Hakan Romantschuk
Hakan Romantschuk, a chemical engineer, is Director of Planning of the Oy Tampella Ab engineering works in Tampere, Finland.
Spent liquor from the manufacture of sulfite pulp can now be turned into single-cell protein animal feed. The author, closely involved in development of the process in Finland, describes how it is done and what the advantages are.
A process has been developed and put into commercial operation in Finland for the manufacture of single-cell protein (SCP) animal feed using the spent liquor of sulfite pulp mills. Called the Pekilo process after the commercial name of the product, it is designed to operate at unit sizes smaller than SCP plants which use petroleum paraffins and methanol feed stocks. The Pekilo process also contributes significantly to eliminating problems of water pollution from sulfite spent liquors.
The first commercial Pekilo plant recently went on-stream at the United Paper Mills pulp mill at Jämsänkoski. It is hoped that this unit, which has an annual capacity of 10 000 metric tons, will produce SCP at significantly lower costs than petroleum-based plants of 100 000-ton annual capacity.
Although sulfite waste liquor is the first raw material to be used in the Pekilo process, the technology is not restricted to the sulfite pulp industry. Other types of waste carbohydrates, for instance soy whey, have been successfully tried on a laboratory scale, and plans call for continued efforts to broaden the applicability of the Pekilo process in the future. Hydrolysis of agricultural waste products is seen as a possible system for producing raw materials for carbohydrate fermentation in the Pekilo process.
Countries which import large quantities of proteins and are looking for ways to increase their domestic protein production may find the Pekilo process useful.
Pekilo SCP is derived mainly from wood sugars. Carbohydrate-laden pulp mill wastes are seldom present in quantities sufficient to sustain an annual production of SCP exceeding 20 000 metric tons. Because of the nature of the raw material it can be obtained at a low cost-or at no cost if the spent liquors are not being used for fuel. However, as the cost of a unit working at this scale cannot justify large investments, the technology must be relatively uncomplicated.
The development of the Pekilo process began with observations by Otto Gadd, of the Finnish Pulp and Paper Research Institute, that certain microfungi could be cultivated in submerged cultures of spent sulfite liquors. The classical method of producing fodder yeast from sulfite spent liquor is, of course, well known to pulp manufacturers, especially in Finland. The prospect of making major improvements in this process by employing a fungus seemed particularly promising, given the rising demand for animal feed and the increased acceptance of SCP as a substitute for more conventional feeds.
The search for the right microfungus started with the testing of more than 300 varieties. As the cost of research increased the investment base was broadened; eight major Finnish companies formed a group called SITU for the purpose of developing Pekilo on a fully commercial scale. Research continued at the Finnish Pulp and Paper Research Institute.
The microfungus finally selected was Paecilomyces variotii, which has the following favourable characteristics:
-It has a high content of crude protein, i.e., 55-60 percent, compared to the common microfungi level of 2540 percent.-It has a satisfactory rate of growth under process conditions.
-It is very easily separated from the liquor.
-There were no signs of toxic effects in test animals.
Following selection of the microfungus, the development of Pekilo went from laboratory to bench-scale fermentation. Through fermentation runs with 0.45- and 1-cubic-metre fermenters, it was established that continuous fermentation could be maintained over periods of several weeks without interference from foreign organisms. During this stage of Pekilo's development it was important to produce enough of it to begin feeding trials at the Finnish Agricultural Research Centre with various animals, mainly pigs, calves and chickens. They proved that Pekilo protein could be used as a partial substitute for soybean meal, fish meal or skim milk powder.
COMPOSITION OF PEKILO SCP PROTEIN
|
|
Percent of dry matter |
|
Protein |
55-60 |
|
Fat |
2-4 |
|
|
g/16 g N |
|
Amino acids |
|
|
Threonine |
4.6 |
|
Valine |
5.1 |
|
Cysteine |
1.1 |
|
Methionine |
1.5 |
|
Isoleucine |
4.3 |
|
Leucine |
6.9 |
|
Tyrosine |
3.4 |
|
Phenylalanine |
3.7 |
|
Lysine |
6.4 |
|
Tryptophan |
1.2 |
|
|
mg/kg dry matter |
|
Vitamins |
|
|
Thiamine |
6 |
|
Riboflavin |
66 |
|
Pyridoxine |
16 |
|
Niacin |
488 |
|
Pantothenic acid |
35 |
|
Biotin |
2 |
|
Folic acid |
12 |
The Pekilo process - from start to finish
At the same time a preliminary engineering study indicated that the Pekilo process could be commercially competitive and profitable. On the basis of this, one of the SITU group companies, United Paper Mills Ltd., decided to build a Pekilo plant with a rated capacity of 10 000 tons annually and to attach it to its Jämsänkoski pulp mill, which has an annual production capacity of 100 000 tons of calcium-based sulfite pulp. The spent liquors of this plant were usually evaporated and burned as fuel in a boiler.
Finally, in 1973, a licence agreement was signed between the SITU group and the Oy Tampella Ab engineering works in Tampere. Under this agreement, Tampella received exclusive worldwide sales rights to the Pekilo process and the right to grant sub-licenses. Tampella also took over responsibility for further engineering development work in order to make the process more widely applicable on commercial scales. Tampella is also the contractor for the Jämsänkoski plant.
The simplicity of the Pekilo process is striking. In SCP production a dilute solution of the carbohydrates constituting the raw material is normally given a preparatory treatment before fermentation. But if spent sulfite liquor is used as the carbohydrate source the only preparatory treatment necessary is contact with steam in a reaction tower in order to remove sulfur dioxide. After cooling, the liquor goes into a fermenting vessel, together with other nutrients essential for the support of the organism.
In the fermenting vessel, compressors are used for intensive mixing and good air contact. After four to five hours, approximately 15 kg of the microfungi are produced per cubic metre of liquor.
Following fermentation the protein-rich organism can be easily separated from the liquor with a drum filter. Efficient washing of the product at this point is possible using a very limited amount of water. In order to minimize the cost of drying, mechanical removal of water using a rotating disc press was introduced. This increases the dry solids in the product to 30 percent before it reaches the conveyor dryer.
It is important to ensure that the only organism being cultivated is Paecilomyces. This means that all the process streams introduced into the fermentation vessel - including the several thousand cubic metres/hour of air - must be free of other organisms capable of reproduction at a rate faster than Paecilomyces. The production unit is intended to operate continuously for 5 000 to 6 000 hours, and maintenance of aseptic operating conditions over such a long period has been a constant and major challenge to the engineers. The experience gained with the pilot unit, however, has been encouraging. It included continuous operation of a 15-cubic-metre fermenter for more than 3 000 hours without infection. During the pilot project several types of dryers, dewatering presses and other equipment were also tested and selected for use in the full-scale plant.
Pekilo protein from the pilot fermenter was also sent for animal feeding tests to the Institute of Veterinary Medicine in Norway, and to the National Institute for Research in Dairying in the United Kingdom.
Once the invariable difficulties of a new plant are overcome, and the Pekilo process becomes truly commercial, it is expected to have the following significant advantages over other carbohydrate fermentation processes:
- A 55-60 percent protein-rich product which also has a favourable amino acid composition.- The aseptic operation produces a final product which is microbiologically well defined.
- The level of impurities originating with the feed stock can be kept very low because of ease of washing in the filter.
