The salads, how to ensure freshness and appearance while respecting the environment?

Foto 1a
Pictures of fresh-cut salads as they are available at the supermarket. Product with large residual free water.

Drying machines for fresh-cut salads have been increasing their production capacity. The challenge for the manufacturers is an ergonomic and compact design to make the maintenance and plant sanitation easy. But the real competition lies in automation and energy consumption in order to reduce processing costs and maintain product margins during the economic crisis.

by Alfonso William Mauro

Treatments applied to fresh-cut vegetables are critical for maintaining the organoleptic and sensorial qualities which are characteristic of the fresh products. Fresh vegetables like salads must undergo a series of physical and chemical processes before sale and consumption. These processes might all be designed so as to not compromise the natural fresh quality and genuineness of the vegetable products. The combined application of these treatments is aimed at blocking the proliferation of harmful microorganisms and slowing down browning and discoloration. In order to do this, vegetable processing plants have being asked to integrate quality controls during processing, handling and sanitizing in order reach the overall goal of food safety. The processing involves several steps that are performed by a chain of machines that sort, cut, wash, dry, remove foreign bodies, chill and package the product. The factors that have an impact on packaged products, making them compliant with regulatory requirements, are related to the growth of microorganisms and deterioration of the product. In addition to the chemical and physical washing, other aspects can have significant impact on the shelf-life. Among these ones there are the damage to the surface of the leafy vegetables during processing, the level of residual free water in the packaging and the temperature of the product during the transport. Damaged parts of the product speed up the growth of microbial agents and the residual free water in the packaging provides fertile terrain for their proliferation, along with the exponential effect that temperature has on the growth. The cold chain from the processing to the final customer is critical and the level of residual free water in the packaging must be at the appropriately low levels. Drying techniques depend upon the variety of salads. For example, a certain level of residual water might be left in packaged rocket and cut salads so that the product does not wilt inside it. Other large and tender leaf varieties (spinach for example) require higher drying levels to be able to be automatically packaged with gravity filling machines. The level of residual free water should be set for each type of product and, above all, should be controlled automatically by the drying machine. In the picture two samples representing different drying levels for packaged salads are reported, as they were found at supermarket.

Foto 1b
Pictures of fresh-cut salads as they are available at the supermarket. Product with low residual free water.

To run the delicate production step of the drying, there are three different technologies: centrifuge systems, de-watering systems and air drying tunnels. Energy costs related to high production capacity and level of automation in these machines weigh heavily on the operating costs. These aspects have a large impact on the profitability of the final product which is highly subject to price pressure. In this paper, the three drying methods mentioned above are explored. In particular, the operating factors to be controlled and their influence on the productivity and the energy consumption are described, in order to give elements of consciousness to choose the most appropriate technology on the market depending on the effective needs.

Details of centrifuge basket.

Centrifuge systems
Centrifuge systems can be manually or automatically loaded and are used to remove the free water that is on the surface of the product after washing. There are several types of centrifuges on the market depending on the type of product to be processed. The operating principles are basically the same for each of these types. The centrifuge operates mechanically, with the product placed in a stainless steel basket which spins at a progressively higher speed, pushing the product out towards the walls and forcing the water to slide off it and then is collected in a chamber between the basket and the outer structure of the machine. After the drying cycle, the basket spins more slowly and the product is unloaded onto a discharge belt and moved towards the next processing step.

Foto 2b
Fresh-cut salads production line with a centrifuge.

The hourly productivity of a centrifuge depends on the size of the basket as compared to residual damage to the product as illustrated in Figure 1.

Figura 1
Figure 1. Maximum hourly productivity of a vertical centrifuge as a function of the basket diameter (data based on rocket, first cut, with a residual free water of 22% after washing and of 8% after centrifuge. Test data supplied by F.lli Naddeo srl).

Energy consumption of centrifuge systems is not closely related to hourly productivity, as can be seen in the example reported in Figure 1. To dry rocket at an hourly production rate of 300 kg/hour starting with an initial residual free water level of 22% (exiting from a washing pool for fresh-cut salads) to a final level of 8%, 4 kW are required, that is equivalent to 0.013 kWh/kg. Some machines are currently available with systems that recover energy during the braking with energy savings of around 25%. In addition, systems with a nearly horizontal basket have limited the effect of the gravity over the vertical wall that limits the surface adhesion, which allows to use longer basket walls with a corresponding increase in hourly production and a continuous processing.

De-watering systems
De-watering systems are characterized by continuous drying on a variable speed belt along which a set of shaker trays with perforations, air intakes and blowers allow the product on the belt to lose its water by gravitational force or air jet.

Foto 3
Details of a dewatering system.

The shakers and perforations remove the largest drops of water. The air blowing system and perforations also serve to rotate the product on both sides. The air intake completes the removal of remaining water. The drying occurs with the air blown near the belt, physically removing the drops of water on each leaf (not the liquid film). Drying times depend on the speed of the air and residual free water as reported in Figure 2.

Figura 2
Figure 2. Drying times as a function of inlet air velocity for fixed target of residual free water on the leaves.

Energy consumption for a determined level of drying depends on different factors. Typically a de-watering process for 500 kg/h of product with a residual free water level of 13%-9% requires a power of 14-18 kW, corresponding to 0.028-0.032 kWh/kg of product.

Air tunnel drying system with integrated cooling. View of the outlet of the system during the drying of 800 kg/h of spinach with a residual free water of the final product equal to 2.5% and a product temperature of 7°C ( Courtesy of f.lli Naddeo srl).
Air tunnel drying system with integrated cooling. View of the outlet of the system during the drying of 800 kg/h of spinach with a residual free water of the final product equal to 2.5% and a product temperature of 7°C (Courtesy of f.lli Naddeo srl).

Air tunnel systems
Today the most advanced drying system is the air tunnel. The air tunnel system works continuously, provides for drying with warm and dry air. It is frequently integrated with a cooling system. A thermodynamic process takes place inside the system, usually in a closed circuit, like that shown in Figure 3. Warm dry air at thermodynamic state 1 is blown across the product one or more times and this partially dries the product. The air progressively becomes more humid during the process, similarly to an adiabatic saturation process. After that, the air is regenerated through the cooling process with mechanical de-humidification from point 2 to point 3 and then heated up again to point 1. Energy consumption depends primarily on the number of passages of air over the product, the speed of the air blown on the product, and the system for the energy conversion. Systems with highest performance available on the market today have energy specifications equivalent to 0.058 kWh/kg for a production rate of 800 kg/hours of spinach with a residual free water of 12% at the inlet and of 2.5% at the outlet.




Figura 3
Figure 3. Thermodynamic process in a closed circuit system for drying fresh cut salads. 1-2 process line; 2-3 de-humidification; 3-1 heating.

Comparison of drying technologies: advantages and disadvantages

Centrifuge systems
The centrifuge system is compact, requires few energy, around 0. 013 kWh/kg for the present technology (almost half the consumption of a de-watering system and one quarter of a tunnel). Baby leaf products and high volume products like leaf lettuce, chard or spinach, show visible physical damages when processed in a centrifuge and, at the same time, a large residual free water that greatly limits shelf-life of the packaged product and also the appeal to the final consumer. Generally, this technology cannot achieve residual free water levels lower than 5% to 8% of the weight of the product, depending on the product and the season. Vertical centrifuges are not able to be used for continuous processes and they require loading systems that take up 6-7 meters of linear space on the processing line. Furthermore, increasing the diameter of the centrifuge, it does not have an effect on the productivity. Systems with almost horizontal baskets can offer quasi-continuous processing and higher levels of productivity. Not all parts of the centrifuge system are easy to be inspected for sanitation.

Dewatering systems
They are continuous systems and are compacter than centrifuge ones (including the related accessories). All parts are generally visible and therefore are simple to wash. Energy consumption is greater than that one of a centrifuge but lower than a drying tunnel. These systems are usually open, so drops of nebulized water are dispersed into the ambient with the subsequent increase in the latent heat load on the cooling system of the plant (especially in the case where the de-watering system includes a system of warm air injection). De-watering systems have the great advantage of not causing physical damage to the product even at high processing rates. The level of residual drying of the product depends on the design of the system but it can also be costly to achieve residual free water as low as 12% – 9%. For these reasons, this type of system can be used for baby leaf products and for continuous processing, while the centrifuge cannot, but it cannot reach the desired rates of drying on a stand-alone basis.

Air tunnel systems
The greatest advantage of tunnel systems is that they are able to achieve residual free water levels of less than 4%, and they can theoretically reach the total removal of the residual water on the product, which is not possible with the other two systems. Furthermore, they do not damage the product. The addition of these factors increases the quality of the packaged product and considerably extends the shelf-life. On the other hand, the tunnel system has a higher energy consumption when compared to other drying systems. Tunnels are relatively compact with respect to the productivity that they can realize, but they should work with a constant flow of product with a maximum inlet residual of free water equal to 12%. For this reason, they might be installed in series with a centrifuge or, better yet, a de-watering process, in order to avoid physical damage of the products and to work continuously. In addition, they require a cooling system downstream to bring the product to the temperature level required for refrigerated storage or direct transport after packaging. The tunnel system also offers the possibility to dry very fragile and delicate products and attains a better quality of the product. Maintenance and cleaning operations are quick and easy, and reduce personnel costs. Also this system can be easily integrated into facilities with cooling systems. It is important, however, to note that since the tunnel involves a continuous process, it might be designed appropriately. Factors that should be considered are the relationship of the surface to the volume of the processed product (related to the fluid-dynamic concept chosen for this process), energy recovery and the performance coefficient of the energy conversion system. Also, it is important to adequately monitor the process. Unlike centrifuge and de-watering processes, it is not possible to stop and re-start the machine for a short period. Therefore during the change of the product over the line (washing operations), it is vital that the system automatically limits its performance, using an inverter on the process fans and heat pump compressor, in order to save energy. Today the most advanced systems are able to adapt their operation to real processing needs. In other words, they monitor the process adapting the capacity of the energy conversion system to the real operation. The same happens for the fresh products. When they come from the field, their residual water depends on the atmospheric conditions and there is the risk to over-dry the product and, in addition, to reduce the weight. In such cases the energy consumption is needlessly higher. To avoid this situation is required a partial load regulations for the heat pump in relation to the monitoring of the process controlling the fans speed in accordance with the real-time needs of the process. For direct expansion heat pump systems, partial load conditions is allowed by inverters which regulate the fans’ and compressors’ speeds, reducing the capacity up to 50% of the nominal value. In the case of wide variability of the load, it is necessary to use indirect expansion systems with thermal storage that allow the heat pump to turn itself on and off, if needed. An example of an indirect expansion system using continuous monitoring systems and algorithms to control the real operating conditions is introduced below. An indirect expansion drying tunnel for fresh product was built by the F.lli Naddeo Company and its partners based on a system able to process under partial load conditions.

Figura 4
Figure 4. Drying of rocket. Current frequency of the fans for an indirect expansion tunnel drying system (data from F.lli Naddeo srl compaby).

The control system allows each module fan to speed according to the intensity of the process. Also, each fan turns off if the process during the previous module as an intensity below a threshold value (case related to the fact that the product has been dried). Figure 4 reports an example of the fan current frequency versus time according to the described algorithm. This innovative control system has allowed the efficient adaptation of the energy consumption of the machine to the real process needs and has resulted in a significant reduction of the average energy consumption.