Influence of water activity on growth and activity of Aspergillus niger for glycoamylase production in solid-state fermentation

Growth of Aspergillus niger and glucoamylase production correlated well with the water activity of the substrate (wheat bran plus corn flour) in a solid-state fermentation. Both were maximal at an initial water activity of 0.936. Glycoamylase reached 550 units/g dry substrate after 96 h.The authors are with the Biotechnology Unit, Regional Research Laboratory, CSIR, Trivandrum-695 019, India     

Modelling the effect of temperature and water activity on growth of Aspergillus niger strains and applications for food spoilage moulds

AIMS: To develop a model for the combined effect of water activity (a(w)) and temperature on growth of strains of Aspergillus niger, and comparison with data on food spoilage moulds in the literature. METHODS AND RESULTS: An extended combined model describing the growth of two strains of A. niger, as a function of temperature (25-30 degrees C) and a(w) (0.90-0.99) was developed. The growth rate (micro) was expressed as the increase in colony radial growth per unit of time. This extends the previous square root model showing the relationship between temperature and bacterial growth rate developed by Ratkowsky et al. (1983) and the parabolic relationship between the logarithm of the growth rate and a(w) developed by Gibson et al. (1994). A good correlation between the experimental data and the model predictions was obtained, with regression coefficients (r(2)) > 0.99. In addition, the use of this model allowed predictions of the cardinal a(w) levels: a(w(min)), and a(w(opt)). The estimation of the minimum a(w) levels (a(w(min))) was in accordance with data in the literature for similar and a range of other Aspergillus and related species, regardless of the solutes used for a(w) modification. The estimation of the optimal a(w) (a(w(opt))) and the optimal growth rate (micro(opt)) were in good agreement with the experimental results and data from the literature. CONCLUSIONS: This approach enables accurate prediction of the combined effects of environmental factors on growth of spoilage fungi for rapid prediction of cardinal limits using surface response curves. SIGNIFICANCE AND IMPACT OF THE STUDY: This approach is a rapid method for predicting optimal and marginal conditions for growth of a wide range of spoilage micro-organisms in relation to interacting environmental conditions and will have applications for improving shelf-life of intermediate moisture foods.     

Influence of the interaction of temperature and water activity on the production of ochratoxin A and the growth of Aspergillus niger, Aspergillus carbonarius and Aspergillus ochraceus on coffee-based culture medium

In the present study, the effect of temperature and water activity on fungal growth and ochratoxin production on coffee-based medium was assessed. Optimal growth of three Aspergillus strains was observed in the same ecological conditions, namely 30 degrees C and 0.99 water activity. Maximal daily growth is 11.2, 6.92, and 7.22 mm/day for Aspergillus niger, Aspergillus carbonarius, and Aspergillus ochraceus, respectively. However, ecological conditions for optimal ochratoxin production vary according to the toxinogenic strain, with water activity as a limiting factor. Such an ochratoxin A production is inhibited at 42 degrees C and 0.75 water activity. Correspondence between laboratory tested water activity and that measured on a sun-dried ripe cherry batch shows that the first 5 days of drying are critical for fungal growth and ochratoxin A production. Accordingly, artificial drying of cherries at temperatures above 42 degrees C will impede not only fungal growth but also contamination with ochratoxin A.     

Comparison of dichloran rose bengal chloramphenicol and Sabouraud dextrose agar with cycloheximide and chloramphenicol for airborne mold sampling

The more the mold species isolated on a culture medium, the more the sampling environment is represented accurately. According to the sampling purpose, it is crucial to use the best culture medium for mold. However, no study is available regarding the comparison of dichloran rose bengal chloramphenicol (DRBC) and Sabouraud dextrose agar with cycloheximide and chloramphenicol (SDA-CHX-CHL) culture media in terms of their application for airborne sampling, isolation, and identification of fungi. Airborne mold samples were impacted onto both DRBC and SDA-CHX-CHL, simultaneously using single-stage Andersen sampler. The limit of detection (LOD) value for airborne mold count was 7 CFU m^?3 (1 colony growth on the Petri dish). The total mold counts (TMC) ranged between <7 and 504 CFU m^?3 (med 56 CFU m^?3) and <7 and 1218 CFU m^?3 (med 259 CFU m^?3), collected on SDA-CHX-CHL and DRBC, respectively. Significantly higher TMC were observed on DRBC than on SDA regardless of the sampling environment (i.e, indoor or outdoor) (p < 0.05). Among the most predominant mold genera, observation frequencies of Penicillium spp. and Aspergillus spp. on both culture media were found to be more than 70%. Observation frequencies of Cladosporium spp., Alternaria spp., and yeast were found to be higher in samples collected on DRBC than those on SDA-CHX-CHL. Finally, DRBC was found to be superior to SDA in terms of both number of colonies and number of genera isolated from the air.     

Combined effect of water activity depression and glucose addition on pectinases and protease production by Aspergillus niger

he combined effect of depressed water activity (a) and glucose addition on the production of extracellular pectinases and protease by Aspergillus niger 148 was studied. A differential response in the production of enzyme activities was observed. Glucose (0.7%) acted as catabolite repressor for all the enzymes at a=1 and at a=0.975 for pectinesterase, pectinlyase and protease. For polygalacturonase at a=0.975 a synergistic stimulating effect was observed. By depressing a a pectinase complex containing mainly pectinlyase and polyglalacturonase could be obtained.     

Morphometric evaluation of the specific growth rate of Aspergillus niger grown in agar plates at high glucose levels

Development of surface grown cultures of Aspergillus niger no. 10 was studied at two experimental levels: (a) following the time course of the biomass density (X [=] mg cm(-2)) and fitting the data by the logistic expression, which yielded a macroscopic specific growth rate expressed as mu(obs) = (dX/Xdt)[1-(X/X(max))](-1); and (b) measuring morphometric parameters like the specific elongation rate (k) of the germ tubes and their diameters (D(h)), the colony rate of radial extension (u(r)), and the mean length of distal hyphae (L(av)) to estimate the specific growth rate with the following proposed expression: mu(calc) = u(r)ln2[L(av)ln(L(av)/D(h))](-1). Increases in the initial glucose concentration (10, 40, 70, 120, 200, and 300 g L(-1)) caused reductions in the specific growth rates, the elongation kinetics of the germ tubes, and the hyphal diameter, nevertheless, u(r) and X(max) presented parabolic behavior, showing their maxima in the interval of 90 to 120 g L(-1) of glucose. The overall macroscopic effect of the tested concentrations of glucose on surface grown cultures of A. niger was to produce densely packed and slowly extending colonies, where changes in hyphal lengths and diameters were significant. There was good agreement between mu(obs) and mu(calc) values. Hence, this work validates a kinetic model based on morphometric data to estimate the specific growth rate of molds, obtained from dry weight data, using mold cultures grown in the same solid medium i.e., agar plates. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 287-294, 1997.     

Esterase activity of Aspergillus niger in continuous culture

In citrate limiting medium the esterase activity of Aspergillus niger had a maximum value at the lowest dilution rate (D=0.013 h^-1) and at all higher dilution rates progressively decreased in activity. In glucose limiting medium the esterase activity values were always lower than in citrate limiting medium and did not show much variation with varying dilution rate. Electrophoresis of cell free extracts from all dilution rates revealed a multimolecular esterase profile only at D=0.013 h^-1 in citrate limiting medium, which was also the only dilution rate to support good conidiation. The increase in esterase activity at D=0.013 h^-1 was observed cytochemically to occur in the phialides. No cytochemical esterase staining occurred in the vegetative cultures at all other dilution rates.     

Synergistic action of both Aspergillus niger and Burkholderia cepacea in co-culture increases phosphate solubilization in growth medium

Co-inoculation of the fungus Aspergillus niger and the bacterium Burkholderia cepacia was undertaken to understand the interaction between different species of phosphate-solubilizing microorganisms (PSM). PSM were inoculated in a single or mixed (A. niger-B. cepacia) culture. During 9 days of incubation, microbial biomass was enhanced, accompanied with increases in the levels of soluble phosphate and titratable acidity, as well as increased acid phosphatase activity. Production of acids and levels of phosphate solubilization were greater in the co-culture of A. niger-B. cepacia than in the single culture. The quantity of phosphate solubilized by the co-culture ranged from 40.51 ± 0.60 to 1103.64 ± 1.21 μg PO(4) 3- mL(-1) and was 9-22% higher than single cultures. pH of the medium dropped from 7.0 to 3.0 in the A. niger culture, 3.1 in the co-culture, and 4.2 in the B. cepacia culture. On the third day of postinoculation, acid production by the co-culture (mean 5.40 ± 0.31 mg NaOH mL(-1)) was 19-90% greater than single cultures. Glucose concentration decreased almost completely (97-99% of the starting concentration) by the ninth day of the incubation. These results show remarkable synergism by the co-culture in comparison with single cultures in the solubility of CaHPO(4) under in vitro conditions. This synergy between microorganisms can be used in poor available phosphate soils to enhance phosphate solubilization.     

Rock phosphate solubilization by Aspergillus niger grown on sugar-beet waste medium

Solubilization of rock phosphate by Aspergillus niger was studied in solid-state fermentation on sugar-beet waste. This combination was selected after testing three agroindustrial waste materials, namely rice hulls, sugar-beet waste and alperujo. Sugar-beet waste was the best substrate for fungal growth with 69% mineralization, followed by rice hulls and alperujo. The fungus was successfully cultivated on sugar-beet waste supplemented with 3.0 g/l rock phosphate, acidifying the medium and thus decreasing the pH to 3–3.5. Solubilization of insoluble phosphate increased during the first half of the process, reaching a maximum of 292 g phosphate/ml, although a part of it was probably consumed by the mycelium.     

黑曲霉对黄曲霉生长、产毒及黄曲霉毒素B1的影响

目的研究黑曲霉对黄曲霉生长、产毒的抑制作用及对AFB1的降解作用。方法将黑曲霉分别与黄曲霉、AFB1共同培养,定期测定培养液pH、菌丝体干重、黄曲霉孢子数、AFB1含量。结果黑曲霉与黄曲霉混合培养时,黄曲霉孢子数、AFB1含量均比单独培养的低,2组之间差异有统计学意义(P<0.05),抑制率达到68.06%~91.52%;加入黑曲霉后,AFB1含量降低,实验组与对照组之间差异有统计学意义(P<0.05),降解率为46.19%。结论黑曲霉既能抑制黄曲霉生长、产毒,又能降解AFB1。     

肉桂醛、柠檬醛抑制黑曲霉生长的比较研究

黑曲霉是一类极易通过饲料、食品、粮油霉变而具致病性的有害真菌。与物理和化学方法抑制黑曲霉生长相比,生物抑菌剂抗黑曲霉生长具有药效久、无抗药性并安全健康的优点。本实验采用天然肉桂醛、柠檬醛作为抑菌剂,以正常生长的黑曲霉为对照,分别采用牛津杯法、气体扩散法比较对黑曲霉生长效果的影响。结果表明,柠檬醛作用所形成的抑菌圈显著大于肉桂醛作用所形成的抑菌圈,且在同一浓度下柠檬醛对菌丝体形态和孢子囊形态的抑制比肉桂醛显著,而气体扩散法抗黑曲霉效果优于牛津杯法。     

Water and water activity in the solid state fermentation of cassava starch by Aspergillus niger

Summary During the solid state fermentation (SSF) of cassava starch by Aspergillus niger estimations were made of total water, consumed water and the residual water remaining in small quantities after 23 h. A theoretical calculation based on the Ross equation showed that the water activity (a _w) of the substrate decreased to 0.85 towards the end of the culture. Such low values were assumed to be inhibitory to growth. The a _w of the substrate was increased when sugarcane bagasse was used as a high water retention capacity support. Higher growth rates and substrate conversion to biomass were obtained with this system, confirming that water availability is a critical factor in the SSF of starch substrates.Abbreviations A, B Experimental constants - a _w Water activity - H₂O_c Consumed water - H₂O_R Residual water - H₂O_T Total water - IDW Initial dry weight - IMC Initial moisture content - OUR Oxygen uptake rate - S Substrate dry weight - Sc Substrate conversion: consumed substrate/initial substrate - S _H Amount of sugars hydrolysed - SSF Solid state fermentation - X Biomass dry weight - W ^* Amount of solids/g of water