Process for the Production of Bulgur

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Bulgur Plant in the United Arab Emirates

Bulgur, a popular and one of the oldest foodstuffs made of wheat, even came before bread. It was prepared by ancient Babylonians, Hittites and Hebrew populations over 4,000 years ago. For this reason there are many different ways of spelling, such as bulgur (Germany/Europe), bulgar / burgal / burghoul (Turkey), burghol / borgol (Arabia), Pourgouri (Greece). In the Bible bulgur was called arisah.

It is particularly strongly represented in the Middle East, the Balkan States and the Asian area. Due to its durability, its good storage and flow properties as well as its nutritive value and easy preparation, bulgur met with great interest in many parts of the world. In comparison with other wheat products bulgur belongs to the nutritionally most valuable dishes. Different bulgur types and fractions sizes are used depending on the way of preparation or purpose of use.

With the plant installed by Amandus Kahl / Schule cooking times of 3 to 7 minutes are reached depending on the product size.

Process for the production of bulgur

The wheat types used are very important for the production of bulgur. The protein, ash, vitamin and mineral contents must be determined first before deciding on the corresponding process. The nutrient losses must be kept as low as possible. The plant for the production of bulgur can be divided in the following groups: Cleaning, soaking, cooking, drying, cooling, tempering, whitening, groating, classifying, bagging (Fig. 1).

Fig. 1: Plant groups

In order to determine soaking time, soaking temperature, starch modification degree, thiamine losses, swelling factor, water absorption, cooking time, cooking temperature, and drying parameters of the corresponding wheat type, on the basis of which the plant was designed, tests were carried out at the pilot plant (Fig. 2) in Reinbek.

Fig. 2: Pilot plant

With the pilot plant of Amandus Kahl / Schule Canadian durum wheat (type CWRS) with a bulk density of 0.82 kg/l was processed. The soaking time was about 40 to 60 minutes and the soaking temperature was below the gelatinization temperature of wheat starch. The cooking time was between 15 and 20 minutes depending on the product. The cooking temperature could be adjusted from 110 to 130 °C. The moisture content after soaking is 38 to 45 % H20 depending on the product type. The starch modification degree is between 40 and 50 % depending on the product type, measured according to the AMG method. It is important not to add too much water during the soaking process so that the caryopsis will not swell too much and damage the grain. Otherwise substances contained in the grain will escape (vitamin losses).

As the soaking water will be reused by the industrial plant, it was already tested in the pilot plant for e.g. suspended matters, odour, COD value, BOD 5-value, conductivity and pH-value, nitrogen, ammonia, nitrite, and nitrate, and the corresponding wastewater treatment plant has been designed accordingly.

Practical implementation of the test results

After evaluation of all parameters tested in the pilot plant, the industrial plant could be designed which was to be installed in an existing eight-storey building (Fig. 3).

Fig. 3: Building of the bulgur plant

A plant for an input capacity of 2.5 t/h was projected on the assumption that cleaned raw material without shrivelled grain, chalky grain, foreign seeds and impurities would be available, i.e. a homogeneous input product. During planning and realisation of the plant it had to be considered that the already existing maize mill would continue production during assembly of the bulgur plant. This was a great challenge for the local personnel, since the maize mill is in the same building.

The plant works automatically and is monitored and controlled by the operators in the control room (section see Fig. 4).

Fig. 4: Section of the control room

Corresponding units for monitoring of the plant were installed. The cooker with a diameter of 2.5m and a length of approximately 10m had to be elevated by 30m and transported into the building. The product-contacting parts both in the wet and in the dry area are made of stainless steel. The exhaust air is discharged via filtering systems. In order not to fall below the dewpoint in the drying plant, the exhaust air of the fluidized bed drier and the belt drier was mixed.

Soaking section

The number of the soaking tanks is determined by the input capacity and the total cycle time of the product to be soaked. The cycle includes feeding of the tank with product, filling of the tank with water, soaking, discharge of the water, discharge of the product as well as purging of the tank (Fig. 5, exemplary scheme).

Fig. 5: Exemplary scheme of the soaking section

The design of the tank must ensure uniform distribution of the soaking water, in order to obtain a homogeneously soaked product. The water is circulated during the soaking phase and the temperature is checked and kept constant (Fig. 6). In order to use as little soaking water as possible, the swelling factor must be taken into account when designing the tanks. The soaking water is cleaned in the water treatment plant for reuse. Only as much fresh water is added to the soaking water as is absorbed by the product.

Fig. 6: Soaking section

To have always the same product quantity in the tank, the corresponding product quantity is added pneumatically via a weighing system. The pneumatic system ensures gentle transport of the product, so that neither broken grains nor many fines are produced which might clog the screens in the soaking tank.

Cooking section

One of the most important parts of the bulgur plant is the cooking unit. The continuous cooker, also called autoclave, of 2.5 m in diameter and 10 m in length is equipped with several belt sections (Fig. 7 + 8), where the product is cooked uniformly as it is turned several times. Unlike the batch system, the continuous cooker does not produce lumps. The cooker is provided with corresponding control devices for observing the product in the cooker. This allows a fast reaction by modifying the cooking time and the cooking temperature.

Fig. 7: Cooker during assembly

Fig. 8: Cooker shortly before commissioning

The layer level in the cooker can be adjusted any time by means of a layer level weir. The complete interior of the cooker is equipped with an automatic purging system so that the formation of moulds and deposits is avoided during shutdown of the plant. The steam is distributed uniformly in the complete interior by means of corresponding equipment installed in the autoclave. Due to the special design of the belt the steam can reach the product uniformly on all sides. The product is fed and discharged via special locks.

Drying/Cooling section

After cooking, the product is conveyed via a steam discharge belt to the fluidized bed drier, where it is pre-dried to a moisture content of approximately 30%. The fluidized bed drier (Fig. 9) consists of several drying zones, in which different drying temperatures can be adjusted. The chosen temperatures should not be too high in order to avoid cornifications/crustifications at the product surface.

Fig. 9: Fluidized bed drier

In the downstream belt drier (Fig. 10) the product is dried to a residual water content of approximately 13 %. Before the product can be conveyed into the tempering silos, it is cooled to about 5°C above ambient temperature in a belt cooler.

Fig. 10: Belt drier

Tempering

Tempering is defined as stress, moisture and temperature equalization in a substance without outer influences, only via the factor time.

The longer the product dwells, the harder and glassier it becomes and the more suitable it is for the treatment in the following shelling / whitening machine and groat cutters. The residence time in the tempering silo should be at least 1 to 2 days.

Whitening / Shelling

Shelling/whitening of the bulgur wheat is necessary for removing the outer coat of the grain kernel which is rich in crude fibres and for obtaining a lighter colour. As a result the energy content is increased. The "shelling depth" must not be exaggerated, however, since the valuable minerals and vitamins in the outer zone of the grain kernel are to be preserved.

After a corresponding tempering time the product is shelled in the whitening machine VPC (Fig. 11).

Fig. 11: Whitening machine VPC

The vertical shelling/whitening machine works with conical whitening disks and integrated air openings in-between. The smaller diameter is at the top side, the larger diameter at the bottom. The product is fed vertically from above by means of an integrated draw-in screw in the shelling chamber. The outlet is controlled by a motor-driven counterpressure disk which also determines the filling degree of the machine. This guarantees uniform shelling. During the whitening/shelling process the product is cooled by means of air. An aspiration system serves for discharge of the bran.

The whitening/shelling degree is between 10 and 20% depending on customer requirement. For reaching the best possible whitening effect, product moisture, product hardness and circumferential speed play a decisive role.

Groating

The consumer does not want whole grain kernels, which still have to be crushed, but a groated product without meal which is immediately available for food preparation. Besides, the groat is needed in different granular sizes depending on the dishes to be prepared. Contrary to corrugated rolls, a groat cutter serves for the production of uniform products of the same granular size with a very low meal content.

After the whitening/shelling process the product is cut by several groat cutters. The machines are equipped with perforated drums and pinwheels. The lower half of the rotating drums is surrounded by a knife cage. The grain kernels falling through the drum holes in their longitudinal axis are cross-cut by the knives. The pinwheels prevent the holes from clogging. The spacing of the knives and the cutting angle can be modified and determine the granular size of the final product. The cutting effect is also determined by the moisture content and the hardness of the kernels as well as the throughput.

Classifying

By means of a plansifter following the groating process the groated product is classified in three fractions. Depending on the type of preparation, it is distinguished between fine, medium and coarse. Prior separation of the very low meal content has already taken place.

Bagging

After classification, the bulgur is ready for storage and can be bagged or packed, as requested by the customer.

Final product

With a final moisture content of less than 10% bulgur has excellent storage properties and remains nonperishable for a very long time. The good storability is caused mainly by the fact that the microflora of the kernel is inactivated to a large extent during the treatment in the autoclave. The storability is also favoured by the low moisture content. The storability is about 9 months at a temperature of 20°C and about 4 months at 30°C.

Processing of the wheat results in a starch modification of the final product between 50 and 52 %, measured according to the enzymatic AMG method.

The hydrothermal treatment of the wheat reduces the cooking time of the finished products (fine, medium, coarse) to 3 to 7 minutes.

The loss of crude proteins of the tempered product is between 3 and 5 % in comparison with the raw material.

As in every cooking process, the available vitamins are reduced by autoclavation due to the thermal effect and post-treatment.

The vitamin B1 / B2 value is 40 to 50 % and 40 to 45 % depending on the cooking time and whitening degree.

Summary

The plant installed by the company Amandus Kahl / Schule can be operated round-the-clock for the production of bulgur.

Furthermore, the plant can be used for the reduction of anti-nutritive substances in oilseeds and legumes, for the modification of starch, the increase of the by-pass protein (UDP), the reduction of germs in spices/grain and for the production of a coffee which is gentle on the stomach (acid reduction).

With the addition of some components the plant can also be used for the production of instant rice and parboiled rice.

Product tests can be carried out at the pilot plant.