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Brazilian Journal of Chemical Engineering

Print version ISSN 0104-6632On-line version ISSN 1678-4383

Braz. J. Chem. Eng. vol. 16 n. 1 São Paulo Mar. 1999 



1Instituto Mauá de Tecnologia, Estrada das Lágrimas 2035, 09580-900, Tel.: (011) 741 3119, Fax.: (011) 741 3131, São Caetano do Sul, SP, Brazil.


(Received: April 24, 1998 ; Accepted: March 15, 1999)



Abstract - When the moisture content of the fermenting medium significantly decreases during laboratory-scale solid-state fermentation tests, the quantity of water to be periodically added to the medium in order to control its moisture content may be evaluated from the water evaporation rate of the non-inoculated medium.
Keywords: Solid-state fermentation (SSF), Rhizopus oryzae.




Solid-state fermentation (SSF) processes were almost ignored in Western countries during about forty years. This situation has changed in the last fifteen years owing to the high potential of SSF techniques in socioeconomic and/or profit-economic applications such as composting of wastes, ensiling of grasses, upgrading of ligno-cellulosic products or staple foods, and the production of enzymes, organic acids and fermented foods (Murthy et al., 1993). Among the factors that affect the results obtained in SSF processes, the moisture content of the fermenting medium plays an important rôle. Depending on the process, the moisture content must be maintained in a given optimum interval in order to obtain the desired results. Among the methods proposed to control the above parameter (Narahara et al., 1984; Thiemann, 1985; Daubresse et al., 1987; Ahmed et al., 1987; Durand et al., 1988; Menezes et al., 1989; Crooke et al., 1991; Soccol et al., 1994; Kolicheski, 1995), the periodic addition of water to the fermenting medium is frequently adopted (Thiemann, 1985; Menezes et al., 1989) mainly in laboratory-scale tests when the moisture content significantly decreases as fermentation proceeds. Based on the assumption that the moisture content decreasing rate of the fermenting medium is not greater than the water evaporation rate of the non-inoculated medium, the method proposed in this paper permits to evaluate the volume of water to be periodically added to the medium during the experiments.



Rhizopus oryzae ATCC 10404, cultivated on potato-dextrose-agar medium at 27ºC, was used in the experiments. The solid matrix was a sterilized (120ºC; 20 min) mixture of wheat bran (200 g) and ground corn (13 g). The nutrient solution was prepared by dissolving urea (6.4 g), citric acid (4.7 g), KH2PO4 (3.7 g) and MgSO4.7H2O (1.6 g) in 190 mL of previously sterilized (120ºC; 20 min) distilled water. The inoculum was prepared by suspending spores and hyphae of R. oryzae in the nutrient solution. The above suspension was carefully added to the solid matrix in order to obtain a well homogenized mixture. The incubation temperature and the relative humidity of the air were 27ºC and 90-93%, respectively.

The fermentation tests were carried out in non - perforated trays (Test 1) and in Petri dish covers (Test 2) under the following experimental conditions.

Test 1: tray diameter, 18 cm; initial mass of inoculated medium, 220g (average layer thick, 2.5 cm); initial moisture content of the medium, 53.7%; test duration, 32 h.

Test 2: dish cover diameter, 9 cm; initial mass of inoculated medium, 20g (average layer thick, 0.9 cm); initial moisture content of the medium, 47.9 %; test duration, 48 h.

With the only purpose to determine the water evaporation rates, non inoculated media (220 g in the tray, and 20 g in the dish cover) were incubated at 27ºC and was periodically weighed. The moisture contents were measured by drying the medium at 100-110ºC until it reached constant weight.



The experiments carried out to measure the water evaporation rates (tests duration, 50 h) lead to equations (1) (when the tray was used) and (2) (when the Petri dish cover was used), where M is the mass (g) of the system at time t (h), and r is the correlation coefficient.

M = 397.78 - 1.03 t (r = -0.9997) (1)

M = 72.17 - 0.17 t (r = -0.995) (2)

The above equations show that the water evaporation rates were 1.0 g.h-1 when the tray was used, and 0.17 g.h-1 when the Petri dish cover was used. Based on the above values, the moisture content of the fermenting medium was controlled by carefully distributing on the fermenting medium surface 8.0 mL of water at 8 h time intervals in Test 1, and 1.0 mL of water at 6 h time intervals in Test 2. In Test 1, the moisture content varied from 53% to 59% (average of 9 measurements, 56.0%; standard deviation, 2.3 %). In Test 2 the moisture content varied from 46% to 53% (average of 12 measurements, 49.4 %; standard deviation, 2.6 %). Depending on the process, the results obtained in a preliminary test may indicate the necessity of a better control of the medium moisture content by periodically addition of lower volumes of water, mainly when the microbial activity produces relatively high quantities of water.



The authors acknowledge the technical assistance of Renato Piplovic.



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