![35 Kortei et al./IFRJ 23(1): 34-39
its drying characteristics as well as quality of the final
dried product.
Drying kinetics of different mushroom species,
such as button and oyster mushrooms, have been
reported in previous studies (Pal and Chakraverty,
1997; Giri and Prasad, 2007; Addo et al., 2009;
Wakchaure et al., 2010; Tulek, 2011). However,
the influence of irradiation as a pretreatment on
the kinetics of solar-dried mushrooms has not yet
been studied. This study therefore investigates the
influence of gamma irradiation pre-treatment on
drying kinetics of oyster mushrooms (Pleurotus
ostreatus) dried in a tunnel solar dryer.
Materials and Methods
Mushroom species and growth parameters
Oyster mushrooms (Pleurotus ostreatus)
originally from Mauritius, were cultivated on
Triplochiton scleroxylon sawdust composted for 28
days and supplemented with 1% CaCO3
and 10%
rice bran as described by Obodai et al., (2003). This
was carried out at the Mushroom Unit of the Council
for Scientific and Industrial Research (CSIR)-Food
Research Institute, Accra, Ghana. Growth and
harvesting of mushrooms was from the period of
September to December, 2013. Mature mushroom
harvested 2 days after primodia emergence were used
for the study.
Irradiation of mushroom materials
Forty (40) grams of mushroom slices (6.8±0.51
mm thick) were packed into polythene containers
and irradiated at doses of 0.0 kGy (control), 0.5 kGy,
1 kGy, 1.5 kGy and 2 kGy at a dose rate of 1.7 kGy
per hour in air at room temperature (28±1o
C) from a
cobalt 60 source (SLL 515, Hungary). Doses were
confirmed using the ethanol-chlorobenzene (ECB)
dosimetry system at the Radiation Technology
Centre of the Ghana Atomic Energy Commission,
Accra, Ghana. For each dosage, a total of 520 g of
mushrooms was irradiated.
Drying experiments
The oyster mushrooms slices (6.8±0.51 mm
thick) were dried using a tunnel solar dryer designed
and fabricated by the CSIR-Food Research Institute,
Ghana. Prior to solar drying, the mushrooms were
pretreated with gamma irradiation under the different
dose treatments aforementioned. Mushrooms,
weighing 150 g (in triplicates) were spread in a single
layer on a wire mesh and loaded into the solar tunnel
dryer. Drying was conducted between the hours of
0900to1700hrseachday.Moisturelossduringdrying
was determined by measuring the loss in weight of
samples at 30 min interval, with an electronic balance
(Kern 510, Kern and Sohn, GMbH, Germany).
Sampling and weighing was done until a constant
weight was attained (Akonor and Tortoe, 2014).
Both experimental and control samples were dried
simultaneously under the same weather condition.
At the beginning and ending of each experimental
run, moisture content of mushrooms was determined
by standard methods (AOAC, 1990). Mean drying
temperature and relative humidity over the drying
period were 53.2±6.4°C and 30.7±5.8% respectively.
Dried mushrooms were sealed air-tight and stored in
rigid polypropylene containers.
Mathematical modeling
Moisture ratio (MR) of mushrooms for thin layer
drying was calculated as follows:
(1)
Where Mt
= Moisture content (%) after time t; Me
= Equilibrium moisture content and Mo
= Initial
moisture content. However, due to varying relative
humidity and temperature during drying and the
fact that Me
is very small, compared to Mo
and Mt,
it
could be neglected, thus simplifying (1) according to
Yaldyz and Ertekyn (2001) and Goyal et al., (2007)
as:
(2)
Experimental data for moisture ratio vs. drying
time were fitted to 5 drying models, commonly used
to describe the thin layer drying kinetics of perishable
fruits and vegetables, by Non-linear regression
(Statgraphics Centurion 15.1). Models used were;
Lewis [MR = exp (-kt)], Page [MR = exp (-ktn
)],
Henderson and Pabis [MR = a exp(-kt)], Diffusion
model [MR = a exp(-kt) + (1 – a) exp(-kbt)] and
Wang and Singh [MR = 1 + at + bt2
]. In these models,
a and b are dimensionless drying coefficients while k
and n are drying constants (min-1
).
The main criterion for selecting the best model
to describe the drying curves was the coefficient of
determination (R2
). Also, the reduced chi square (χ2
)
and the Root Mean Square Error (RMSE) were used
to determine the goodness of fit between predicted
and experimental data. High R2
and low χ2
and RMSE
correspond to a better goodness of fit (Akpinar et al.,
2003). The χ2
and RMSE were calculated from the
following formulae:
(3)](https://image.slidesharecdn.com/korteimodellingdryingkinetics-160316225222/85/Modeling-the-solar-drying-kinetics-of-gamma-irradiation-pretreated-oyster-mushrooms-Pleurotus-ostreatus-2-320.jpg)
Modeling the solar drying kinetics of gamma irradiation-pretreated oyster mushrooms (Pleurotus ostreatus)
- 1. © All Rights Reserved *Corresponding author. Email: [email protected] International Food Research Journal 23(1): 34-39 (2016) Journal homepage: http://www.ifrj.upm.edu.my 1 Kortei, N.K., 2 Odamtten, G.T., 3 Ayim-Akonor, M. and 4* Akonor, P.T. 1 Graduate School of Nuclear and Allied Sciences, Department of Nuclear Agriculture and Radiation Processing, P. O. Box AE 1, Atomic, Accra 2 Department of Botany, University of Ghana, P. O. Box LG 55, Legon-Accra 3 Animal Health and Food Safety Division, C.S.I.R.-Animal Research Institute, Ghana. P. O. Box AH 20, Achimota, Accra 4 Food Processing and Engineering Division, C.S.I.R- Food Research Institute, Ghana, P. O. Box M 20, Accra Modeling the solar drying kinetics of gamma irradiation-pretreated oyster mushrooms (Pleurotus ostreatus) Abstract Oyster mushroom slices (Pleurotus ostreatus) were exposed to ɣ-radiation as a pretreatment and solar dried to investigate the influence of irradiation on drying kinetics. Processing conditions included exposure of mushrooms to 0 kGy (control), 0.5 kGy, 1.0 kGy, 1.5 kGy and 2.0 kGy of ɣ-radiation at a dose rate of 1.7 kGy/h and drying at a mean temperature of 53.2±6.4°C. Experimental drying data were fitted to 5 thin layer drying models by non-linear regression. Irradiation was observed to enhance the drying rate of mushroom slices, with higher doses causing faster moisture removal. Drying characteristics of slices exposed to lower dosages were best described by Page’s model (R2 =0.9878, 0.9967, 0.9925 correspondingly for “control” (0.0 kGy), 0.5 and 1.0 kGy while the Diffusion model best fit the data for those exposed to higher doses of radiation (R2 =0.9938, 0.9890 for 1.5 and 2.0 kGy respectively). Deff ranged from 1.88 to 2.44 x 10-08 and increase from “control”, 0.5 kGy, 1.0 kGy, 1.5 kGy to 2.0 kGy. Irradiation of mushrooms as a pretreatment for drying increases moisture diffusivity and drying rate with higher doses having the most effect. Introduction Mushrooms play a vital role in the biosphere and their production represents the most efficient bioconversion of a wide range of lingo-cellulosic waste materials including sawdust and corn cobs into expensiveproteins.Theyareoneofthehighestprotein producers per unit area and time (Kortei, 2011) and are nutritionally well endowed with essential amino acids, minerals and vitamins (Akindahunsi and Oyetayo, 2006; Kumari et al., 2011). Mushrooms are also known to possess medicinal properties because they contain bioactive compounds such as triterpenoids, lectins and steroids (Lindequist et al., 2005; Singh et al., 2012). Freshmushroomshavebeenreportedtostorefrom 1 to 3 days at ambient conditions because of their high moisture content and high transpiration rate (Mahajan et al., 2008). Therefore, it is necessary that they are marketed soon after harvest, or preserved with special care to maintain its wholesomeness. In this regard, several techniques, including solar drying, have been suggested to improve their shelf stability and enhance its economic potential. Solar drying is accomplished by exposing the produce to air in a chamber which is heated by concentrating the sun’s energy with an insolation material. This reduces moisture content of the produce and stabilizes it by lowering the rate of chemical reactions and its susceptibility to microbial attack. Dehydrated mushrooms are used in several preparations, including soups and stews (Martinez- Soto et al., 2001). Albeit one of the most widely employed drying techniques, solar drying may be slow and impart certain undesirable quality changes to the final product.As a result, pre-treatments have been applied to products before drying. These pretreatments, usually chemical, improve drying rates and or prevent undesirable changes associated with drying. Other pretreatments such as irradiation have been employed in food dehydration (Wang and Chao, 2002; Yu and Wang, 2005). Irradiation technology has proved effective in sterilizing and also extending the shelf life of food by delaying or eliminating biological processes. Its application in certain fruits and vegetables as a pre-treatment for drying have been shown to boost drying rates by altering the structure of tissues (Wang and Chao, 2002; Wang and Du, 2005). Application of irradiation prior to drying mushrooms may also result in changes that will affect Keywords Gamma irradiation Mushrooms Solar drying Drying kinetics Article history Received: 29 November 2014 Received in revised form: 26 June 2015 Accepted: 2 July 2015
- 2. 35 Kortei et al./IFRJ 23(1): 34-39 its drying characteristics as well as quality of the final dried product. Drying kinetics of different mushroom species, such as button and oyster mushrooms, have been reported in previous studies (Pal and Chakraverty, 1997; Giri and Prasad, 2007; Addo et al., 2009; Wakchaure et al., 2010; Tulek, 2011). However, the influence of irradiation as a pretreatment on the kinetics of solar-dried mushrooms has not yet been studied. This study therefore investigates the influence of gamma irradiation pre-treatment on drying kinetics of oyster mushrooms (Pleurotus ostreatus) dried in a tunnel solar dryer. Materials and Methods Mushroom species and growth parameters Oyster mushrooms (Pleurotus ostreatus) originally from Mauritius, were cultivated on Triplochiton scleroxylon sawdust composted for 28 days and supplemented with 1% CaCO3 and 10% rice bran as described by Obodai et al., (2003). This was carried out at the Mushroom Unit of the Council for Scientific and Industrial Research (CSIR)-Food Research Institute, Accra, Ghana. Growth and harvesting of mushrooms was from the period of September to December, 2013. Mature mushroom harvested 2 days after primodia emergence were used for the study. Irradiation of mushroom materials Forty (40) grams of mushroom slices (6.8±0.51 mm thick) were packed into polythene containers and irradiated at doses of 0.0 kGy (control), 0.5 kGy, 1 kGy, 1.5 kGy and 2 kGy at a dose rate of 1.7 kGy per hour in air at room temperature (28±1o C) from a cobalt 60 source (SLL 515, Hungary). Doses were confirmed using the ethanol-chlorobenzene (ECB) dosimetry system at the Radiation Technology Centre of the Ghana Atomic Energy Commission, Accra, Ghana. For each dosage, a total of 520 g of mushrooms was irradiated. Drying experiments The oyster mushrooms slices (6.8±0.51 mm thick) were dried using a tunnel solar dryer designed and fabricated by the CSIR-Food Research Institute, Ghana. Prior to solar drying, the mushrooms were pretreated with gamma irradiation under the different dose treatments aforementioned. Mushrooms, weighing 150 g (in triplicates) were spread in a single layer on a wire mesh and loaded into the solar tunnel dryer. Drying was conducted between the hours of 0900to1700hrseachday.Moisturelossduringdrying was determined by measuring the loss in weight of samples at 30 min interval, with an electronic balance (Kern 510, Kern and Sohn, GMbH, Germany). Sampling and weighing was done until a constant weight was attained (Akonor and Tortoe, 2014). Both experimental and control samples were dried simultaneously under the same weather condition. At the beginning and ending of each experimental run, moisture content of mushrooms was determined by standard methods (AOAC, 1990). Mean drying temperature and relative humidity over the drying period were 53.2±6.4°C and 30.7±5.8% respectively. Dried mushrooms were sealed air-tight and stored in rigid polypropylene containers. Mathematical modeling Moisture ratio (MR) of mushrooms for thin layer drying was calculated as follows: (1) Where Mt = Moisture content (%) after time t; Me = Equilibrium moisture content and Mo = Initial moisture content. However, due to varying relative humidity and temperature during drying and the fact that Me is very small, compared to Mo and Mt, it could be neglected, thus simplifying (1) according to Yaldyz and Ertekyn (2001) and Goyal et al., (2007) as: (2) Experimental data for moisture ratio vs. drying time were fitted to 5 drying models, commonly used to describe the thin layer drying kinetics of perishable fruits and vegetables, by Non-linear regression (Statgraphics Centurion 15.1). Models used were; Lewis [MR = exp (-kt)], Page [MR = exp (-ktn )], Henderson and Pabis [MR = a exp(-kt)], Diffusion model [MR = a exp(-kt) + (1 – a) exp(-kbt)] and Wang and Singh [MR = 1 + at + bt2 ]. In these models, a and b are dimensionless drying coefficients while k and n are drying constants (min-1 ). The main criterion for selecting the best model to describe the drying curves was the coefficient of determination (R2 ). Also, the reduced chi square (χ2 ) and the Root Mean Square Error (RMSE) were used to determine the goodness of fit between predicted and experimental data. High R2 and low χ2 and RMSE correspond to a better goodness of fit (Akpinar et al., 2003). The χ2 and RMSE were calculated from the following formulae: (3)
- 3. Kortei et al./IFRJ 23(1): 34-39 36 (4) Where N= Number of observations; z = Number of constants in the model; MRexp and MRpre are experimental and predicted moisture ratios respectively. Effective moisture diffusivity Generally, diffusion is assumed as the dominant transport mechanism during drying and the rate of moisture movement is therefore described by an effective diffusivity value, Deff (m2 /s), which is related to MR by equation 5 (5) Where t=drying time (min) and L= Half thickness of slices (m). The effective moisture diffusivity was obtained by plotting the experimental drying data in terms of lnMR against time, t (min). From equation 5, a plot of lnMR against drying time, t, gives a straight line with slope, K, where (6) Results and Discussion Drying profiles Drying curves from the drying experiment is displayed in Figure 1.As shown, exposure to radiation influenced rate of moisture loss in the mushrooms during drying, such that irradiated slices dried faster than the “control” (0.0 kGy). Among the mushrooms exposed to ɣ-radiation, the rate of moisture loss directly corresponded to radiation dosage, with those exposed to high levels of gamma rays drying faster. 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 affirms 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 (Srikiatden and Roberts, 2006). 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). Non-Linear regression modeling Table 1 summarizes the outcome of the non- liner regression modeling using 5 thin layer drying models, and these were compared based on their R2 , χ2 and RMSE. All 5 models showed very good fit (R2 >0.9) to the experimental data. Nevertheless, the Page and Diffusion models were the best to describe drying kinetics of mushrooms under the different experimental conditions. Drying kinetics of slices exposed to lower radiation dosages (0.5 – 1.0 kGy) was quite similar to the “control”. Under these experimental conditions, the Page’s model resulted in the highest R2 and lowest χ2 and RMSE and best suited its description. The three indices were 0.9878, 0.0382, and 0.0014 for the control, 0.9967, 0.0184 and 0.004 for 0.5 kGy and 0.9925, 0.0274 and 0.0008 for 1.0 kGy. The Diffusion model best predicted drying behavior of mushrooms exposed to ɣ-radiation in excess of 1.0 kGy. Drying characteristics of mushrooms Figure 1. Influence of irradiation on drying rate oyster mushrooms Figure 2. Model fit for control (A), 0.5 kGy (B) and 1.0 kGy (C) using Page’s model
- 4. 37 Kortei et al./IFRJ 23(1): 34-39 slices from this group were therefore dissimilar from the earlier group, which includes the “control”. Drying characteristics of mushroom slices in this study are quite different from observations made in some previous studies. In these earlier studies, drying characteristics were best described by Wang and Singh model (Arumuganathan et al., 2009) Logarithmic model (Wakchaure et al., 2010) and Midilli et al model (Tulek, 2011). Differences in variety and or processing conditions may account for the contrasting outcomes. Figures 2 and 3 compare the experimental moisture ratios to those predicted by the Page’s (for control, 0.5 kGy and 1.0 kGy) and Diffusion models (for 1.5 kGy and 2.0 kGy). These models showed very good fit between the experimental and predicted moisture ratios, confirming the suitability of these models for describing solar drying of ɣ-irradiated mushrooms. Effective moisture diffusivity The effective moisture diffusivity (Deff ) describes the rate of moisture movement in food (Okos et al., 2007). Deff varied between 1.88 and 2.44 x 10-08 m2 /s for the control and mushrooms treated with 2.0 kGy of ɣ-rays. The moisture diffusivity in the differently treated mushrooms increased with increasing dosage of ɣ-irradiation (Figure 4). Differences in effective diffusivities may be attributed to the extent of tissue disruption that may have occurred in mushrooms as a result of irradiation. Gamma irradiation causes breakage of fibrous structure and enlarges the pores therein (Akram et al., 2012) thus facilitating moisture removal. High diffusivity values as a result of increasing radiation exposure further emphasize the enhancement of moisture removal by this processing technology. Deff results obtained in this study were comparable to the generalized range of 10-9 – 10-12 for most foods (Labuza and Altunakar, 2007) higher than 1.55 – 4.02 x 10-09 m2 /s reported for milky mushrooms (Arumuganathan et al., 2009) and 9.62 – 1.56 x 10-09 m2 /s reported for oyster mushroom (Tulek, 2011) but lower than 9.21 x 10-08 m2 /s to 1.49x10-07 m2 /s for white button mushrooms (Wakchaure et al., 2010). These variations are likely to result from varietal and conditional differences adopted in these various studies. Conclusion The effect of irradiation as a pre-treatment prior to drying oyster mushroom slices was manifested in reduced drying time. Pre-treated slices dried faster Table 1. Drying models and selection criteria for best fit
- 5. Kortei et al./IFRJ 23(1): 34-39 38 than the control, with increasing dosage resulting in shorter drying time. Among the 5 thin layer models, Page’s model best predicted the drying characteristics of slices exposed to lower doses of ɣ-radiation, while Diffusion model gave the best results and adequately described the behavior of slices that received higher doses (1.5 and 2.0 kGy). Moisture diffusivity ranged between 1.88 and 2.44 x 10-08 m2 /s and was higher among the pretreated mushroom slices. Gamma irradiation appears to be a suitable pretreatment for drying mushrooms. References Addo, A., Bart-Plange, A. and Boakye, D.M. 2009. Drying characteristics of cap and stem of mushroom. Journal of Science and Technology 29: 88-95. Akindahunsi A.A. and Oyetayo, F.L. 2006. Nutrient and antinutrient distribution of edible mushroom, Pleurotus tuber-regium (Fries). LWT Food Science Technology 39: 548–553. Akonor, P.T. and Amankwah, E.A. 2012. Thin layer drying kinetics of solar-dried Amaranthus hybridus and Xanthosoma sagittifolium leaves. Food Processing and Technology 3: 174-178. Akonor, P.T. and Tortoe, C. 2014. Effect of blanching and osmotic pretreatment on drying kinetics, shrinkage and rehydration of chayote (Sechium edule) during convective drying. British Journal of Applied Science and Technology 4: 1215-1229. Akpinar, E.K., Bicer, Y. and Yildiz, C. 2003. Thin layer drying of red pepper. Journal of Food Engineering 59: 99–104. Akram, K., Ahn, J.J. and Kwon, J.H. 2012. Identification and characterization of Gamma-irradiated dried Lentinus edodes using ESR, SEM, and FTIR analyses. Journal of Food Science 77: 690-696. AOAC. 1990. Official methods of analysis. 13th edn. Washington DC: Association of Official Analytical Chemists. Arumuganathan, T., Manikantan, M.R., Rai, R.D., Anandakumar, S. and Khare, V. 2009. Mathematical modeling of drying kinetics of milky mushroom in a fluidized bed dryer. International Agrophysics 23: 1-7. Doymaz, I. and Gol, E. 2011. Convective drying characteristics of eggplant slices. Journal of Food Process Engineering 34: 1234-1252. Giri, S.K., and Prasad, S. 2007. Drying kinetics and rehydration characteristics of microwave-vacuum and convective hot-air dried mushrooms. Journal of Food Engineering 78: 512–521. Goyal, R.K., Kingsly, A.R.P., Manikantan, M.R. and Ilyas, S.M. 2007. Mathematical modeling of thin layer drying kinetics of plum in a tunnel dryer. Journal of Food Engineering 79: 176-180. Kortei, J.N.K. 2011. Growing oyster mushrooms (Pleurotus ostreatus) on composted agrowastes; An efficient way of utilizing lignocellulosic materials, p. 8-10. Germany: Lambert Academic Publishing. Kumari, D., Reddy, M. S. and Upadhyay, R. C. 2011. Nutritional composition and antioxidant activities of 18 different wild Cantharellus mushrooms of Northwestern Himalayas. Food Science and Technology International 17: 557-567. Labuza,T.P. andAltunakar, B. 2007. Diffusion and sorption kinetics of water in foods. In Barbosa-Canovas, G.V., Fontana, A.J., Schmidt, S.J. and Labuza, T.P. (Eds). Water activity in foods, Fundamental applications, p. 215-238. Oxford: Blackwell Publishing. Lindequist, U., Niedermeyer, T.N.J. and Julich, W.D. 2005.The pharmacological potential of mushrooms. Evidence Based Complementary and Alternative Medicine 2: 285-299. Mahajan, P.V., Oliveira, F.A.R. and Macedo, I. 2008. Effect of temperature and humidity on the transpiration rate of the whole mushrooms. Journal of Food Engineering 84: 281–288. Martinez-Soto, G., Ocanna-Camacho, R. and Paredes- Lopez, O. 2001. Effect of pretreatment and drying on the quality of oyster mushrooms (Pleurotos ostreatus). Drying Technology: An International Journal 19: 661- 672. Obodai, M., Cleland-Okine, J., and Vowotor, K. A. 2003. ComparativestudyonthegrowthandyieldofPleurotus ostreatus mushroom on different lignocellulosic by- products. Journal of Industrial Microbiology and Biotechnology 30: 146-149. Okos, M.R., Campanella, O., Narsimhan, G., Singh, R.K. and Weitnauer, A.C. 2007. Food dehydration. In: Heldman, D.R. and Lund, D.B. (Eds). Handbook of Food Engineering. 2nd edn. p. 600 – 744. Boca Raton: Figure 3. Model fit for 1.5 kGy (A) and 2.0 kGy (B) using diffusion model Figure4.Effectivemoisturediffusivityofdriedmushrooms
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