Food and beverage manufacturers are presented with several challenges. Organisations strive to minimise production and supply-chain costs, realise a perfect delivery efficiency and meet a multitude of regulatory needs merely to stay reasonably competitive. Processing efficiency in both waste and energy management is a key component in reducing production costs and creating more value from ingredients. However, increasing efficiency is almost never a single straightforward solution, but a delicate trade-off between cost, energy and product quality. As such, to make the most appropriate decisions, a deeper understanding is needed of the effects of processing parameters on both product quality and capacity.
The processing temperature is one of the most important factors in multiple processing steps. Higher processing temperatures will result in larger heat flows, thus increasing capacity. Additionally, in pasteurisation processes, higher temperatures will more effectively destroy harmful microorganisms, ensuring product safety. On the other hand, lower processing temperatures generally decrease energy consumption and decrease the heat load on the product. The latter is especially important in protein-rich formulations, since proteins can denature at temperatures as low as 40°C. Not only will proteins lose their functionality when denatured, potentially negating their use in specific products, they also tend to aggregate with each other and equipment surfaces. The consequent fouling has a large effect on runtime and the amount of cleaning required. It is therefore important to take all these consequences into account when determining appropriate processing temperatures.
Another important factor in processing efficiency is the water content of the product, . Removing water from product streams is one of the main sources of energy consumption in the food processing industry. Some production processes revolve around the addition of water followed by removal of that same water (e.g. starch production). Less water usage in processes is therefore a direct improvement in energy efficiency. On the other hand there is a difference in energy usage for different water removal units:
- Membrane concentration generally requires 20-40 MJ/tonns of water removed
- Falling film evaporators, depending on number of stages and steam supply, require around 55-500 MJ /tonnes of water removed
- Spray drying requires 3500-4500 MJ/tonnes of water removed.
It is clear from these numbers that if water removal can be moved from more energy intensive drying processes to less intensive drying, up to 99% energy savings can be achieved. Naturally there are limits to the amount of dry matter in a processing stream that certain equipment can handle, but even minimal changes will have a significant effect with the large throughputs generally used in food processing plants.
A final factor that stronlgy affects processing efficiency is the humidity or moisture content of the process air used for drying. The maximum moisture content of air at atmospheric pressure differs with temperature, from close to zero moisture per amount of dry air at temperatures below 0 °C up to an infinite amount at 100 °C (pure steam). As such, the capacity of spray dryers is highly dependent on the outlet temperature of the air. However, this outlet temperature is usually bound by the moisture content of the outgoing powder. Therefore, the only ways to improve capacity is to build a bigger spray dryer with more air or to decrease the humidity of the ingoing air. Naturally, the latter is the only option for optimization of an existing spray dryer. Dehumidification treatments with silica or zeolites are options for decreasing and/or standardizing the humidity of the drying air. Another option, which requires much less investment, is to match the dryer capacity to the outside air humidity, i.e. increase ingoing product flow when the outside humidity becomes lower. This can have a large impact on capacity as the daily variation in humidity can be quite significant.
The effect of product-process interactions on efficiency
Besides processing conditions, product-process interactions can also determine the process efficiency, as already mentioned in the form of protein denaturation. Another important interaction is the relation between product powder stickiness and drying air humidity in a spray dryer. The equilibrium between the moisture content of dry matter and air is described by a sorption isotherm. This captures the behaviour of moisture becoming harder to evaporate at higher dry matter content due to binding and adsorption energies. The stickiness of powders depends on moisture content and temperature, becoming more sticky at higher moisture contents and at higher temperatures. Since spray dryers approach ideal mixing, the outgoing powder is in near-equilibrium with the outgoing process air. Therefore, the stickiness of the powder depends on outgoing air humidity and temperature. If the powder becomes too sticky during spray drying, fouling will occur, leading to shorter runtimes and longer cleaning times. By modelling the drying process, the sorption isotherm and the dependence of powder stickiness on moisture and temperature, the optimal conditions for the spray dryer can be set to meet the requirements for non-sticky conditions, optimal capacity and final product moisture content.
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