Man has been consuming mushrooms as food, medicine and even as intoxicant, since time immemorial by collecting them from the wild.
Mushroom-hunting is still a game or a hobby with many. The appearance of mushrooms in the wild is, however, uncertain and seasonal depending upon many factors like habitat and climate etc. The need and greed to ensure the regular availability of mushrooms impelled the mankind to domesticate some of the most delicious mushrooms. It started with the domestication of paddy straw mushroom (Volvariella volvaceae) and black ear mushroom (Auricularia polytrica) in China, but undoubtedly it was the introduction and cultivation of the common white button mushroom (Agaricus bisporus) in the limestone caves in France in late eighteenth century which can be truly termed as biggest milestone in the history of mushroom production.
The production and consumption of mushrooms is increasing very fast throughout the world, mainly due to greater and greater awareness of their
nutritive and medicinal attributes, besides, of course, unique flavour and texture; consumption of such fancied items is also a natural corollary to the general economic development of a country and, needless to say, the world
economies are booming. World production of mushrooms is estimated about 12 million tonnes and the annual growth rate is still above 8 %.
Currently, about twenty species of mushrooms are being commercially cultivated world over, but significant production is of the button mushroom (Agaricus bisporus), Shiitake (Lentinula edodes), Oyster mushroom (Pleurotus spp.), Black ear mushroom (Auricularia polytricha) and paddy straw mushroom (Volvariella volvacea).
. Most damaging post harvest changes in mushrooms vary with species—it is blackening in the button mushroom, cap-opening in the paddy straw mushroom and mucilage in the oyster mushroom, which affect their marketability significantly. Weight loss is very serious problem in all the mushrooms as these contain very high moisture (85-90 %) and are not protected by the conventional cuticle.
Due to very high moisture and rich nutritive value, microbial spoilage in mushrooms is also a problem. In case of the button mushroom all the four most deleterious changes namely, browning, veil-opening, weight loss and microbial spoilage ask for the utmost post-harvest care. Needless to say that these changes are also accompanied by changes in the nutritional and medicinal attributes of these mushrooms.
Utmost post harvest care of mushrooms is needed not only for the fresh market but also for the processing, as most of these changes are irreversible.
Gluts and distress sales are not uncommon in mushroom marketing specially during the peak months when seasonal produce hits the market in a big way. Withholding of the fresh mushrooms at any point of the chain— grower, wholesaler or the retailer—, is neither feasible nor advisable as it may result in further deterioration in quality leading to the total loss. Information about proper post harvest care and processing of such a perishable commodity is therefore of vital importance to keep the wheels of this industry moving at the right speed; with the adoption of proper packaging, storage and processing technologies, problems in marketing, like seasonal gluts and distress sales, can also be ameliorated.
A sizeable production of the button mushroom and almost entire production of other mushrooms still comes from the small seasonal growers although many commercial units produce button mushroom throughout the year under the controlled conditions.
The problem of gluts exacerbates during the so-called peak season (December-February). Serbia and Brazil are predominantly markets for the fresh mushrooms, with very little quantity sold as canned mainly for the institutional consumers. Almost entire export is in the canned form accompanied with mushroom pickles as a bye-product of the canneries. But future is going to witness greater contribution from the processing sector, both as stored and really processed mushrooms. Improved postharvest practices for the storage and processing of mushrooms including value-addition, readymade or ready-to make products will not only be demanded but will add to the returns to the growers as well as processors.
Two most common post harvest practices and aspects of mushrooms are: proper packaging and storage for the fresh mushrooms; and processing for long-term storage as well as value addition. Market for the fresh commodities is likely to continue; reverse trend has already started in the countries where processed products were being consumed.
Therefore most important of all, it is the proper packaging and storage of the fresh mushrooms which should receive the attention of all the players in the field—researchers, growers and traders. Besides canning, drying, steeping and pickling currently resorted to for the long-term storage and trade, it is the production and consumption of the readymade or ready-to-make value-added mushroom products which have, of late, been receiving the attention of the mushroom research and industry.
Mushroom-based soup powder, noodles and biscuits are already on the shelves. Technologies for ready-to make mushroom pizza, mushroom curry in retortable pouches, nuggets, ketchup, preserve in sugar syrup (murabba) have been developed. Adoption of proper post harvest practices of the storage and processing may partially ameliorate the problems of marketing of mushrooms during the peak periods.
STORAGE OF FRESH MUSHROOMS
Obviously, fresh mushrooms need to be properly stored to retard post harvest deterioration till these are consumed. Needless to reiterate that the refrigeration or cold-storage is the most essential part of the post harvest care of all the horticultural commodities including mushrooms.
Pre-treatments, if any, packing and pre-cooling precede the refrigerated storage in most cases.
Cooling and refrigeration
Cold-preservation of mushrooms is the most important aspect of the storage and can be classified in two categories: refrigeration and freezing.
Household and commercial refrigerators usually run at 4–70C. Cold or chill storage may use a slightly lower temperature (–1 to –40C), depending upon the freshness of the mushrooms to be refrigerated.
Freezing is done at a temperature of below -180C. Chill storage will preserve perishables for days or weeks and frozen storage (deep freezing) will preserve for months or even years. Refrigeration has certain advantages over freezing as it takes less energy to cool mushrooms to just above its freezing point than to freeze it. The
temperature of the button mushroom after picking, which varies between 15 and 180C, rises steadily during the storage due to respiration and atmospheric temperature and the heat causes deterioration in quality; in addition, the respiratory rate increases with the increase in the storage temperature leading to a vicious cycle.
It has been estimated that mushrooms at 100C have 3.5 times higher respiratory activity than those at 00C, which necessitates immediate shifting of mushrooms to the refrigerated zone. Hence the heat should be removed immediately after the harvest and the temperature of mushrooms should be brought down to 4-50C as quickly as possible.
Low temperature retards the growth of microorganisms, reduces the rate of postharvest metabolic activities of the mushroom tissues and minimizes the moisture loss. The choice of the cooling system depends upon the quantity to be handled; it may be a refrigerator for a small grower or consumer a cold room with all the facilities for a commercial grower. Forced-chilled air, ice-bank or vacuum cooling systems are the other systems in vogue at commercial level.
The size and shape of the packs play important role in the selection of the cooling room system and design. Packs with more than 10 kg mushrooms or with 15 cm thick layers of mushroom cause problems. Vertical flow of air is more suitable for cooling.
The mushrooms should not be stored in the same cooler along with fruits as the gases produced by fruits cause discolouration of mushrooms. As the simple forced-chill air-cooling system is time consuming, the vacuum cooling is becoming popular. To ensure high quality mushrooms in the market place with enhanced shelf-life, these must be cooled as quickly as possible after picking and kept cool throughout the cold chain.
Storage under low temperature is an excellent method for restricting deterioration of harvested mushrooms for a limited period of time. The maturation and textural changes in button mushrooms were slowed down at 00C ensuring the maintenance of excellent quality. The shelf-life of the button mushroom was about 14-20 days at 10C, about 10 days at 60C and 2 to 3 days at 200C.
Also, polyphenol oxidase activity and respiration rate were enhanced at 200C storage. Baker et al. (1981) observed that in button mushrooms, forced air cooling
resulted in a weight loss of about 2.5 per cent within 15-30 min. Minamide et al. (1985) reported that hydro cooling of button mushroom near their freezing point for 3 h within 6 h of harvest, packing in 100 per cent nitrogen gas (N2) for 2 h and then transferring to room temperature (200C) preserved them for 15 days. Chopra et al. (1985) recommended 100 gauge polythene bags with 0.5 percent venting area for packing button mushroom in case of refrigerated storage. Nichols (1985) recommended optimum temperature and relative humidity for storage of button mushrooms as 0-20C and 85-90 per cent respectively. Saxena and Rai (1988) however, reported the adverse effects of over-ventilation of polythene packs; mushrooms were best preserved in non-perforated 100 gauge polypropylene bags kept at 50C. Varszegi (2003) conducted an experiment to determine the relationship between the bacterial growth on mushroom cap and the pre- cooling methods (forced wet cooling and vacuum cooling) and found that vacuum cooling provided the longest period of time needed to reach the maximum value of microbial population and this method was found beneficial for the quality. Blanching for a short period is absolutely essential for producing good quality frozen mushrooms. Steam blanching for 3 min prior to freezing recorded retention of qualities of oyster mushroom also .
In vacuum-cooling, the water in cell walls and interhydral spaces of mushrooms gets evaporated under low pressure, and the evaporative cooling lowers the temperature from the ambient to 20C in 15 to 20 min.
Vacuum-cooling is a uniform and faster process, where mushrooms are subjected to very low pressure and water evaporates giving off the latent heat of vaporization. The vacuum cooled mushrooms have superior colour than conventional-cooled mushrooms. The major drawback of the system is the high capital cost and loss of fresh weight of the produce during the process of cooling. Filling and emptying the cooling chamber adds to the marketing cost. However, air spray-moist chillers can also cool the mushrooms rapidly. The temperature can be lowered by 16-180C in an hour without any moisture loss.
With a view to reduce the weight loss during the conventional vacuum cooling, ice bank cooling of mushrooms is now in vogue where a stack of mushrooms is passed through forced draft of chilled but humidified air from the ice bank (Water body maintained at sub zero temperature).
Radiation preservation offers a method of “cold sterilization” where the mushrooms may be preserved without marked change in their natural characters. Low dosages of γ-radiation could be used to reduce the microbial contamination and extend the shelf-life of mushrooms. However, irradiation should be given immediately after harvest for optimum benefits. Various types of beneficial effects of radiation have been observed in preserving the button mushroom and oyster mushroom.
Irradiation has been found to delay the maturation i.e. development of cap, stalk, gill and spore and also reduce the loss of water, colour, flavour, texture and finally the quality losses. Cobalt 60 (Co60) has been used as a common source of γ rays. A dose of 400 krad gave whiter buttons than the controls when the atmospheric temperature during growth and subsequent handling was slightly lower than 200C
A dose of 10 kGy (Kilo Gray) is reported to completely destroy microorganisms. Enhancement in shelf-life of Agaricus bisporus upto a period of 10 days was achieved by application of gamma rays close to 2 kGy and storage at 100C. Irradiation reduces the incidence of fungal and bacterial infection and also retards the breakdown of mannitol and trehalose.
However, the loss of flavor components has been noticed in irradiated mushrooms. But amino acids in fresh mushrooms were better preserved by γ irradiation and this
showed that irradiation at low levels proved better than irradiation levels of 1 & 2 kGy. Benoit et al. (2000) investigated the effect of gamma irradiation on some biochemical parameters of the mushrooms: higher doses significantly reduced the rate of respiration compared to samples irradiated with 0.5 kGy and non irradiated mushrooms. Ionizing treatments significantly increased phenylalanine ammonia-lyase (PAL) activity and total phenolic concentration. Koorapati et al. (2004) evaluated the effect of electron-beam irradiation on quality of white button mushroom and observed that irradiation levels above 0.5 kGy prevented microbesinduced browning. They recommended that irradiation at 1kGy was the most effective in extending the shelf-life of mushroom slices.
A study was conducted by Escriche et al. (2001) to determine the effect of ozone on post harvest quality of mushroom. Ozone treatment (100mg / h) of mushrooms prior to packaging increased the external browning and reduced the internal browning rates. The ozone treatment exhibited no significant differences in terms of texture, maturity index and weight loss of mushrooms.
Packaging plays very important role in handling, marketing and consumption of the produce and products, protects the quality during the storage and transport, keeping in retail and storage with the consumer.
Packaging of mushrooms from the production site up to the consumer including packaging for export market is an important aspect of post harvest handling. Generally, the see-through packaging increases the consumer confidence in the product. If the packaging and storage is not done properly, mushrooms not only deteriorate in their saleable quality but also in nutritional quality due to enzymatic changes.
The oyster mushrooms are harvested, stem-cut and adhered straw, if any, is removed. The cleaned mushrooms are packed in polypropylene bags of about 100 gauge thickness with perforations having vent area of above 5 per cent. Though the perforation causes slight reduction in weight during the storage, it helps maintain the freshness and firmness of the produce. Storage of dhingri at very low temperatures especially in nonperforated polypacks results in condensation of water, sliminess of the surface and softening of the texture. Cooling with positive ventilation is desirable i.e. cold air.
Paddy straw mushrooms are packed in polythene bags as well as tray packs. As very low temperature storage causes frost injury and deterioration in quality, the best way of storage is at 10-150C in polythene bags with perforations.
Mushrooms packed in bamboo baskets with an aeration channel at the center and dry ice wrapped in paper placed above mushrooms, is in practice for transportation. Packing in should be directed through the packed produce.
For transporting oyster , the fruit bodies are stacked in trays or baskets. Few poly pouches containing crushed ice are kept along with mushrooms if refrigerated transport is not available. The tray is then covered with thin polythene sheet with perforation. The pre-packed polythene packs with perforations may also be transported in this way. Wooden cases for transport by rail or boat is practiced in China.
Paddy straw mushrooms can be stored more effectively at the button stage than at any other stage. At temperatures below 100C, however, the mushrooms liquefy rapidly, irrespective of the packaging and stage of development (button or umbrella stage), due to chilling injury.
Button mushrooms are packed in many ways as per the retail, wholesale and transport requirements. Gormley and MacCanna (1967) conducted a study to identify the various quality attributes of fresh mushrooms preferred by the consumers. Two most important quality factors that affect the sale of fresh button
mushroom at the retail shop are the whiteness and stage of maturity i.e. the unopened button stage.
. * Quality factors in fresh mushrooms
|S. No.||Quality factor||Consumer emphasis*|
|2.||Degree of maturity||3|
|3.||Free from viral disease||3|
|8.||Size and shape||2|
* (3= very important, 2 = less important, 1 = not very important)
The most common method of packing in developing countries like Nigeria is small polyethylene or polypropylene packets containing mostly 200 or 400 g of mushrooms and generally these small packets are stored for a few days by retailers or consumers. Quantities more than this have a tendency to loose their acceptability due to price factor.
For transportation, these small packs are stacked in large containers. Keeping in view the family-demand and price, mushrooms are also packed in plastic punnets (trays) overwrapped with PVC film. Packaging of the button mushrooms for retail market, by the seasonal farmers is admittedly, primitive: washed mushrooms packed in thin (<100 gauge) polypropylene bags, hand sealed and unlabelled. Of late, some improvements with respect to labelling and machine-sealing have been introduced. Some firms have introduced unwashed mushrooms in plastic trays over-wrapped with PVC films; six trays of 200 g each in the printed card board boxes.
In American and European countries, where peat is used as the casing soil and sprays are in the nature of ‘washing down’, the mushrooms are not washed before packaging. However, in India and some other countries, button mushrooms, which are often soiled due to the use of FYM-based casing soil, need to be washed before sale or processing. Consumers have developed a preference for the washed mushrooms, though enlightened people still go for the unwashed.
Though washed mushrooms deteriorate faster than unwashed but washing becomes necessary to remove soil particles if non-peat casing soil is used. Mushrooms are trimmed with stainless steel knives immediately after harvesting and washed to remove the adhering casing soil and other foreign materials and then they are packed in suitable containers.
Increased water content in the mushroom, however, results in slogging and spoilage by bacteria. Generally plain water is used for washing of the mushrooms, which makes the mushrooms free from the adhering casing soil only, but does not impart any whiteness to the mushroom. To improve and maintain whiteness many pretreatments have been tried: dipping of mushrooms in dilute solutions of hydrogen peroxide (1:3) for half an hour and then steeping in 0.25 per cent citric acid solution containing 550 ppm sulphur dioxide showed significant positive effect. Small growers wash the mushrooms in solution of other reducing agents also to retard the browning caused by polyphenol oxidase.
Sun-drying is the cheapest and oldest method among various drying methods. It is a very simple operation, where no fuel or mechanical energy is required. However, it is completely dependent on weather and it is not possible round-the-clock and round the- year. Even though the quality of the product is affected by the environmental factors, due to free availability of heat source, it is considered to be the lowest-cost method of drying.
Mushrooms are spread over the trays or sheets and kept in open under the sun; favourable atmospheric conditions are above 250C temperature, with less than 50 per cent relative humidity and high wind velocity. Sun dried product contains more than 10-12 % moisture and should therefore be oven dried at 55-600C for 4-6 h to further reduce the moisture to 7-8 % to avoid any spoilage during storage. The dried product regains to a large extent its flavor and texture after rehydration. The technique has however, been not used for the button mushroom. Other mushrooms are generally sun-dried by resource-poor growers.
However ,oyster mushroom could be sundried during the days with high temperature (above 250C), low humidity (less than 50 percent RH) and high wind velocity.
The fruit body of the oyster can be beaded in a thin wire or thread and hanged in the air in direct sunlight forefficient dehydration and freeness from dust. The weight of the end product of this method had 10-12 percent of its original weight. They also recommended that the sundried product should be oven dried at 55- 600C for 4-6 h before packing in airtight packs.
Rama and John (2000) reported that when the temperature during sun-drying ranged from 21.60C to 35.70C, time taken was 14 h for the pretreated oyster mushroom and 12 h for the untreated oyster mushroom, to reach 5-6 per cent moisture level. They also reported that the dehydration ratio, shrinkage ratio and rehydration ratio of the sun dried product was 10.64, 0.19 and 2.21 respectively. The colour of the final product varied from brown to creamy white.
Cabinet air drying
Cabinet air drier which is also known as tray-drier (Fig. 20) consists of series of trays placed in the plenum chamber and hot air at constant air flow rate is allowed to pass through this plenum chambers.
However, the conventional oven drying method results in dark brown coloured product with a tough texture in case of the button mushroom; other mushrooms give reasonably acceptable product. Due to little air movement inside the conventional oven, the evaporated water condenses on fruit bodies increasing the
mucilage and deteriorating the quality. Large quantity of condensed water may also damage the equipment. Hence, a cabinet drier with circulated air supply was found to be superior. This process utilizes mechanical means for ventilating natural/hot air through mushrooms to accomplish the removal of moisture. Its features are
- The rate of drying can be controlled by adjusting the temperature of hot air ventilating through the mushroom. The process, therefore, makes possible the reduction of temperature and moisture stress developed during the drying process.
- Mushrooms can be dried irrespective of weather conditions, day or night, as the process does not depend on any natural sources like sunlight.
- The process is automatic and requires unskilled labour except a trained person to operate the dryer.
- There are practically no losses to insects, birds and rodents in the process.
- It, however, requires fuel and electrical or mechanical power to drive the air blower, elevators etc. therefore the cost of drying per kg of mushroom is higher compared to sun drying.
- It requires less space for operation. The drying temperature of 550C in the plenum chamber has been found to give the end product with the desired qualities of texture, colour and rehydration.
Home made cabinet drier is essentially a galvanized box of size 90 x 60 x 90 cm with perforated iron sheet at the bottom. The sides and the top of the box are fixed in a wooden frame and the whole thing is supported on an angle iron stand of about 38 cm height. There are two slits (5 cm x 3.7cm) along the top end of the two long sides and about 10 cm below the top. These slits are provided with metallic shutters for opening and closing.
About seven trays (87 cm x 60 cm) can be stacked on supports in staggering positions. The perforated iron plate at the bottom of the cabinet can be heated by means of a charcoal oven or oil stove.
Bano et al. (1992) reported that the mechanically-dried oyster mushrooms packed in air-tight containers have more than one year of shelf-life. Rama and John (2000) studied the mechanical drying of oyster mushroom. They maintained the temperature at 600C for the first 4 h and later at 500C for the rest of the 11h of drying to reach a moisture content of 5-6 per cent except the samples without any pretreatment which took only 8 h. The dehydration ratio, shrinkage ratio and rehydration ratio of the mechanically dried product were 9.89, 0.2 and 2.61 respectively. The colour of the dried product varied from brown to creamy white. Dehydrated oyster mushrooms should be packed in foil-laminated pouches for better storage stability Conditions of dehydration of tropical paddy straw mushrooms have been standardized by Pruthi et al. (1978). For the inactivation of peroxidase and catalase prior to dehydration of mushrooms, optimum time of water blanching was found to be 3-4 min and that of steam blanching as 4-5 min.
Removal of water from a substance by sublimation from the frozen state to the vapour state is known as freeze-drying.
Freeze-drying takes place in three stages: water present in the product is removed by formation of ice crystals; the ice crystals are then removed from the outer surface of the material by sublimation; after removal of all the ice, the little quantity of water left is then removed by evaporation in the freeze dryer.
In a freeze-drying system, original shape and size can be retained and the shrinkage, which is a problem with other drying methods, is almost negligible.
The product can be stored up to 6 months without any change in its quality and appearance. However, this is a very costly and energy intensive process and the venture depends upon the demand and price for such products.
Fluidized-bed drying is the process of removing moisture by exposing commodities into a high velocity of hot air. The velocity of the hot air should be such that it should not throw out but keep the material in the fluidized condition. Fluidized-bed drying, besides providing high quality product, reduces drying time also. A laboratory model fluidized bed drier is shown, the cap is yet to be placed on top of it. Singh et al. (2001) studied the drying characteristics of the fluidized bed drying of the button mushroom and found that quality of the dehydrated mushrooms was significantly influenced by the pretreatments as well as temperature; the samples treated with 1 per cent KMS, 0.2 per cent citric acid and 3 percent salt solution and dried at 500C gave satisfactory results.
Suguna et al. (1995) designed a batch type fluidized bed drier of 6 kg / batch capacity and obtained a satisfactory shelf-life of 5 months. The optimum temperature was found to be 500C with an air flow rate of 35 m3/ min.