Technical profile: continuous process to cook and stabilize cereals

by Teresa Acklin
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   Contributed by suppliers, technical profiles feature new technology, products, specific applications or proprietary concepts. This material was prepared by Favini Impianti S.r.l., Bergamo, Italy.

   An innovative process to cook and stabilize cereals and legumes using a high pressure steam heat treatment has been developed by Favini Impianti. The innovation includes a new continuous retort, designed, patented and manufactured by the company. The process assures a high degree of product uniformity and simplifies management through automation of control operations that up to now were carried out in batch.

   Cooking is the key operation in the preparation of several semifinished and finished products, such as toasted soy, cereals flakes, beans, chickpeas, stabilized legumes, pre-cooked wheat and couscous.

   During heat treatment, a product's chemical and physical properties are altered. Starch structure is modified, making subsequent cooking easier and faster; aromatic characteristics are elevated; color changes; and elimination of microbes assures long lasting product preservation. Each product and each kind of utilization by the customer requires different treatment conditions.

   The difficulties of this process arise from the need to maintain a high degree of uniformity during treatment in order to obtain repeatable and constant color, taste, texture and performance during subsequent cooking. Most important is avoiding overtreatment that would result in unacceptable burning or blackening of the product.

   The heat exchange kinetic is obviously affected by the product's relative moisture, which is not merely a parameter of the raw material to be processed. In fact, relative moisture changes constantly during cooking because of steam condensation on the surface of product particles. Furthermore, it is necessary to eliminate the excessive condensate, which is the main cause of lumps and overcooking (starch chain denaturation).

   Some moisture gradients (condensate percolation) will always exist in traditional batch treatment plants where a thick product layer must be conditioned. These gradients are the main cause of the above-mentioned burning and, generally, of a lack of product uniformity within a batch.

   Because of the difficulty of reproducing process parameters in a sequence of several batches, producers face problems when a customer requires a standard product that consistently must comply with a specific quality level.

The Plant.

   The process plant that allows continuous and easily controlled heat treatment consists of:

   • a retort in which the process parameters and conditions can be easily modified and then maintained at constant levels (see Table 1 on page 46);

   • one load and one unload valve, which are able to feed the retort continuously without modifying pressure and temperature conditions inside the unit;

   • all ancillary equipment contained inside the retort to eliminate condensation, including equipment for feeding and distributing steam, water and/or additives and for product conveying and distribution.

   The retort, completely produced in stainless steel, is made up of a tubular shell with two hinged covers, making them accessible. Everything is perfectly insulated.

   Inside this shell, the woven wire belt conveys the product from the load to the unload valve. The belt is supported by a tubular frame sliding on a track, making belt extraction easy for maintenance and cleaning.

   A set of spreading blades assures the mixing of the product and maintains the prefixed layer. The belt speed determines the retention time of the product inside the retort.

   A manifold spreads the steam at the pre-set temperature. Because a higher degree of moisture can be necessary to accelerate treatment for some products, nozzles on the conveying net have been designed for the distribution of water or solutions with additives.

   Each part of the unit is easily accessible and constructed according to the highest quality standards. The ability to fully extract the sliding frame that supports the belt allows thorough cleaning of both the shell and the belt itself.

   The plant complies with safety rules under the ISPELS patent and meets all requirements according to the European Machinery Directive (Directive 89/392/EEC Amended).

   For the operator accustomed to tackling the complexity of a batch process, it is not difficult to see the great advantage deriving from a continuous process.

   The cereals or legumes are conveyed from the storage silo through a proportioning valve and enter the retort, undergoing the required heat treatment; the product then is immediately ready for the next operation.

   Inside the retort, the product, stirred by the conveyor belt, comes in contact with the steam at the required pressure. The excess condensate drops to the bottom of the unit, from which it is removed to avoid overtreatment and formation of lumps.

   Management of the process is completely automatic. A pressure regulator maintains fixed pressure and temperature conditions. An auxiliary temperature indicator allows further monitoring. A variable-speed drive that can be controlled from the outside enables belt speed setting, and product layer thickness is controlled by the exterior inlet valve.

   As shown in the technical details in Table 2, system dimensions are moderate, requiring only enough space clear of fixed machinery to extract the belt supporting carriage.

   Obviously, production capacity is influenced by many factors. Table 3 outlines actual results obtained using the continuous process.

   The Favini process plant offers a series of unquestionable advantages:

   • continuous process;

   • elimination of heating and cooling transients;

   • reduction in processing time, resulting in energy savings;

   • even and repeatable batches with high treatment quality;

   • elimination of contact between condensate and product;

   • uniform stabilizing/sterilizing treatment of product (see Table 3).

   The experience of Favini Impianti in plant production and process management is considerable. Two plants have been operating at full capacity for more than a year.

Table 1 — Process parameters

Treatment temperature100°C to 147°C
Working pressure1 to 3.5 bars
Retention time2 to 50 minutes
Product layer thickness5 to 15 centimeters
Water and/or additives flow10 to 100 liters/hour

Table 2 — Technical specifications

Length 8 meters (plus 8 m belt extraction space)
Width 2 m
Height1.5 m
Proof pressure5 bars
Installed power1.1 to 2.2 kW
Production capacity100 - 2,500 kg/h
Product moisture, in11% to 22%
Product moisture, out15% to 30%

Table 3 — Product results, dehusked oat

Total microbial aerobical charge 320,000/g 30/g
Mildews60/g less than 10/g
Yeasts120/gless than 10/g
It is possible to eliminate salmonella and escherichia coli in the final product. Similar results have been achieved with other cereals. The process also can be used for spice sterilization.