage it mechanism and cause cavitation. There are three kinds of equipment in terms of size: laboratory, pilot and industrial and in terms of processing requirements it is categorized into batch and continuous processes.
Fig 1. Vacuum frying equipment
Vacuum frying process
The vacuum-frying process consists of several steps. These steps include fruit preparation, peeling and slicing, pre-treatment, vacuum-frying process, and removal of excess oil. Vacuum frying usually uses raw materials as fresh fruits and vegetables. However, fruit paste also can be used by preparing a dough made up with fruit pulp and starch or flour. Slicing of the fruit has a large influence on the final product characteristics. Fruit and vegetables could be sliced into thin pieces from 1.5-to 7.5-mm thickness that need a relative short frying time. Pre-treatments can be used to further improve quality attributes of the fried product, such as oil content, appearance, texture, taste, and retention of nutrients and phytochemicals.
The pre-treatments commonly used are blanching, pre-drying, impregnation, and freezing. Blanching is used to minimize enzymatic browning and also to pre-gelatinize starch. Pre-drying is used to reduce the initial water content before frying and thus reduce frying time. Osmotic dehydration is used to introduce salt or sugar to reduce initial water content. Application of anti-browning agents like tartaric acid, cysteine, and calcium chloride are used to prevent non-enzymatic browning in banana. Freezing can be used to create a porous and spongy matrix in vacuum-fried fruit. Dipping the fruits in a solution of hydrocolloids such as guar gum and xanthan gum, pectin, carboxymethyl cellulose (CMC), gum arabic, and sodium alginate is a common fruit pre-treatment before vacuum frying to improve product quality attributes
Fig 2. Vacuum frying process
After the pre-treatment, fruits are ready to be fried. In a small-scale fryer, the process will start by placing the fruits inside a basket and placed in the vacuum chamber after which the vacuum pump is started. After the oil has reached the desired temperature and the chamber has the desired pressure, the basket is submerged in the oil to start the frying process. At the end of the frying time, the basket is lifted from the oil and shaken or spun to drain the surface oil. Then the pressure is gradually increased, and the product is centrifuged to eliminate part of the surface oil.
Packaging and storage of vacuum fried chips
Vacuum fried products can be packaged in low density polyethylene, polyethylene terephlate or aluminium foil laminate bags with nitrogen flushing and would have an acceptable shelf life at ambient conditions. However, the two important qualities that dictate the shelf life of vacuum fried products were crispness and rancidity. Since aluminium foil laminate bags have lower oxygen and water vapor transmission rates, it is therefore highly recommended for use with nitrogen flushing in packaging of vacuum fried products.
Advantages of vacuum frying
- Vacuum fried products has low moisture content (<6%) and low water activity (aW<0.3)
- The absence of air during frying may inhibit oxidation including lipid oxidation, enzymatic browning; therefore, the color and nutrients of fried samples can be largely preserved.
- The products are crispy and retain original colour, taste and odour as of the natural foods.
- Reduced oil content in the fried product and it has less adverse effect on oil quality.
- Likewise vacuum frying is beneficial to some fruits or vegetables with intense odour such as durian.
- Reduced acrylamide formation in fried chips
Disadvantages of vacuum frying
In financial terms, the investment cost of vacuum frying process is much higher than that of deep frying. This is because the vacuum frying technique is basically designed for large-scale industry. There is a lack of vacuum fryer design for small scale production. But this technology offers significant benefits such as improved product safety and quality cooking oil and low oxidation in product due to low temperature processing