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 (Skjoldebrand 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 2 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

(Dagerskog and Sorenfors, 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 (Skjoldebrand 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

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).

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