Food Biochemistry and Food Processing (2 edition)

BLBS102-c39 BLBS102-Simpson March 21, 2012 14:20 Trim: 276mm X 219mm Printer Name: Yet to Come

39 Minimally Processed Foods 749

there is dipole rotation and ionic polarization resulting in volu-

metric heat generation in the food (Metaxas and Meredith 1993).

The extent of energy absorption by the food sample depends on

its dielectric loss factor. The higher the loss factor, the more

energy can be absorbed in the material. Chemical composition,

including water and salt, and process temperature are among

the factors that determine the materials loss factor. Although RF

and MW heating are forms of dielectric heating, they are distinct

from technological point of view. Both systems are operated at

specific bands, namely frequencies between 0.003 and 300 MHz

for RF and between 300 and 300,000 MHz for microwaves (Ra-

maswamy and Tung 2008). RF systems have longer wavelengths

compared to MW systems. Thus, the penetration depth is typi-

cally smaller for MW heating. In general, the efficiency of power

utilization is far lower in a RF generator than a microwave unit,

although the initial capital cost per KW of power output may

be higher. Selection of RF or MW heating usually depends on

product physical properties and required process conditions for a

particular application. For large products where greater penetra-

tion depth is required and control of uniformity of heating is not

a major issue, RF offers a good solution. However, microwave

systems are preferred where uniformity of drying and moisture

control is essential.

The earlier applications of RF heating in fruit and vegetable

processing was carried out for vegetable dehydration and quality

assessment of juice from orange, peach, and quince (Kinn 1947,

Moyer and Stotz 1947, Demeczky 1974). Nelson and Trabesli

(2009) explained how dielectric properties of fruits affect their

heating characteristics. RF-based treatments have been widely

used in postharvest operations (Ikediala et al. 2000, Wang et al.

2006, 2009a). Wang et al. (2006) proposed the use of radio fre-

quencies at various levels to control insect infestation without

compromising quality aspects. Sosa-Morales et al. (2009) noted

that RF and MW treatments can be highly efficient in reducing

the heating time required for pest control, as compared to con-

ventional thermal treatments. Water-assisted RF heating treat-

ment was used to control the infestation of apples by codling

moth, and it was reported to be an efficient technique, which

also sustained the fruit’s quality (Wang et al. 2005, Wang et al.

2006). Further, RF electric field has been used in deactivating

E. colibacteria in apple and orange juice (Geveke et al. 2007).

Despite the clear advantages of RF heating, the problem of un-

even heating imposes a big hurdle in its wide acceptance. There

is ongoing effort to mitigate some of these problems using com-

puter modeling (Birla et al. 2008). New and advanced RF sys-

tems are being developed to improve heating applications.

MW-based processing systems are more widely used for ei-

ther domestic or commercial food preparations. There has been

considerable advance in the design, power, and cost features of

these systems since their introduction for small scale heating

applications. The reliability of MW-assisted treatment of fruits

and vegetables has been rated high over the conventional heating

methods (Vadivambal and Jayas 2007). MW-assisted drying has

proved to be an excellent technique for processing the raw fruits

and vegetables. The technique was used to study drying behavior

of various fruits like apples, mangoes, and carrots (Wang and Xi

2005, Orsat et al. 2006, Zhang et al. 2006, Vadivambal and Jayas

2007). Various fruits and vegetables have been reviewed to as-

sess the effect of varying microwave conditions on banana slice,

cranberries, and carrots (Drouzas and Schubert 1996, Yong-

sawatddigul and Gunasekaran 1996a, 1996b, Tein et al. 1998).

Also, there has been use of vacuum for assisting microwave dry-

ing to remove the moisture. Chou and Chua (2001) reported that

it was necessary to combine MW drying with vacuum in order to

improve drying performance. Many other authors (Table 39.1)

have combined MW with other heating techniques in order to

reduce process energy and produce high-quality products.

There are some indication of nonthermal and enhanced

thermal effects of microwave processing (Ramaswamy et al.

2002, Datta and Anantheswaran 2004). However, it is generally

Table 39.1.The Use of Microwave Drying on Different Fruits and Vegetables

Fruit or Vegetable Type of Treatment References

Carrots MW & vacuum Regier 2005, Wang and Xi 2005, Orsat et al. 2007

Potato MW & convective, MW & vacuum Bondrauk et al. 2007, Reyes et al. 2007, Markowski et al. 2009,

Song et al. 2009

Strawberry MW & vacuum, MW & convective Bohm et al. 2006, Charangue et al. 2008, Contreras et al. 2008,

Wojdylo et al. 2009

Mushroom MW & vacuum, MW & convective Orsat et al. 2007, Yang et al. 2008, Askari et al. 2009, Giri and

Prasad 2009

Spinach MW, MW & convective Ozkan et al. 2007, Dadali et al. 2008, Karaaslan et al. 2008,

Dadali and Ozbek 2009

Peach MW Wang and Sheng 2006

Orange MW, MW & vacuum, MW & convection Ruiz et al. 2003, De Pilli et al. 2008, Guo et al. 2009

Apple MW, MW & vacuum, MW & convection Han et al. 2009, Huang et al. 2009, Tarko et al. 2009, Witrowa

and Rzaca 2009, Li et al. 2010

Pumpkin MW, MW & vacuum, MW & convection Alibas 2007, Wang et al. 2007a, Nawirska et al. 2009

Banana MW, MW & vacuum, MW & convection Mousa and Farid 2002, Kar et al. 2003, Pereira et al. 2007

Note:The objectives of researches shown may differ.