Drying and rehydration of oyster mushroom

FOOD/FEED SCIENCE AND TECHNOLOGY

Drying and rehydration of oyster mushroom

Giannini Pasiznick Apati^I,II; Sandra Aparecida Furlan^I,*; João Borges Laurindo^II

^IUniversidade da Região de Joinville; Campus Universitário, s/n,; 89223-251; Joinville - SC - Brasil

^IIUniversidade Federal de Santa Catarina; C. P.: 476; 88040-900; Florianópolis - SC - Brasil

ABSTRACT

Dehydration and rehydration processes of Pleurotus ostreatus fruiting bodies were investigated in this work. Mushroom samples were dehydrated at 40, 50 and 60 ºC, using drying air with relative humidity of 75 %. The rehydration was investigated at different temperatures of immersion water (25, 55 and 85 ºC) and different immersion times (30, 75 and 120 minutes). The best rehydration occurred for the samples dried at 40 ºC. The rehydration could be done in water at room temperature, during 30 minutes. Water sorption isotherms of samples were determined at 30, 40 and 50 ºC. Both GAB and BET models satisfactorily represented the experimental data of moisture sorption of dried mushrooms.

Key words:Pleurotus ostreatus, dehydration, rehydration, moisture sorption isotherms

RESUMO

Processos de desidratação e de rehidratação de cogumelos da espécie Pleurotus ostreatus foram avaliados neste trabalho. Os cogumelos foram desidratados a 40, 50 e 60 ºC, com umidade relativa do ar de 75 %. O processo de rehidratação foi avaliado para diferentes temperaturas de água de imersão (25, 55 e 85 ºC) e diferentes tempos de imersão (30, 75 e 120 minutos). A melhor temperatura de secagem foi 40 ºC, levando em consideração a melhor rehidratação dos cogumelos desidratados nesta temperatura. A rehidratação pode ser feita em água a temperatura ambiente, por 30 minutos. Isotermas de sorção de umidade de amostras foram determinadas a 30, 40 e 50 ºC.Tanto o modelo de GAB quanto o de BET representaram satisfatoriamente os dados experimentais de isoterma de sorção de umidade.

INTRODUCTION

Large amounts of wastes are generated by the agricultural sector, forest exploitation and food industries, which are treated or sent to sanitary landfills. Searching the equilibrium between the social, economic and environmental aspects, the reuse of agricultural wastes has taken on an extremely important dual purpose: elimination or reduction of wastes from the environment and giving them added value through the production of low cost food (Villas-Bôas et al., 2002). One of the strategies developed for the disposal of large amounts of lignocellulosic wastes is the production of edible mushrooms (Chang and Miles, 1992). Recent studies have employed agricultural and forest wastes to optimize the biological, chemical and physical parameters for mushroom cultivation (Leifa, 2002; Chaves, et al., 2004; Gern, 2005; Furlan et al., 2006). Pleurotus spp., commonly known as oyster fungi, is a common primary degrader of wood and vegetable residues (Zadrazil and Kurtzman, 1981). It can be found in tropical and subtropical regions and in rainforests, and can also be artificially cultivated (Maziero et al., 1992). Appreciated because of its delicious taste, these fungi contain high quantities of proteins and carbohydrates, minerals (calcium, phosphorus, iron, etc), and vitamins (thiamin, riboflavin and niacin) as well as low fat content (Sturion and Oetterer, 1995; Justo et al., 1998; Manzi et al., 1999). According to Silveira et al. (2006), the energy values of P. ostreatus DSM 1833 is between 139.36 and 213.05 kcal/100g of mushrooms.

For many reasons, the fungi of Pleurotus genus have been intensively studied in many different parts of the world: high gastronomic value, ability to colonize and degrade a large variety of lignocellulosic residues, shorter growth time when compared to other edible mushrooms; little environmental control, few diseases and pests attack fruiting bodies, simple and cheap cultivation technique (Jwanny et al., 1995; Patrabansh and Madan, 1997).

The mushrooms of the Pleurotus genus are more delicate and sensitive than the Agaricus genus and they start deteriorating immediately within one day after the harvest. Once deteriorated, these fruiting bodies can cause severe gastrointestinal discomfort. Under ideal climatic conditions, shelf life of these mushrooms is about 10 days, their quality being affected predominantly by storage temperature. The shelf life can be reduced from 9 days at 2 ºC to 3 days at 18 ºC (Lukasse and Polderdijk, 2003). Therefore, cooling the fresh mushrooms can be an alternative regarding their distribution and sale, thus increasing their shelf life (Villaescusa and Gil, 2003). For long periods of conservation, the traditionally used method for Pleurotus genus mushrooms is the convective drying at 45-65 ºC (Pal and Chakraverty, 1997; Arora et al., 2003). Dehydration is a classical method of food conservation, based on the principle that the reduction of the water activity of the product must be conducted until defined levels that guarantee the microbiological and physicochemical stability (Cao et al., 2003; Krokida et al., 2003; Lewicki and Jakubczyk, 2004).

The dehydrated mushrooms can be rehydrated by water immersion before the consumption. The rehydration characteristics of dried products are used as a quality parameter and indicate if physical and chemical changes occurred during the drying process due to process conditions, pre-treatments and sample composition (Funebo and Ohlsson, 1998; Lewicki, 1998). Funebo and Ohlsson (1998) studied the rehydration capacity of Agaricus bisporus fruiting bodies after hot-air dehydration, followed or not by microwave at the end of the dehydration process. The power of the applied microwaves was measured through the temperature of the food center. Rehydration capacity was better using hot-air dehydration without the use of microwaves.

Based on the above discussion, this work dealt with the evaluation of a conservation process for Pleurotus ostreatus fruiting bodies. Mushroom dehydration was performed in an oven with forced air circulation and their rehydration capacity was investigated. Due to their importance in food storage, water sorption isotherms were determined for fresh whole mushrooms, dehydrated mushrooms and powdered dehydrated mushrooms.

MATERIALS AND METHODS

Microorganism and maintenance

P. ostreatus DSM 1833, was used in this work. The culture was kept in Petri dishes containing WDA medium (1 L of wheat extract, 20 g of dextrose and 15 g of agar) at 4 ºC. The wheat grains were washed and cooked in boiling deionised water for 10 min at a ratio of 1:2 (wheat grain mass: water volume), then the liquid drained were used as a wheat extract (Furlan et al., 1997).

Inoculum production (spawn)

Wheat grains were used as substrate and growth support. The cooked grains, obtained as described above were supplemented with 0.35% of calcium carbonate and 1.3% of calcium sulphate (dry mass basis) and then packed (250 g) in polypropylene (PP) bags (18x30 cm). The bags were sterilized at 121 ºC for 1 h, cooled to room temperature and inoculated with three 15 mm diameter agar disks containing the Pleurotus mycelium taken from the Petri dishes. The bags were incubated at 30 ºC, in the absence of light, for 15 days or until complete colonization of the grain surfaces by the mycelium. The bags containing the solid inoculum were kept for three months at 4ºC for further use.

Culture medium and environmental conditions for fruiting body formation

Banana straw, consisting of banana plant leaves, was used as cultivation substrate. The banana straw were ground into 2 to 5 cm particles, dried at 60 ºC for 1 h and packed in raffia bags. The bags containing the straw were kept under immersion in water for 12 h according to Madan et al. (1987). Afterwards, 150 g (dry weight) of the substrate were transferred to transparent PP bags (50µ thick). The substrate was supplemented with 5 % of rice bran (dry weight), closed, sterilized in an autoclave at 121 ºC for 1h, cooled to environmental temperature under UV radiation for 30 minutes, inoculated with 10 % (dry weight) spawn and homogenized. The bags were incubated at 30 ºC until total colonization. The bags were transferred to the cultivation room (24 m²) for fruiting body production (27 ºC, light deviation between 500 and 1000 lux, 12 h a day, and air humidity equal to 88% were automatically controlled and the air exchange was ensured by air conditioning). Primordia were induced by making small perforations in the bags. Fruiting bodies were harvested with a scalpel as described by Sturion (1994).

Drying

After harvest, 50 g of mushrooms were dehydrated in an oven with forced air circulation at 40, 50 and 60 ºC and relative air humidity of 75 %. The samples were placed in Petri dishes and hanged to a weighing apparatus, to determine the drying curves. The sample mass was registered each 15 minutes. The determination of the sample humidity was carried out by weighing the samples before and after drying at 90 ºC for 48 h (gravimetric method).

Rehydration

The rehydration parameters were evaluated using an experimental design 3^3-1, in which the drying temperature (40, 50 and 60 ºC), the water immersion temperature (25, 55 and 85 ºC) and the water immersion time (30, 75 and 120 minutes) were the investigated variables. This resulted in nine experiments, which were carried out in duplicate. In each experiment 2.0 g dried mushroom were immersed in 100 mL of water. After rehydration, excess water was drained, the samples dried with absorbent paper and weighed again.

Water sorption isotherms

Water sorption isotherm experiments were performed at 30, 40 and 50 ºC for mushrooms fresh, dehydrated mushrooms and powdered dehydrated mushrooms, using the saline solutions static method (Rizvi, 1986). Two grams of sample were placed in porcelain crucibles kept in desiccator and periodically weighed until constant weight. After 25 days, the mushrooms reached equilibrium. The GAB (Guggenheim, Anderson and De Boer) and BET (Brunauer, Emmett and Teller) models, given by equations 1 and 2, respectively, were used to represent the experimental isotherms. The models' parameters were determined by a non-linear regression, using the software Statistica^® 6.0 for Windows^® (Statsoft, Tulusa, UK).

where, a_w is the water activity, X_m is the value of the monolayer moisture in dry basis (gw/g), C is the sorption heat related to the adsorbed monolayer, K is the sorption heat related to the multilayers and X_e represents the equilibrium moisture, in dry basis.

Statistical analysis

The drying tests were carried out in triplicate and the mean values and standard errors were calculated using the software Origin^® 7.0. In order to investigate the parameters that influenced the rehydration process, the software Statistica^® 6.0 was used to the Pareto analysis with 5% significance level.

RESULTS AND DISCUSSIONS

Drying

Figure 1 (a) presents the drying curves for P. ostreatus fruiting bodies. The curves' slopes indicate the presence of a small drying period at constant rate, at the beginning of the process. It was better observed in the drying rate curves (Fig. 1b). The duration of constant rate were about 65 minutes at 40 ºC, 35 minutes at 50 ºC and 25 minutes at 60 ºC. Martínez-Soto et al. (2001) had also observed a very small period of constant drying rate (5 minutes) during P. ostreatus fruiting body drying. However, some authors have not observed the existence of a constant rate period during dehydration of P. florida (Arora et al., 2003). As expected, an increase in the drying speed with the increase of drying temperature was observed. This implied in a reduction of 43.7 % in the process time when the drying temperature changed from 40 to 60 ºC and a reduction of 28.6 % when the temperature changed from 50 to 60 ºC. Similar results have been found in the literature (Arora et al., 2003; Krokida et al., 2003) for A. bisporus and P. florida fruiting body drying. Pal and Chakraverty (1997) found reduction of about 40 % in the process time, when the drying temperature of P. ostreatus fruiting bodies was increased from 45 to 60 ºC. The visual quality of the dehydrated mushrooms was not affected by the drying temperatures investigated in this work.

Rehydration

For mushrooms, besides the investigation of the drying times and rates, the product rehydration capacity and quality should also be evaluated. Figure 2 presents the visual comparison between fresh, dehydrated and rehydrated mushrooms. It was observed that the rehydrated mushrooms had good appearance, even so they did not recover the same appearance of fresh mushrooms, because they were not completely rehydrated and suffered some changes under the drying conditions.

According to Pareto charts presented in Figure 3, the rehydration capacity was related to the drying temperature, which presented significant effects. The immersion time in water and the water rehydration temperature did not present a significant influence on the fruiting body rehydration capacity. However, the drying temperature increase had a negative influence on the fruiting body rehydration capacity. The rehydration capacity decreased with increasing drying temperature, which could be associated to the stronger mushroom structure deformation at higher temperatures. According to Foust et al. (1982), increasing drying temperature leads to the increase of water liberation rate, then promoting important structure deformations in the biological material. Thus, considering the rehydration performance, the lower drying temperature (40 ºC) could be suggested for Pleurotus dehydration.

Water sorption isotherms

The parameters of GAB and BET models for 30, 40 and 50 ºC, as well as the correlation coefficients (R), are given in the Table 1. Both models represented satisfactorily the experimental data of moisture sorption. It was observed that K values in the GAB model had always been very near to 1, which could make this model equivalent to BET model. There are no reports in the literature, concerning the use of BET model for mushroom sorption isotherms. However, Belarbi et al. (2000), testing GAB and BET models for several date species, observed that in some cases, the BET model could represent the experimental data as well as GAB model.

Figure 4 presents sorption isotherms obtained at 30, 40 and 50 ºC. The equilibrium humidity decreased with the increase of temperature. This was because the kinetic energy associated with the molecules of water present in the food increased with increasing temperature (Shivhare et al., 2004). At high water activities, this was no longer be observed. Significat differences in the equilibrium humidity of fresh, dehydrated and powdered dehydrated mushrooms were not observed, independent of the temperature used.

CONCLUSIONS

The drying temperature is a very important process variable in mushroom drying. The best temperature for P. ostreatus fruiting bodies drying process was 40 ºC, taking into consideration the best rehydration of the fruiting bodies dried at this temperature. The rehydration could be done in water at room temperature during 30 minutes. Both GAB and BET models represented satisfactorily the experimental data of moisture sorption.

ACKNOWLEDGEMENTS

Giannini Pasiznick Apati would like to thank CAPES/Acafe for the financial support. João Borges Laurindo thanks CNPq for PQ research scholarship.

Received: March 17, 2008; Revised: February 12, 2009; Accepted: October 01, 2009.

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Publication Dates

• Publication in this collection
  06 Aug 2010
• Date of issue
  Aug 2010

History

• Reviewed
  12 Feb 2009
• Received
  17 Mar 2008
• Accepted
  01 Oct 2009