20 Infrared processing C Skjöldebrand, ABB and Lund University, Sweden 20.1 Introduction: the principles of infrared heating Sir William Herschel discovered infrared – or heat radiation – in the 1800s when he was attempting to determine the part of the visible spectrum with the minimum associated heat in connection with the astronomical observations he was making In 1847 AHL Fizeau and JBL Foucault showed that infrared radiation has the same properties as visible light It was being reflected, refracted and was capable of forming an interference pattern (Encyclopedia Internet, 2000) There are many applications of infrared radiation A number of these are analogous to the use of visible light Thus, the spectrum of a substance in the infrared range can be used in chemical analysis much as the visible spectrum is used Radiation at discrete wavelengths in the infrared range is characteristic of many molecules The temperature of a distant object can also be determined by analysis of the infrared radiation from the object Medical uses of infrared radiation range from the simple heat lamp to the technique of thermal imaging, or thermographs It has also been used for drying dye and lacquer for cars, glue for wallpaper, paper in paper machines, and dye to plastic details, as well as shrinkage of plastics and activation of glue in the plastic industry, etc The electromagnetic spectra within infrared wavelengths can be divided into parts; long waves (4 mm to mm), medium waves (2–4 mm) and short waves (0.7–2 mm) The short waves appear when temperatures are above 1000°C, the long waves appear below 400°C and the medium waves appear between these temperatures The electromagnetic spectrum is shown in Fig 20.1 (Anon, The Infrared Handbook) For food the technique has been used in many applications, as the long waves are one of the main heat transfer mechanisms in ordinary ovens or other heating equipment Short waves are new for the food industry 424 The nutrition handbook for food processors 0.38 0.76 Short Visible wave light IR Gamma Radiation X-rays designations rays Wavelength Ultra violet rays mm Medium wave IR mm Long wave IR Infrared rays Radio waves nm mm mm m km 10–9 10–6 10–3 10–0 103m Fig 20.1 The electromagnetic spectrum (Anon, The Infrared Handbook) In the USSR in the 1950s AV Lykow and others reported the results of their theoretical and experimental studies of infrared drying (Ginzburg, 1969) In the 1960s W Jubitz carried out substantial work on infrared heating in East Germany and in France M Dáribéré and J Leconte did some work on different applications of infrared irradiation in various industries During this time IS Pavlov in the Soviet Union carried out a lot of work on infrared heating and food Long wave radiation was already used in the United States during the 1950s in many industrial food processes During the early 1970s there were many discussions concerning finding new methods for industrial frying/cooking meat products (Skjöldebrand, 1986) Deep fat frying, the process most often used in industrial frying, was criticised because of the fat and flavour exchange and surface appearance Also, environmental and nutritional aspects had to be considered The consumer also wanted products more like the ones cooked at home One of the new techniques discussed was near infrared heating (NIR) or short wave infrared heating This technique is used in the car industry for drying coatings, as well as the paper and textile industries Thus, like many other processes in the food industry, infrared heating was transferred from other industries Therefore, why has short wave infrared radiation not been used before? The answer is that there was a lack of knowledge about many of the factors concerning this process The radiators, the reflectors and the different systems for cassettes were developed during the 1960s but there was not very much knowledge about the optical properties of the foodstuffs and how these develop during processing The problems then were also braking the radiators and cleaning the equipment During the 1970s and 1980s most of the research work on food was carried Infrared processing 425 out in Sweden at the Swedish Institute for Food and Biotechnology (SIK) (Dagerskog and Österström, 1979; Skjöldebrand, 1986; Skjöldebrand and Andersson, 1989) In this chapter the application of infrared processing in the food industry will cover the following areas: • • • • Examples of applications in the food industry; The infrared process and its impact on quality; The infrared process and its impact on nutrition; Future outlook 20.2 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 (Skjöldebrand, 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 (Skjöldebrand 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 Österström, 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 mm) and intermediate IR (around 426 The nutrition handbook for food processors Table 20.1 The calculated penetration depths for crust and crumb for radiators used in baking ovens (Skjöldebrand et al, 1988) Power level (%) Maximum energy wavelength 100 1300 75 1320 50 1410 Table 20.2 1979) Spectral range (nm) 800–1250 1250–2500 800–2500 800–1250 1250–2500 800–2500 800–1250 1250–2500 800–2500 Penetration depth Crumb Crust 3.8 1.4 1.9 3.8 1.4 1.9 3.8 1.4 1.8 2.5 0.6 1.2 2.5 0.6 1.1 2.5 0.6 1.1 Measured penetration depths for some foods (Dagerskog and Österström, Penetration depth Product Potato Potato Pork Bread Radiation source lmax(mm) 1.12 1.24 1.12 1.12 Wavelength range (mm) l < 1.25 1.25 < l < 1.51 l > 1.51 4.76 4.17 2.38 6.25 0.48 0.47 0.28 1.52 0.33 0.31 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 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; Infrared processing • • • • • • 427 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 0.500 W/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: 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 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 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 (Skjöldebrand 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 428 The nutrition handbook for food processors • 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 (Skjöldebrand 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 20.3 Infrared processing and food quality The improvements in bread quality from using the baking oven at SIK mentioned earlier were achieved using short wave infrared heating (Skjöldebrand et al, 1994) Suggestions have been made that radiant heating elements should be operated at temperatures between 1200 and 1800°C as only wavelengths longer than mm are effective in developing colour in particular Successful results have also been reported for several frying applications (Dagerskog, 1978) Four different frying methods were compared in work carried out by researchers at Lund University and at SIK The product studied was a meat patty Infrared processing 429 (Dagerskog and Sörenfors, 1979) They studied convection, deep fat frying, contact and long wave IR, and found that from pure heat transfer considerations the four techniques gave almost equivalent results if appropriate frying conditions were used The surface crust of the meat was, however, rather different for the four methods, influencing the sensorial experience For IR and convection frying the crust was similar with burned areas and a skin-like surface, but the periphery of the meat patty was browned first during convection heating in contrast to IR heating, where the surface area was browned first For flavour, no difference among the methods were found, except for deep fat frying which gave a significantly stronger ‘off-flavour’, probably due to the absorbed fat The juiciness of the IR fried meat patties was rated by the panel because of the exceptionally long frying times needed, which in turn was due to the recipe The results also showed that it was very important to have the recipe tailor-made for the technique used In this study it was also reported that, as expected, the surface crust became darker as intensity levels grew higher However, the total impression scores indicate an optimum at intermediate levels The optimum levels often coincided for both texture, flavour and brightness When parboiling at a similar degree of heat treatment as compared with conventional technology, the infrared treatment required a shorter time (83%) with lower weight losses (50%) The flavour, colour and texture of the infrared braised meat were claimed to be far superior (Asselberg et al, 1960) Researchers testing a baking oven in a Swedish bakery during the 1980s found that the colour for ordinary wheat bread was very good and acceptable but the evenness could be better This was due to the fact that radiation cannot be distributed evenly to all bread sides in an optimal way The volume and the porosity were very good and comparable to ordinary baked bread (Skjöldebrand et al, 1988) The products tested were sweet rolls, buns, baguettes, white bread loaves and rye bread loaves It is, however, difficult to use steam in a baking oven when using NIR as the water molecules absorb the waves Weight losses for some of the baked products are about the same when baking in an NIR oven as in a conventional oven However, the baking times are reduced by 25–30% when using short wave infrared radiation as a heating technique The reduction depends on geometry and thickness of the product In general it was found that volume increased in bread baked in an infrared oven compared to bread baked in a conventional oven Colour is comparable if flat bread is baked It was found that as energy levels reach 100% white spots occur on the bread surface, due to ungelatinised starch Also, big pores or even holes in the crumb can occur when too high energy levels are used in a baking oven based on infrared heating However, as there is a fast response when changing energy levels, the heat transfer may be controlled to get an optimised colour on the surface An industrial process for pre-cooking of bacon in a continuous infrared oven at Swift & Company has been investigated by Hlavacek (1968) Electric resistance heaters below the seamless stainless steel belt supplemented the 288 kW of infrared radiant heating from overhead quartz lamps The frying time was 2–3 430 The nutrition handbook for food processors minutes and pre-cooked bacon was found to taste as good or better than freshly fried bacon The results showed that the final moisture content and sensory quality of the product heated by the two-stage process were higher than those heated by conventional methods In Taiwan IR has also been used for dehydration of fish Over 90% of the far IR dried products were of a higher quality than currently marked sun dried products (Wei-Renn-Lein and Wen-Rong-Fu, 1997) 20.4 Infrared processing and nutritional quality There are very few studies in the literature on the infrared process and its impact on food nutrients However, some of it is reported here Comparisons between conventional heating techniques and infrared heating give some hints as to their effect on the nutritional value of heated products Recent studies of intense IR radiation treatment on nutritional value and antinutritional factors of cereals (corn, rice, brown sorghum) and common beans show that digestibility and energy values were not altered significantly but protein quality decreased (Keya-EL and Sherman U, 1997) No anti-nutritional factors were found in rice Tannin in sorghum was denatured extensively by IR treatment Small amounts of aflatoxins in corn and sorghum trypsin inhibitors in common beans were destroyed Mackerel were dried in IR radiation at 180°C for 40 minutes and the results revealed better nutritional values than after conventional treatment (Shyue-BinHo, En-Chie-Lin, Fu-Jin-Wang and Sheu-Der-Wu, 1996) The Maillard reaction in the crust of bread or meat can be better controlled when using NIR heating than using grilling or frying or in an ordinary baking oven This is due to the fast response when changing the energy levels of radiators However, too high energy transfer at the start of the process may give white spots of ungelatinised starch As the IR heating technique in most applications shows shorter drying, frying or heating times for food nutrition, spoilage is less than that of most conventional techniques The spoilage can also be controlled in a better way Knowing the kinetics of the chemical reaction of different nutritional components and using mathematical models based on knowledge about the IR heating temperature and mass transfer is one way to optimise the nutritional value of the ready-made product (Skjöldebrand, 2000) 20.5 Future trends The infrared technique of heating foods shows a lot of advantages compared to conventional techniques These cover both process aspects as well as nutritional and quality aspects However, the technique is not widely used in the industry • More knowledge about the interaction between processes and products needs to be gained The relationship between raw material properties and how these are affected by the process to obtain the desired properties in the end product Infrared processing • • • • • • 431 should be studied These are necessary for the success of using new techniques like NIR or short wave infrared heating equipment IR heating should be particularly useful for continuous baking, drying and grilling as well as for surface pasteurisation All different heating techniques have their own limitations concerning application areas and possibilities Good background knowledge may combine the techniques in the most optimal way More knowledge is important for the success of the heating technique The use of IR technology in the food industry is quite limited today, and the available equipment is not optimised for the various heating operations along the processing lines for baking, drying, etc Its application is certain to grow as food equipment manufacturers begin to realise its full potential Along with the development of process control and information technology the IR technique will show its full potential with fast regulation of radiators and rapid heat transfer IR heating will certainly fulfil its role in the requirements of flexible production units With the development of new products the heating technique will be important and used New flavour can be created via both the recipe and the heating technique The combination of different heating techniques is probably a meaningful road to examine for heating of food in the most optimal way (Wählby, 2002) There are already several types of combination ovens available on the market; essentially these types of ovens are based on microwave technology with conventional technology added From a food quality point of view this is an interesting field – how to optimise combinations of heating technologies The IR technology should probably be used at the start of the heating procedure or a stepwise combination of different power levels should be designed In drying the NIR technology should be used and with a stepwise change from high to low power levels The behaviour of food has been explored quite a bit, but the introduction of new sensing techniques (e.g gas sensing, IR, radio and microwave) shows that there are several areas of research to be continued By using the new sensing technologies, it may be possible to let food control the process on-line To be able to this, a good relationship between these sensor outputs and food quality is necessary When this is known the quality and the nutritional value of food can be controlled better and the correct heating technique can be selected 20.6 References afzal t m and abe t (1998), ‘Diffusion in potatoes during far infrared radiation drying’, Journal of Food Engineering 37(4), 353–65 anon, The Infrared Handbook, Philips asselberg e a, mohr w p and kemp j g (1960), Food Technology 14, 449 432 The nutrition handbook for food processors bolshakov a s, bouskov v g, kasulin g n, rogov f a, skryabin u p and zhukov n n (1976), Effects of Infrared radiation rates and conditions of preliminary processing of quality index on baked products, 22nd European meeting of meat research workers, Malmö, Sweden dagerskog m (1978), Stekning av livsmedel pHD thesis, Gothenburg (In Swedish) dagerskog m and österström l (1979), Infrared radiation for food processing A study of the fundamental properties of infrared radiation Lebensmittel Wissenschaft u Technologie 12, 237–42 fellows p (1988), Food Processing Technology Principles and Practice Ellis Horwood, Chichester, England and VCH, Weinheim, Germany ginzburg a s (1969), Application of Infrared Radiation in Food Processing Leonard Hill, London hallström b, trägårdh c and skjöldebrand c (1988), Heat transfer and food products Elsevier Science, London hlavacek r g (1968), Food Proc 28, 51 keya-el and sherman u (1997), ‘Effects of a brief, intense infrared radiation treatment on the nutritional quality of maize, rice, sorghum and beans’, Food and Nutrition Bulletin 18(4), 382–7 shyue-bin-ho, en-chie-lin, fu-jin-wang and sheu-der-wu (1996), ‘The study on processing smoked mackerel slices by far infrared heating’ Food Science – Taiwan (In Chinese) 23(6), 801–8 skjöldebrand c (2000), ‘Infrared heating’ in Thermal technologies in food processing Chapter (Ed Philip Richardsson) p 208–28 Woodhead Publishing Ltd, Cambridge, England skjöldebrand c (1986), Cooking by Infrared Radiation In Proceedings from Progress in Food Preparation processes, p 157–73 skjöldebrand c and andersson c g (1989), ‘A comparison of infrared bread baking and conventional baking’ Journal of Microwave Power and Electromagnetic energy 24, 91–101 skjöldebrand c and andersson c g (1987), Baking using short wave infrared radiation Proceedings from Cereals in a European Context, First European Conference on Food Science and Technology Chapter 7.6 (Ed ID Morton) Ellis Horwood, Chichester, UK, p 364–76 skjöldebrand c and falk c (1990), Development of a simulation programme for IR heating Engineering and Food Vol Physical properties and process control (Eds Spiess WEL and Schubert H) Proceedings of the Fifth International Congress on Engineering and Food 28 May–3 June 1989, Cologne Federal Republic of Germany London p 869–86 skjöldebrand c and schmidt m (1985), Kortare bakningstider med ny teknik Bröd konditorn 11.22 (In Swedish) skjöldebrand c, ellbjär c, andersson c g and eriksson t (1988), ‘Optical properties of Bread in the Near Infrared Range’ Journal of Food Engineering 8, 129–39 skjöldebrand c, van den hark s, janestad h and andersson c g (1994), Radiative and convective heat transfer when baking dough products, Proceedings from the Fourth Bath Food Engineering Conference, University of Bath, UK 19–21 September wei-renn-lein and wen-rong-fu (1997), ‘Small fish dehydration by far infrared heating’ Food Science Taiwan 24(3), 348–56 (In Chinese) wählby u (2002), Heat and Mass Transfer in Impingement Cooking – Aspects of Food Quality PH D thesis, Gothenburg, Sweden