Influence of cultivation practices on yield and post-harvest quality of oyster mushrooms (pleurotus ostreatus)

Abstract

Previous studies have indicated that oyster mushrooms do not last long under normal storage; the shelf-life can be increased only by storage at cold temperatures and wrapping in polyethylene plastic. Currently, mushrooms regardless of species, can be stored for only a few weeks before they deteriorate rapidly. Studies were conducted in a growth chamber and cold storage facility to determine how the crop responds to agronomic and post-harvest practices such as wheat straw leachate, alternative substrates, extract drainage, pasteurising methods, use of 1-NAA, storage temperatures and drying techniques. The research was performed at the Agricultural Research Council-Tropical and Subtropical Crops (ARC-TSC) at the Mbombela (Nelspruit) experimental farm, Mpumalanga Province, South Africa.

The potential growth inhibition rate of oyster mushrooms by means of wheat straw leachate was determined in growth rooms. Fresh and stored wheat straw was boiled for extraction at five levels of dilution treatments [undiluted extract; 3:1; 1:1; 1:3 ratios and dH20 control]. The 3:1 ratio encouraged mycelial growth when fresh wheat straw was used, while the 1:1 ratio increased growth with wheat straw that had been stored for 3 to 6 months. However, when using wheat straw as a growing medium in oyster mushroom production, storage of the straw for at least 3 to 6 months prior to spawning is necessary in order to leach out compounds that are capable of inhibiting mycelial growth.

Another follow-up experiment was conducted in a custom made hut-house, where potential growth inhibition of oyster mushrooms by stored wheat straw leachate was determined. Stored wheat straw was drained using five levels of treatment (no drain; 1 h; 2 h; 3 h and 4 h drain) during spawning. Wheat straw leachate drained for 1 h and 2 h during spawning increased the cumulative number of clusters, caps and fresh mass (g) of the oyster mushrooms.

For effective pasteurisation of mushroom substrates, soaking, boiling and steaming was carried out including the use of NaOCI (household bleach - Jik) and gypsum. Both boiling and steaming expedited the rate of colonisation and prevented attacks by parasitic fungi (Trichoderma spp.) which resulted in increased yield and biological efficiency of oyster mushrooms.

Thatch grass (natural pasture) and wood chips were evaluated together with wheat straw (control) as alternative substrates. Both the wheat straw and thatch grass produced a high cumulative number of clusters, caps and fresh mass (g) of oyster mushrooms. Thatch grass can therefore be used as an alternative substrate for inoculating oyster mushrooms, with subsequent increases in clusters, caps or yield and biological efficiency.

Results from different levels of the plant growth hormone auxin (0; 0.1; 0.5; 1; 1.5 and 2 mmol 1-NAA) indicated that 1 mmol 1-NAA significantly increased the growth, yield and biological efficiency of oyster mushrooms during the cooler April – June and the warmer September to November months. There was need to apply 1 mmol 1-NAA during cooler and warmer months, as it did significantly improve the production of oyster mushrooms.

To extend the shelf-life of oyster mushrooms, storage under 2°C (70% RH) and 5°C (70% RH) in covered containers maintained the initial fresh mass (50 g), as well as good quality and colour, without any physiological disorders noticeable during the evaluation periods. The significance of the study is that it suggests that oyster mushrooms can be kept in storage for more than 4 weeks under cold storage and still be usable by consumers without the loss of quality.

Various types of drying techniques, in combination with preservatives [i.e. sun drying; electric dehydration; drying at an ambient (room) temperature (21℃) – no additives; sodium metabisulfite; and lemon juice], were used to investigate the optimum treatment to extend the shelf-life of oyster mushrooms. Oyster mushrooms that dried with or without preservatives under an electric dehydrator or under the sun dry maintained good quality, colour, and lower moisture content and lasted for six months (still usable by consumers) when stored at ambient (room) temperatures.

Keywords: Biological efficiency, leachate, mycelia, physiological disorders, shelf-life, substrate, wheat straw