Developments In Post-Baking Drying
Radio frequency (RF) heating is commonly used with conventional baking ovens to remove the final moisture of partially dried bakery products, such as breakfast cereals, crackers, biscuits and sponge cake, as well as to increase the production capacity of existing baking ovens.
By Francien Machielse
The competitive forces in the marketplace for biscuits, crackers and breakfast cereals are greater today than ever before. Radio frequency post baking is a well known method that allows bakers to better control the final moisture content of their products, to reduce checking problems, increase productivity and improve product quality. More importantly, new studies have proven that besides the known benefits, radio frequency is a very efficient method to drastically reduce acrylamide formation in baked products.
In fact, conventional ovens do not operate effectively when a product’s moisture content decreases, because of the low rates of heat transfer caused by the low thermal conductivity of dry foods. Moreover, increasing the time required to dry the centre of the product can cause excessive surface browning thereby changing the organoleptic properties of the product. Baked products with excess moisture in the centre can be transferred to RF heaters to efficiently remove the moisture without changing the organoleptic properties of the product.
Post-baking Processes
Additionally, by exiting the oven with a higher moisture content, producers can significantly increase their production on the same line as the last portion of the traditional baking oven has the least effective heat transfer properties. In particular, RF equipment can be fitted after the baking oven to reduce uniformity and stabilise the residual moisture content of the products coming out; for this reason it is commercially known as “post-baking” drying equipment.
Established in 1978, Italy-based Stalam is a major player in the development, design and manufacture of equipment where dielectric electromagnetic fields are used for a variety of heating and drying processes on raw material, intermediate and finished industrial products. James Farley, sales manager of the food division at Stalam, explains: “A Stalam RF dryer consists of two distinct main parts: the generator and the applicator. The applicator, which is inside the drying tunnel, is a system of two opposing electrodes (plates or metallic grid, horizontally placed), which represents an electrical capacitor.
The RF generator creates an electro-magnetic field between the upper and lower part of the capacitor, alternating at a frequency of about 27 MHz. This means that the polarity of the electrical charge of each plate is continuously changing (from positive to negative and vice versa) about 27 million times per second. It is well known that water molecules are made of two distinct electrical polarities; positively charged ‘hydrogen’ and negatively charged ‘oxygen’.
Therefore, when they are placed between the alternating charge of a capacitor system, the water molecules will ‘re-orient’ to face the opposite charge, 27 million times per second. This rapid oscillating movement causes the water molecules to heat up due to the friction created. This heat causes in turn the increase of the temperature and the evaporation of the water. This is what happens when a product, still humid at the outlet of a baking oven, enters an RF tunnel. The biscuit, being of different molecular structure, is not affected by the RF field and does not heat up, except by conduction from heated water present within it (this effect will not cause any over-baking or colouring of the product).” Stalam offers a complete range of generators from 10 to 250 kW rated RF output power to meet all customers’ production needs.
Equipments
In post-baking drying there are several issues that separates Stalam machines from the standard RF equipment, according to Farley: “Our patented double polarity system and the way we control the power applied to the product are quite revolutionary for the industry. The double polarity system consists of each upper and lower electrodes being formed with a series of rods at opposite polarity, thus delivering independently a horizontal electrical field above and below the product being dried.
Because of this method of RF field shaping between the upper and lower electrode, power is efficiently coupled in the product without the need for excessively high electrode voltages, and results in a better power distribution within the biscuit. And you can avoid electrical discharge ‘arcing’ from the electrode causing damage to the product or the conveyor belt and a longer working life of triode and generator components due to the absence of ‘arcing’ stresses caused by higher voltages.”
Unlike standard RF dryers, where the up and down movement of the upper electrodes control the power, the Stalam power control circuit is located in the generator compartment, and regulates output power independently of the electrode position. Because it is not necessary to continuously move the oven electrode up and down for power regulation, the risk of the upper electrodes crashing through limit switches due to a drive system malfunction is completely removed. Also, due to the absence of moving parts in the fixed electrode system, minimum oven maintenance is required. Oven cleaning is a simple operation carried out with the electrodes in place.
Increasing production and lowering moisture are often the main reasons for installing an RF machine, but product quality is an added benefit. The formation of significant levels of the probable carcinogen acrylamide (AA) in starch-rich foods or cereal-based products, subjected to intense heating, has been widely reported. Maillard-type reactions, in the presence of asparagine, have been shown as major reaction pathways of AA formation. As a consequence, AA formation is closely related to the development of the desired sensory properties (colour, flavour, texture) of heated foods.
At present, the most reliable approaches to reduce AA levels in bakery products involve the use of sodium or potassium carbonate, instead of ammonium carbonate, as a raising agent, because of its ability to reduce both the pH of the dough and the formation of reactive compounds; reduce baking temperatures and the reduction of acids or protein-based ingredients to food formulations. However, many of these strategies, especially those based on product acidification or low-intensity heating processes, often lead to the production of foods with poor sensory properties. Recently, the University of Udine in Italy studied the influence of RF heating on AA formation in bakery products. Its findings suggest a further technological strategy for minimising AA formation.
Researcher Monica Anese of the University of Udine explained: “To this purpose, leavened cakes and short dough biscuits were baked to a final moisture of 3.5 per cent and 3.0 per cent respectively, by means of conventional convection heating or different combinations of conventional and RF heating. Results showed that, with respect to the baking process, which was carried out in an air-circulating oven, the application of RF heating in the last stages of the process resulted in a promising strategy to lower the AA levels in the bakery products. The best results were obtained for products that were moved from hot-air baking to RF heating when their moisture was still fairly high, around 10 per cent. Results also suggested that, when very low values of moisture were required, this technological intervention was especially suitable to thin bakery products such as biscuits, as opposed to thick products, such as leavened cakes, because of excessive browning in the internal portion as a consequence of the RF heating.”
James Farley comments on the study: “By combining an RF dryer with conventional heating and exiting the baking oven when the product still had an excess of moisture, the efficacy of the RF-assisted baking process as a mitigation strategy was confirmed by the results relevant to short dough biscuits. The biscuits subjected to the RF baking process had a 35 per cent lower AA content than those obtained using conventional heating only. In another example, a leading Italian manufacturer of grissini sticks – F.Lli Milan – showed that by exiting the conventional oven at 4.5 per cent moisture and removing the residual water to 2.5 per cent, the amount of acrylamide in the breadsticks was 10 to 20 times lower than the values obtained from the conventional oven alone. In addition to the significantly lower AA values, the manufacturer was able to increase production on the same line due to exiting the oven at a higher moisture content, and control the coloring of the grissini resulting in a superior product.”
In an industry where the processes are traditional, equipment companies are continuously coming up with new ways to innovate, resolve customer difficulties and promote efficiency. As product quality and health benefits prove to be most important to consumers, where post baking drying is concerned even scientists come to the companies aid to get it just right.