Research Article

1. Sci. Agri.
11 (3), 2015: 132-138
© PSCI Publications
Scientia Agriculturae
www.pscipub.com/SA
E-ISSN: 2310-953X / P-ISSN: 2311-0228
DOI: 10.15192/PSCP.SA.2015.11.3.132138
Combined effect of solar drying and gamma radiation on the
microbiological quality of mushrooms (Pleurotus ostreatus
(Ex.Fr.)
Kortei N.K*1
, Akonor P.T2
, Appia A.H.K2
, Mill S.W.O3
, Annan SNY3
, Armah JNO3
, Acquah SA3
1. University of Ghana, Graduate School of Nuclear and Allied Sciences, Department of Nuclear Agriculture and Radiation
Processing, P. O. Box LG 80, Legon.
2. Food Research Institute- Council for Scientific and Industrial Research, P. O. Box M20, Accra.
3. Ghana Atomic Energy Commission, Radiation Technology Centre, P. O. Box AE 1, Atomic, Accra.
*Corresponding author email: nii_korley_1@yahoo.com
Paper Information A B S T R A C T
Received: 11 June, 2015
Accepted: 23 July, 2015
Published: 20 September, 2015
Citation
Kortei NK, Akonor PT, Appia AHK. Mill SWO, Annan
SNY, Armah JNO, Acquah SA. 2015. Combined effect of
solar drying and gamma radiation on the microbiological
quality of mushrooms (Pleurotus ostreatus (Ex.Fr.).
Scientia Agriculturae, 11 (3), 132-138. Retrieved from
www.pscipub.com (DOI:
10.15192/PSCP.SA.2015.11.3.132138)
The microbiological quality of dried Pleurotus ostreatus stored at room
temperature was evaluated after solar drying and gamma irradiation at low
doses of 0 (control), 0.5, 1, 1.5 and 2 kGy at a dose rate of 1.7 kGy per
hour in air from a 60CO (SLL 515, Hungary) batch irradiator. Moisture
content attained after drying was 12-14% at an average drying rate of 0.2
g/gmin-1. Initial microbial counts prior to drying ranged between 1.2- 5.3,
0.95- 1.4, 0-1.39 and 0 -0.8 log CFU/g for Total Viable Count, B.cereus,
Yeasts and Molds respectively. Final counts after solar drying and
irradiation recorded an average reduction of 0.55, 0.38 log cycles for TVC
and B.cereus respectively. However, Yeasts and Mould counts increased
after 5 days. There was a general negative linear correlation between solar
drying as well as irradiation and microbial quality. The use of solar drying
and gamma irradiation was effective in decontaminating mushrooms to
acceptable international levels (ICMSF).
© 2015 PSCI Publisher All rights reserved.
Key words: Solar drying, gamma irradiation, microbial count, drying rate
Introduction
Radiations are present in every part of our lives as we encounter them in our natural environment. Common types of
radiation include radio frequency, visible light, infra red light, microwave, cosmic and ultraviolet rays.
Exposure to sunlight is considered as the most important cause of “natural disinfection” in our environments.
According to Deller et al, (2006), the germicidal action of sunlight has long been recognized. However, its ecological
implications and the potentials for practical applications need to be researched more thoroughly since it is found in large
quantities in the tropics and therefore needs to be fully exploited to the benefit of the local inhabitants. Due to its readily
available and low cost nature, it is used for drying all types of food stuffs. Drying is one of the oldest technologies employed in
the processing of agricultural produce and a lot of research has been carried out on the drying of different crops (Bala et al,
2009). Drying is energy intensive and a complex process in which heat and mass transfer occur simultaneously (Kawolongo,
2013). The objective of drying is to remove water to a level such that microbial spoilage and deterioration reactions are greatly
minimized (Doymaz, 2007).
Irradiation of food using ionizing radiation (γ and x-rays or electron beam) is used to inactivate both spoilage and
pathogenic microorganisms and to guarantee the hygienic quality of several foodstuffs (Diehl, 1990). This technology has been
approved as a safe and alternative preservation technique without loss of sensory and organoleptic qualities (Codex
Alimentarius Commission, 1992; WHO, 1994; International Atomic Energy Agency, 2006).
Microorganisms cause a big problem in food processing and packaging because of their serious health implications
associated and therefore stern treatment are required to eliminate them.
The objective of this study is to assess the combined effect of solar and gamma radiations on the microbiological
quality of P.ostreatus.

2. Sci. Agri. 11 (3), 2015: 132-138
133
Materials and Methods
Sample collection
Mushroom Samples
Pleurotus ostreatus mushroom samples were grown on composted sawdust as described by Kortei et al (2014) and
harvested at maturity from the cropping house of the Mycology Unit, Food Research Institute, Council for Scientific and
Industrial Research, Accra, between the periods of February to May 2013.
About 1779g of fresh mushrooms were harvested and weighed on an electronic digital scale (Kern 510, Kern and
Sohn, GMbH, Germany) and collected mushroom material was solar-dried at temperature range of (20- 40 o
C) to a moisture
content of about 12±1%.
Drying of mushroom samples
Drying Procedure
Important parameters affecting the performance of the drier were measured. The k-type thermocouple was used to
measure the drying air temperature along the flow direction of the air inside the drier and a solar meter was used to measure
the global radiation. The relative humidity and temperature of the ambient air were measured with a digital thermometer and
relative humidity meter. The velocity of drying air was measured with an air velocity meter at the outlet of the drier. Weight
loss of the product during drying period was also measured with an electronic balance. The sun dried control samples were
weighed as well. All these data were recorded at thirty minutes (30 mins) interval.
The samples of mushrooms were placed on the wire mesh of the drier in a single layer. Drying was started at about 9
to 10 am. Drying of mushrooms was stopped at about 4 to 5pm. Then samples were collected and kept in a sealed container.
To compare the performance of the gamma irradiated mushrooms and non-irradiated (control) samples of mushrooms were
placed on trays in single layer beside the drier. Both experimental and control samples were dried simultaneously under the
same weather condition. At the beginning of each experimental run, the initial moisture content of mushrooms was measured.
Drying was carried out by using a solar dryer at a temperature of 50-60 o
C to reduce moisture content to about 12±1%
for an average period of 5 days. Dried mushroom parts were cut up and stored in tight-seal polythene containers at room
temperature until needed for microbiological analysis within one hour of collection.
Determination of Moisture content
The moisture content was determined by the gravimetric method of (AOAC, 1995).
Irradiation of mushroom samples
Forty (40) grams of dried mushrooms (Pleurotus ostreatus) were packed into polythene and polypropylene containers
and irradiated at doses of 0 kGy, 0.5 kGy, 1 kGy, 1.5 kGy and 2 kGy at a dose rate of 1.7 kGy per hour in air from a cobalt- 60
source (SLL 515, Hungary). Radiations absorbed were confirmed by Fricke’s dosimetry, which is a reference chemical
dosimeter based on the chemical process of oxidation of ferrous ions (Fe2+
) in aqueous sulphuric acid solution to ferric ions by
ionizing radiation at the Radiation Technology Centre of the Ghana Atomic Energy Commission, Accra, Ghana.
Microbiological analysis
Ten (10) grams of each food sample was mixed with 9 ml peptone water and serial dilutions of each mushroom
sample homogenate were made to 10-3
dilutions. Approximate 0.1 ml aliquot portions of the dilutions were spread onto
duplicate sterile plates of Plate Count Agar (Oxoid, England), Violet Red Bile Agar (Oxoid, England), Bacillus cereus agar
(Oxoid, England) and Dichloran Rose Bengal Chloramphenicol (Oxoid, England) for total mesophilic bacteria, total aerobic
plate count, coliform count, Bacillus cereus and moulds and yeasts respectively.
Cultures were incubated at 37o
C for 24 to 48 hrs. After the incubation time, the different culture plates were examined
for microbial growth. Different morphological attributes of the colonies were observed and recorded. Colonies were counted
using the colony counter (Gallenkamp, England), counts were expressed as colony forming unit per gram of sample
homogenate (cfu/g). Discrete colonies were isolated and purified by repeated sub-culturing. Pure cultures were stored on slants
at 4ºC for further characterization.
Identification of isolates
The bacteria isolates were identified based on standard microbiological methods. Cultural characteristics and
biochemical tests:- catalase, IMViC test, carbohydrate utilization, reaction on Tri-Sugar Iron (TSI) medium, gelatin
liquefaction, starch hydrolysis, nitrate reduction, coagulase, phosphatase production, motility, Oxidase and Urease production
were carried out as preliminary test.

3. Sci. Agri. 11 (3), 2015: 132-138
134
Data Analysis
The microbial counts (colony forming units in standard forms per g) were transformed into logarithms (log10) and
means were determined. Data was analysed with Excell 2003 for Microsoft
Results And Discussions
Drying curves showing the influence of radiation on drying rate of P.ostreatus are presented in Figs. 2 and 3. The rate
of moisture loss was directly proportional to increasing gamma radiation dose as well as time of day. The initial rate of drying
ranged between 0.58- 0.6 g/g min-1
for the initial 30 minutes of drying but reduced gradually to a range of 0.02- 0.05 g/g min-1
by the end of 360 minutes. The observed change in rate of drying could be attributed to the relatively faster loss of moisture
caused by the weakening of inter molecular bonds of H2O molecules from gamma radiation (IAEA, 1995). There was an
observed inverse relationship of ambient temperature and relative humidity with time of day (Fig.4). High rate of moisture loss
in irradiated mushroom may be attributed to the breakdown of tissue structures. Upon exposure to ɣ-irradiation, chitin, which
is the main structural carbohydrate in mushrooms depolymerizes, resulting in loss of firmness (Akram et al., 2012).
Consequently, resistance to moisture migration towards the surface of the product reduces. This observation confirms the
suggestion that food structure is influential in determining moisture transport within food materials (Labuza and Altunakar,
2007). The drying curves showed no constant rate period, suggesting that diffusion is the dominant mode of moisture removal
from the mushrooms. This observation corroborates earlier findings for other products such as white button mushrooms
(Wakchaure et al., 2010), eggplant (Doymaz and Gol, 2011), leafy vegetables (Akonor and Amankwah, 2012).
The state of moisture in a food product is expressed in terms of water activity (aw). It is essential for describing water
availability and mobility in foods (Ayala et al., 2011). Fig.1. shows the chemical ‘stability map’, relating the effect of water
activity (aw) in the food material on the microbial growth. From the stability map, it is evident that during the intermediate
moisture range, many reactions occur causing deterioration (Labuza et al, 1972). It has been reported that microorganisms do
not grow on food products with water activity aw below 0.6 (Labuza et al., 1972; Yan et al., 2008).
Figure 1. Chemical ‘stability map’ of food product (Adapted from Kawongolo, 2013 in Labuza, 1972)

4. Sci. Agri. 11 (3), 2015: 132-138
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Figure 2 .Influence of irradiation on the drying rate of P.ostreatus.
Figure 3.Influence of irradiation on the moisture content of P.ostreatus.
Figure 4. Variations of ambient air temperatures and relative humidity with time of day for solar drying of mushrooms (P.ostreatus).
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
30 60 90 120 150 180 210 240 270 300 330 360
DryingRate(g/gmin-1)
Time (minutes)
2 kGy
1.5 kGy
1 kGy
0.5 kGy
0 kGy
0
10
20
30
40
50
60
70
80
90
100
30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480
MoistureContent(%)
Time (minutes)
2 kGy
1.5 kGy
1 kGy
0.5 kGy
0 kGy
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
9am 10am 11am 12pm 1pm 2pm 3pm 4pm
Temperature(oC)
RelativeHumidity(%)
Time of DayR.H Temp.

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Microbiological counts
There was an observed high initial (0 day) total viable counts of range 1.2- 5.3 log10 CFU/g. However, after
irradiation and drying, total viable counts were reduced by a range of 0.05- 0.7 log cycles (Fig.5). Conversely, non- irradiated
but solar dried P.ostreatus recorded an increase in count from an initial 5.3 to 6.5 log10 CFU/g. There was significant (p<0.05)
differences in microbial counts of irradiated and non irradiated P.ostreatus in many instances.
According to Thompson and Blatchely, (2000), the mechanisms of inactivation of viable microbial cells or spores by
ionizing radiation include both direct and indirect action. Direct action involves absorption of photon energy by a target
molecule in the cells (e.g. DNA) leading to damaged to that target. Indirect action resulted from absorption of photon energy
by a nearby molecule (e.g. water) leading to the formation of highly reactive free radicals (HO, OHO, é) which in turn react
with target molecule in the cells causing its damage. This observed difference could be attributed to the more intense killing
effect of ionizing radiation as it affects directly the microbial DNA (genome) causing damage to fungal or bacterial cells.
Furthermore, the ability of an organism to withstand a physical stress (gamma radiation, solar radiation, heat etc) depends on
how quickly it is able to repair its damaged DNA as a result of denaturing (IAEA- TECDOC, 2006). Results obtained agrees
with data reported by Shurong et al, (2006) who recorded log cycle reductions of 2-3 in packaged tofu after irradiation with 1-3
kGy.
Bacillus cereus enumerated from samples had initial counts of range 0.95-1.4 log10 CFU/g. which reduced to a range
of 0.1- 0.65 log cycles accompanied by its disappearance in some instances after drying and irradiation. Non- irradiated
samples followed same trend as in total viable count and same reasons could be attributed to this observation. Similarly,
Valero et al (2006) reported a reduction in B.cereus population by 1.3- 4.6 log cycles after irradiation of 1.3-5.7 kGy.
Microorganisms differ greatly in their resistance to ionizing radiation. Vegetative cells of bacteria are sensitive to ionizing
radiation; Bacterial spores are found to be more resistant than yeasts, molds and vegetative cell of bacteria (Van Germen et al.,
1999). B.cereus which is also spore forming is associated with a host of food borne illnesses and can be detected in
undercooked meals since their spores are heat resistant. The survival of Bacillus depends on several factors such as nature of
the organism, resistance to a new physical environment and ability to form spores (Godon, 1977; Valero et al, 2006).
Endospores of Bacillus are more resistant than the vegetative cells to harsh weather conditions and even to antimicrobial
treatments (Codex Alimentarius, 2007).
Yeasts counts were initially low ranging 1.29- 1.39 log10 CFU/g for doses 0 and 0.5 kGy. The higher doses (i.e > 0.5
kGy) recorded no yeast count. Yeast counts however, increased after drying and irradiation to ranges of 0.98- 4.37 log10
CFU/g. Dadachova and Casadevall (2008) attributed radiation resistance of yeast spores to melanin content and later
germinated due to favourable environmental conditions might have resulted in this observation. Mould counts were also
initially low ranging 0- 0.8 log10 CFU/g for doses 0, 0.5 and 1.0 kGy. The same pattern of growth and resistance were recorded
as previously reported for yeasts. In a similar study, The role of yeasts and molds in the spoilage of food is well documented
and their growth on foods can cause major problems. Some of molds may produce mycotoxins which could be carcinogenic,
mutagenic, teratogenic, allergic and immunosuppressing (Odamtten et al, 1987; Tournas, 2005; Adu-Gyamfi et al, 2012) to
both humans and animals.
Figure 5. Total Plate Count of gamma irradiated P.ostreatus before and after drying
-1
0
1
2
3
4
5
6
7
8
0 0.5 1 1.5 2
Log10CFU/g
Dose (kGy)
0 Day 5 Day

6. Sci. Agri. 11 (3), 2015: 132-138
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Figure 6. Bacillus cereus of gamma irradiated P.ostreatus before and after drying
Figure 7. Yeasts of gamma irradiated P.ostreatus before and after drying
Figure 8. Moulds of gamma irradiated P.ostreatus before and after drying
-0.5
0
0.5
1
1.5
2
0 0.5 1 1.5 2
Log10CFU/g
Dose (kGy)
0 Day 5 Day
-1
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4
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6
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Log10CFU/g
Dose (kGy)
0 Day
5 Day
-0.5
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1.5
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0 0.5 1 1.5 2
Log10CFU/g
Dose (kGy)
0 Day
5 Day

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Conclusion
Although foodborne illnesses can be barred by good hygiene practices such as the use of Good Manufacturing
Practices (GMP) and Hazard Analysis Critical Control Point (HACCP) application in the chain of food production and
processing, the employment of solar radiations by solar drying and gamma radiation processing to foods will further decrease
the microbial loads to improve its hygienic quality to acceptable international standards (CFS, 2007; ICMSF, 2006).
Acknowledgement
Authors are grateful to Prof. G.T. Odamtten, Mycology Unit, Department of Botany, University of Ghana for his
support. We also thank all the technicians at Radiation Technology Centre, Ghana Atomic Energy Commission.
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