The invention relates to a method of processing a waste material originating from the mushroom culture, especially mushroom bases, into an industrially useful product, such as a feedstuff, by subsequent to removing the casing soil adhering to said bases, further treating them.

Such a process is known from US-A-4,085,233, according to which the mushroom stumps after being washed are broken down to make a puree, which then is centrifuged or filtered in order to remove dirt and impurities. The separated puree then is pasteurized and concentrated in order to obtain a liquid concentrate or it is dried in order to obtain a dry powder that then can be used as a feedstuff. This process requires a lot of energy for concentrating, especially in case the end product should be a dry powder. Besides this, the several compounds being desired to be present in the final feedstuff will be decomposed by the heat treatment. This known process now has been improved by carrying out a method to obtain useful materials from the mushroom stumps or mushroom bases by using a process that requires less energy and with which the compounds for the food can be better obtained from the starting material.

From EP-A-0,357,320 a process is known to obtain an edible material by heating or fermenting the medium that comprises lignin-rich grassy material after removing the specific odor of the digested medium. According to the present process one goes out from a different material, being mushroom stumps or bases from which the adhering soil is removed.

From the Japanese patent applications 2,167,033 and 63,301,754 processes are known to convert the medium from which the mushroom culture bed is formed, by fermentation into a feed for animals. Such culture beds mostly consists of a wood comprising material that is mixed with e.g. soyabean refuse. Also for this Japanse patent applications it is true that the starting material that is used for the fermentation is different from the starting material that is used according to the invention, because according to the present process, mushroom bases are used which remain in the bed after that the mushrooms have been harvested. The process according to the invention is characterized in that said mushroom bases are subjected to a fermentation process for which the

pH-value becomes lower than 5. Preferably the fermentation is carried out by the addition of an acid in order to reach a pH-value of 4-4.5.

In the mushroom production the sufficiently-grown products are harvested after a certain period of time by cutting the upper part (the mushrooms sold on the market) off the mushroom base, which remains behind in the seed bed. The aggregate base material remaining behind in the seed bed constitutes a rather voluminous waste material. At present this waste material is transported to the dumping area. The Dutch production of mushrooms amounts to about 150,000 tons; 15% of this production consists of the waste material containing the mushroom bases, which means an annual amount of waste material of over 20,000 tons. By the process according to the invention the amount of waste material is decreased and a usable end product is obtained. By the fermentation a preserved, hygienic product is obtained that no longer is perishable. A chemical, micro-biological and food-technical quality of the end product is such that this end product has a possitive economical value and can be used in particular as a feedstuff in the meat-pig sector.

The fermentation carried out according to the invention is stimulated by the addition of a biologically easily decomposible carbon source, for which preferably molasses can be used. From the molasses lactic acid can be released by the influence of lactic acid bacteria, so that the pH-value is decreased, which means that in practice for a favourable composition of the starting material, because of the presence in the starting material of molasses, it is possible that by the influence of lactic acid bacteria the pH-value can be obtained without the addition of an extra amount of acid. Besides this, it is possible that the already acidified or fermented suspension of mushroom bases will be added to the mushroom bases which still have to be fermented. Besides this it is prefered that an atmosphere of an inert gas is maintained in the fermentation vessel so that the fermentation takes place excluding oxygen. The invention will be explained in more detail in the following description, wherein reference is made to Figure 1, in which:

Figure 1 is a block diagram of the method of processing mushroom bases. At the mushroom farm the mushroom bases are collected in containers by the grower. Dependent on the quantities of mushrooms that

are produced and the collecting system that is used, plastic containers or containers made of galvanized steel and having a capacity of 200 - 1200 litres are used for this purpose. The collecting frequency depends on the climatic ci cumstances, which may have an effect on the deterioration of the quality of the mushroom bases during the storage period, which is usually at least one week. The filled containers are transported to the central processing plant.

In the central processing plant, illustrated in Figure 1, the mushroom bases are preferably processed without delay. The mushroom bases are supplied via a conveyor belt or a vibrating trough. In Figure 1 block 1 indicates the material supplied, viz. the uncleaned mushroom bases, in a quantity of 1000 kg. The starting material has a dry matter content of 13% and an ash content of 25%, calculated as a percentage of the dry matter content. Via a vibrating trough 2 this starting material is supplied to shaking screens 3 and 4, which are kept in action at a certain vibrating frequency. The mesh of the screen is such that the adhered casing soil is washed away by means of water, which is supplied by high-pressure sprayers mounted above the shaking screens 3 and 4; the mushroom bases, from which the casing soil has been removed, are carried along to a collecting vessel 5. From the source 6 tap water is supplied to the respective shaking screens 4 and 3, so that soiled water, which is collected at the location of the vessel 7, is carried along to the settling tank 8. Part of the partially purified water from the settling tank 8 may be resupplied to the shaking screen 3. From the settling tank 8 casing soil 9 is obtained, in a quantity of 80 kg with a dry matter content of 33.5% and an ash content, calculated as a percentage of the dry matter content, of 56%. Furthermore waste water is obtained from the settling tank 8, in a quantity which is dependent on the quantity of tap water supplied from the source 6, said quantity preferably being 5 - 6 liters. The at least partially cleaned mushroom bases are collected in the collecting vessel 5. The quantity of mushroom bases collected in the collecting vessel 5 amounts to 920 kg, said quantity still containing about 10 kg of casing soil. The dry matter content of the collected material is 11% and the ash content, calculated as a percentage of the dry matter content, is 16%. These cleaned mushroom bases are further processed, in accordance with the essence of the method according to the

invention, for which purpose the mushroom bases, cleaned as to 90% or more, are supplied to a cutting device 11, which may be a cutting-up pump, so that the transport and the cutting up can take place simultaneously. After cutting-up a suspension is obtained, to which an acid and/or molasses are added from the storage vessels 12 and 13 respectively, the suspension thus obtained and the additives are mixed by means of a static mixer 14 an d supplied to the fermentation vessels 15 and 16. In said fermentation vessels 15 and 16 the conversion of the material of the mushroom bases into the animal feedstuff takes place, which conversion or fermentation may take place in various manners and under various circumstances. Finally the end product is discharged by means of the tanker 17.

The fermentation which takes place in the fermentation vessels 15 and 16 results in the material being preserved, this may be achieved by adding an organic acid in order to reduce the pH to a value of 4 - 4.5. The organic acids to be used for this purpose include e.g. lactic acid, formic acid, acetic acid or propionic acid. The fermentation process may be stimulated by adding a biologically easily decomposed carbon source, such as molasses. The addition of molasses may also be omitted when sufficient lactic acid bacteria are present, which lactic acid bacteria bio-chemically convert the carbohydrates present in the mushroom bases into lactic acid. As a result of the addition of molasses the formation of lactic acid is considerably accelerated and the desired low pH-value of 4 - 4.5 is reached within a relatively short period of time. This reduction of the pH-value is necessary in order to prevent the growth of undesirable micro organisms such as pathogenic bacteria, yeasts and fungi and thus to enhance the keeping quality of the suspension. As a result of the reduction of the degree of acidity it also becomes possible to decrease the germ number of in particular Entero bacteriaceae. The additives for fermenting or preserving the suspension on the basis of mushroom bases may also be added in the fermentation vessel. In that case the additive may be molasses. This added molasses functions as a substrate for the coc- and rod-shaped lactic acid bacteria which are present by nature. The acidification may also be accelerated by adding lactic acid bacteria as a grafting substance. Preferably a quick reduction of the pH-value takes place in the beginning of the fermentation process,

so that the lactic acid bacteria can be active in a relatively favourable environment, a low pH-value of the mixture being more favourable for lactic acid bacteria than for other micro organisms. As a result of the low pH-value a preserved product is obtained within a time span of 24 - 48 hours.

Furthermore it is preferred to keep the vapour environment of the fermentation vessels 15 and 16 poor in oxygen, so that fungi and the like micro organisms, which may cause perishing, do not have a chance to develop. In order to accomplish that objective a C0 ₂ or N« atmosphere is preferably used, said inert gases being injected from the container 18.

The feedstuff to be discharged is subjected to a quality check, aimed at the pH-value and the micro-biological quality, whilst at the same time the dry matter content is determined.

The waste water that is removed from the settling tank 8, via the collecting vessel 10, may be discharged into the sewer system without any problem. It is preferred to recirculate the water from the settling tank 8 to the vibrating screen 3, although this water must be checked regularly and, if necessary, changed in order to prevent contamination of this water. According to the method of the invention a useful industrial product, in particular a feedstuff, is thus obtained from an, up to now, large quantity of waste material.