Infrared processing in the food industry

The basic concepts of infrared radiation are:

  • High heat transfer capacity;
  • Heat penetration directly into the product;
  • Fast regulation response;
  • Good possibilities for process control;
  • No heating of surrounding air.

These qualities indicate that infrared radiation should be an ideal source of energy for heating purposes (Skjoldebrand, 1986).

Distinguished from microwave heating, the penetration properties of infrared radiation are such that a suitable balance for surface and body heating can be reached, which is necessary for an optimal heating result. Some empirical work in this field can be found in the literature by Ginzburg for example (Ginzburg, 1969). The penetration properties are important for optimising the system. The penetration depth is defined as 37% of the unabsorbed radiation energy. For short waves, the penetration ability is ten times higher than for long waves. The direct penetration ability of infrared radiation makes it possible to increase the energy flux without burning the surface and thus reduces the necessary heating time that conventional heating methods require. This is especially true for thin products.

In a special study, a method was developed to determine optical properties of bread at different degrees of baking (Skjoldebrand et al, 1988). The results showed that the transmission by the crust was less than in the crumb. Even the thinnest dough sample did not transmit any radiation.

Reflection curves for crust and dough are very similar while reflection for the crumb is about 10-15% less. Table 20.1 shows calculated penetration depths for crust and crumb for radiators used in baking ovens. Measurements have been carried out for other foods and Table 20.2 shows some examples (Dagerskog and Osterstrom, 1979).

In infrared (IR) heating, heat is transferred by radiation, the wavelength of which is determined by the temperature of the body - the higher the temperature, the shorter the wavelength. Present interest in industrial heating applications centres on short wave IR (wavelengths around 1 mm) and intermediate IR (around

Table 20.1 The calculated penetration depths for crust and crumb for radiators used in baking ovens (Skjoldebrand et al, 1988)

Maximum Power level (%) energy wavelength

Spectral range (nm)

Penetration depth Crumb Crust

100

1300

800-1250

3.8

2.5

1250-2500

1.4

0.6

800-2500

1.9

1.2

75

1320

800-1250

3.8

2.5

1250-2500

1.4

0.6

800-2500

1.9

1.1

50

1410

800-1250

3.8

2.5

1250-2500

1.4

0.6

800-2500

1.8

1979)

Measured penetration depths for some foods (Dagerskog and Osterstrom,

Penetration depth

Product

Radiation source

Imax(mm)

Wavelength range (mm)

1< 1.25 1.25 <1< 1.51

1> 1.51

Potato

1.12

4.76 0.48

0.33

Potato

1.24

4.17 0.47

0.31

Pork

1.12

2.38 0.28

Bread

1.12

6.25 1.52

10 mm), since these wavelengths make it possible to start up and reach working temperatures in seconds, while also offering rapid transfer of high amounts of energy and excellent process control. In some food materials, short wave IR demonstrate penetration depths of up to 5 mm.

The most popular industrial applications (for non-food uses) are in the rapid drying of automobile paint and drying in the paper and pulp industry. For paper drying IR has superseded microwaves because it offers superior process control and economy. IR technology has long been underestimated in the food field, despite its great potential. Most applications of IR within the area of food came during the 1950s to 1970s from the USA, the USSR and the eastern European countries. During the 1970s and 1980s SIK did a lot of basic work applying this technique within the area of food. In later years work was carried out in Japan, Taiwan and other countries.

The main part of this work is still of an experimental nature. Applications are mainly in the following areas:

  • Drying vegetables and fish;
  • Drying pasta and rice;
  • Heating flour;
  • Frying meat;
  • Roasting cereals;
  • Roasting coffee;
  • Roasting cocoa;
  • Baking pizza, biscuits and bread.

The technique has also been used for thawing, surface pasteurisation of bread and surface pasteurisation of packaging materials.

The main commercial applications of IR heating are drying low moisture foods (for example breadcrumbs, cocoa, flours, grains, malt, pasta products and tea). The technique is often used at the start of the whole process to speed up the first increase in surface temperature. Such processes are frying, baking and drying. The effect of radiation intensity (0.125, 0.250, 0.375 and 0.500W/cm2) and slab thickness (2.5, 6.5 and 10.5 mm) on the moisture diffusion coefficient of potatoes during far IR drying have been investigated by Afzal and Abe in 1998 in Japan. They found that the diffusivity increased with increasing radiation intensity and with slab thickness. In contrast, activation energy for moisture desorption decreased with increasing slab thickness and resulted in higher drying rates for slabs of greater thicknesses. Some more specific examples will be described below.

20.2.1 Baking

When baking with infrared radiation it seems that short wave radiators should be used. The short wave infrared radiation may be combined with convection for drying the surface with good results.

Ginzburg divided the baking process using infrared radiation into three periods:

  1. The first phase is characterised by an increase in the surface temperature (1-2°C) to 100°C. Very little weight loss occurs during this period.
  2. The second period is characterised by the start of mass transfer. An evaporation zone forms, which moves towards the central parts. Energy is used to evaporate water and to heat the dough.
  3. In the third and final period the central parts have reached 90°C. The temperature increases by a further 8°C at the end of baking. The duration of this period amounts to about 25% of the total time of baking.

When comparing time-temperature relations between infrared radiation and conventional baking it is clear that IR radiation is more efficient both at the surface parts and the central sections. The following results were achieved using short wave infrared heating in the baking oven at SIK (Skjoldebrand et al, 1994):

• The baking time was 25-50% shorter compared to an ordinary baking oven.

The thickness of the product determined the time saving.

  • Energy consumption was comparable to ordinary baking.
  • Weight losses were 10-15% lower.
  • Quality was comparable.

These results show that infrared heating for bakery products is very promising compared to other heating techniques.

Further studies at SIK have shown that baking bread using the short wave infrared heating technique is a very interesting alternative to traditional baking (Skjoldebrand and Andersson, 1987). The baking time can be reduced by 25% and in some cases 50%, depending on the thickness of the product. This is due to penetration of the waves into the product. Depending on the radiators in the oven and their wavelength distribution, the penetration properties of the bread change during baking. At the start they are almost zero, with the crust having poorer penetration depth than the porous water-rich crumb. The baking time reduction is also due to the more effective heat transfer to the surface than occurs in convection or conduction heating. Using short wave infrared radiation may reduce weight losses. In some of the experiments it was found that the water content in the centre had increased during baking, causing a better and longer storage.

20.2.2 Frying meat

Several studies on frying meat by infrared radiation have been carried out by researchers in the former Soviet Union. There have also been studies carried out in Sweden. These studies have shown that maximum transmission falls in the region of the electromagnetic spectrum of 1.2 mm. For wavelengths over 2.5 mm the transmission capacity was negligible (Bolshakov et al, 1976). Consequently it was necessary to use sources with maximum radiation falling in the region of maximum transmission to achieve deep heating of pork. For heat treatment of the product surface, radiators in the region of maximum transmission and reflectance (l > 2.3 mm) had to be used. It was recommended to design a two-stage frying process. In the first stage surface heat treatment was achieved by radiant flux with l at maximum 1.04 mm providing deep heat to the product. The studies showed that the final moisture content and sensory quality of the product were higher when heated by the two-stage process than by conventional methods.

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