Comparative Effect of Steam and Gamma Irradiation Sterilization of Sawdust Compost on the Yield, Nutrient and Shelf-Life of Pleurotus Ostreatus (Jacq.Ex.Fr) Kummer Stored In Two Different Packaging Materials

Abstract

The conventional method of decontamination of the spawn substrate sorghum and compost of ‗wawa‘ (Triplochiton scleroxylon) sawdust by the use of steam sterilization has attendant problems. In this thesis the possibility of using gamma radiation to decontaminate sorghum based spawn substrate and the compost before cultivation of the fruit bodies was determined as well as its effectiveness on selected nutritional, physical and organoleptic properties of the fruit bodies. A survey was carried out using the rapid appraisal method to determine the existing methods of sterilization, use of gamma radiation in food preservation, preference of mushrooms in Ghana by consumers and nutritional and medicinal attributes of the mushroom. The resident mycoflora in the sorghum grains and ‗wawa‘ sawdust as well as in the fresh and dry fruitbodies of P. ostreatus was determined by using Standard Decimal Series Dilution technique on two media (Cooke‘s and DRBC). At the end of the prescribed incubation period of the compost bags, the following parameters were determined: surface mycelia density and contamination, total number of fruit bodies, number of primordial formed, total yield, Biological Efficiency, mycelia growth rate, average stipe length, average cap diameter and mushroom size at each dose applied. The bacterial loads were determined by the conventional microbiological techniques and identification was by the use of the API 20E system. The D10 dose requirement for reduction of fungi and Baccillus cereus were also carried out on the preserved fresh and dry mushroom using Fricke Dosimetry. To ascertain the influence of the different combinations of radiation and steam treatments and packaging material on the nutritional quality of the sporophore formed, proximate analysis and mineral elements composition (Na, Ca, K, P, N, Fe, Zn, Cu, Mg, Pb) were determined by using the International Standard Methods in the Mushroom Industry. Dry curves showing the influence of radiation on drying rate of oyster mushrooms were determined by five non- linear regression models (Lewis, Page, Henderson and Pabis, Diffussion and Wang and Singh by the estimation of R2, X2 and RMSE). The total phenolic contents, flavonoids and free radical scavenging activity DPPH (2, 2‘- diphenyl-1- picrylhydrazyl) as sources of natural antioxidant of stored mushroom in polypropylene and polythene packs were determined using aqueous, ethanol and methanol extracts by Folin- Ciocalteau method. Finally, the influence of gamma irradiation on the colour characteristics were determined by the Hunter L* a* b* and Browning Index (BI) method while textural characteristics were determined mechanically by measuring Texture Hardness (kgf), Fracturability (N), Cohesiveness, Chewiness (N), Springiness (mm), Gumminess, Adhesiveness (kgf.s) and Resilience of fresh, dry and rehydrated mushrooms. The mechanical estimation to textural characters obtained were compared with mean scores on 9-point Hedonic scale of sensory acceptability attributes (appearance, colour, aroma, taste, mouthfeel and overall acceptability of coded samples in two packaging containers. The survey demonstrated the popularity of drum (moist heat) technique of sterilization in Ghana. Majority (64%) of the respondents were dissatisfied with the method of sterilization of compost and spawn substrate with 36% indicated the method was alright by them. Majority (82%) of the respondents have never heard of sterilization of food or mushroom by gamma irradiation technique. Because of the high contamination rate of the steam sterilized compost and spawn substrate, the farmers indicated the need to achieve better sterilization for these starting material for mushroom cultivation. Drying was selected as the most popular preservation method as the fresh fruiting bodies have short shelf-life and P. ostreatus was found to be the most preferred mushroom followed by the termite mushroom (Termitomyces sp.). The medicinal and nutritional value of mushrooms was underscored by this survey. Consumers seem to prefer mushroom on the basis of taste, appearance, texture, aroma or combination of these. A dose of 32 kGy was effective in decontamination of sorghum grains. The fastest rate of mycelia growth was 0.71 cm/day recorded by a single treatment of gamma radiation of 15 kGy on soaked raw sorghum. The slowest growth rate of 0.3 cm/day was recorded on raw non- irradiated and non- autoclaved sorghum grains (nI). The best treatment was the set of experiment which was steamed and irradiated at a dose of 25 kGy (S+I) which produced a growth rate of 11.8 mm/day and colonized completely in 7days. The slowest treatment combination was the set of experiment of non-irradiated sorghum treatment (nI) which produced a mycelium growth rate of 10.0 mm/day and used 13 days to completely colonize and produced poor mycelia density and about 80% contamination. Fungal counts on composted sawdust ranged from 4.72- 5.17 log CFU/g and 3.4- 4.1 log CFU/g on Cooke‘s and Oxytetracycline Glucose Yeast Extract agar respectively. Irradiation was more effective in reducing the fungal load by up to 3 log cycles than steam (1.1 log cycles) with D10 values ranging 5.64±1.12- 5.94±2.06 kGy. Ten encountered species belonging to 4 genera (Aspergillus, Rhizopus, Fusarium and Mucor) were predominated by Aspergillus (A. alutaceus, A. fumigatus, A. flavus, A. niger, A. terreus, A. ustus). Economic yield, biological efficiency, total fresh weight and flush weight of the fruiting bodies were recorded from the various interactions of irradiation and steam treatment of the compost showed significant differences (P<0.05). Autoclaved spawn + steam sterilized sawdust (A+S) of the 32 kGy treated set up recorded the highest yield of 1779g of total fresh weight (economic yield) and corresponding biological efficiency of 99.8%. Autoclaved spawn + non- steamed sterilized (A+nS) and irradiated spawn + non- steamed (I+nS) both recorded the least total fresh weight (economic yield, 0g) and biological efficiency (0%) respectively from the non-irradiated (0 kGy) set up. Microbes such as Staphylococcus aureus, Coliforms and Salmonella were not detected in the fresh and dry mushroom samples but aerobic mesophiles, Bacillus cereus and yeasts and moulds were encountered. The mean D10 values of B.cereus on fresh mushrooms were 3.21± 0.8 kGy (polypropylene bag), 0.76± 0.04 kGy (polythene). Low dose radiation of 1-2 kGy were effective in reducing the contaminants stored in polypropylene and polythene packs sufficiently to achieve the recommended levels of the International Commission for Microbiological Specification for Food (ICMSF). Polypropylene pack was preferred because of its rigidity and aesthetic quality. P.ostreatus contains significant amounts of ash, fat, protein, fibre, carbohydrate, metabolic activity as well as mineral elements (Ca, K, Mg, N, P, Na) and heavy metals (Zn, Mn, Pb, Fe, Cu) to merit its use in medicinal therapy. The drying characteristics of sliced mushroom exposed to low doses (0- 1.0 kGy) could be described by Page‘s Model while the Diffussion Model best fitted samples exposed to higher doses. The Effective Diffusivity (Deff) (which describes the rate of moisture movement in foods) was enhanced by gamma irradiation. The textural profile of fresh and dry mushrooms (hardness, fracturability, chewiness, resilience, cohesiveness measured by instruments varied but data matched the same parameters using sensory analysis by trained panelists. Panelists found no difference between irradiated and the control samples. The colour change of fresh and dried mushrooms measured by L*a*b* values kept in two packaging materials varied little with radiation treatment and Browning Index (BI) was low showing a slow

rate of enzymatic reaction after irradiation. Total phenolic contents of dry mushrooms kept in two packaging materials ranged from 0.56±0.01- 10.96±1.7 mgGAE/g, flavonoids ranged from 1.64±0.05- 8.92±0.6 mgQE/g; DPPH radical scavenging activity also ranged from 7.02±0.1- 13.03±0.04% and IC50 values ranged from 0.08- 0.16 mg/ml. A significant linear correlation was found between values for the total phenolic content and antioxidant activity. The high content of phenolic compounds indicates that these compounds contribute to the high antioxidant activity of P. ostreatus which can be promising candidate for natural mushroom source of antioxidant with high value. Owing to the fact that in most instances there were only marginal differences in nutritional, microbiological and textural properties of the irradiated mushrooms stored in polypropylene and polythene packs, the former is recommended for use as packaging material because of its rigidity and aesthetic value.