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 an essential part of the post harvest care of all the horticultural commodities including mushrooms. Pretreatments, 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 into 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 on 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. 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; also, 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 post-harvest 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 a commercial level. The view of button mushrooms during the refrigerated transport and storage 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 (Rai and Arumuganathan, 2003). Storage under low temperature is an excellent method for restricting deterioration of harvested mushrooms for a limited period.
The maturation and textural changes in button mushrooms were slowed down at 00C ensuring the maintenance of excellent quality (Murr and Morris, 1975 a). Minamide et al. (1980) observed that the shelf-life of the button mushroom was about 14-20 days at 10C, about ten 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 per cent venting area for packing button mushroom in the 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 needed to reach the maximum value of microbial population, and this method was found beneficial for the quality. Blanching for a short period is essential for producing good quality frozen mushrooms. Steam blanching retention of qualities of oyster mushroom also (Das and Pathanayak, 2003).
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 conventionally 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.
To reducing the weight loss during the conventional vacuum cooling, ice bank cooling of mushrooms are 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 (Staden, 1967; Campbell et al., 1968; Wahid and Kovacs, 1980; Roy and Bahl, 1984 a; Lescane, 1994) and oyster mushroom (Roy et al., 2000). 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 (Roy and Bahl, 1984 b). A dose of 10 kGy (Kilo Gray) is reported to completely destroy microorganisms.
Enhancement in shelf-life of Agaricus bisporus up to a period of 10 days was achieved by application of gamma rays close to 2 kGy and storage at 100C 1994). Irradiation reduces the incidence of fungal and bacterial infection and also retards the breakdown of mannitol and trehalose.
However, the loss of flavour components has been noticed in irradiated mushrooms. however, amino acids in fresh mushrooms were better preserved by γ irradiation and this showed that irradiation at low levels proved better than irradiation levels 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 nonirradiated 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 microbe-induced browning. They recommended that irradiation at 1 kGy be 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 (100 mg / h) of mushrooms before packaging increased the external browning and reduced the internal browning rates. The ozone treatment exhibited no significant differences regarding texture, maturity index and weight loss of mushrooms.
Directorate of Mushroom Research