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1. Introduction
Gluconic acid is a mild organic acid that has gained much interest as it has many industrial applications such as in the pharmaceutical, food, animal feed, textile and leather industry (Singh, Pereira, & Singh, 1999). It is also applied as additive in cement to control the setting time and increase strength and water resistance.
Gluconic acid can have further applications for the solubilization of phosphate (Fenice et al., 2000, Rodriguez et al., 2004, Vassilev et al., 2001) and as cement additive (Hustede et al., 1989, Singh, 1976). Gluconic acid is a noncorrosive, nonvolatile, nontoxic mild organic acid so it imports a refreshing sour taste in many food items. In the European Parliament and Council Directive No. 95/2/EC gluconic is listed as a generally permitted food additive (E574). The US-FDA (Food and Drug Administration) has assigned sodium gluconate a GRAS (generally recognized as safe).
The overall demand of this organic acid has been increased for almost 20 years and recently production is amounting to more than 60,000 tons per year and still growing (El-Enshasy, 2003, Singh et al., 1999). Commercially, gluconic acid is produced by three different methods; chemical oxidation of glucose with a hypochlorite solution (Kundu & Das, 1984), electrolytic oxidation of glucose solution containing a known value of bromide (Amberkar, Thadani, & Doctor, 1965), or fermentation process where specific microorganisms are grown in medium containing glucose and other ingredients (Hill and Robinson, 1988, Lee et al., 1998, Shah and Kothri, 1993). The microbial fermentation processes offer attractive techniques for the gluconic acid production to alleviate the problems related to chemical production such as the inevitable side reactions and also to further economize the bioprocess (Singh et al., 1999, Velizarov and Bechkov, 1994).
A wide group of microorganisms particularly filamentous fungi have the ability for gluconic acid production (Cochrane, 1958, Lockwood, 1975). The production of gluconic acid is mainly done in batch cultivation using several species belonging to the following fungal genera, Aspergillus, Penicillium, Fusarium, Mucor and Gliocladium (Lockwood, 1975, Petriuccioli et al., 1994, Rosenberg et al., 1992, Singh et al., 2001a). Among the different fungal genera, it has been reported that the accumulation of large amounts of the gluconic acid and its salts are restricted to certain species of Aspergillus especially Aspergillus niger which considered as the most industrially important gluconic acid producer in fermentation industry (El-Enshasy, 2003, Roukas, 2000, Sankpal et al., 1999, Sankpal et al., 2001).
A large quantity of raw fruit materials during storage undergo decomposition and generate a waste that may cause environmental pollution. Utilization of these waste materials can be a part of environmental pollution control on one hand and production of value added products of commercial significance on the other, thus changing their status from waste to potential provider.
Agro-food byproducts such as grape-must, banana-must and sugarcane molasses contain high concentrations of sugars and can be considered as potential substrates that are easily available and economical waste carbohydrate sources for gluconic acid production by different fungal species. Gamma-irradiation affects the activity of some fungal species during fermentation processes. Chakravarty and Sen (2001) showed that low dose of ionizing radiation on microorganisms is responsible for accelerated enzyme activity.
Gherbawy (1998) showed that the lowest dose of gamma irradiation (1 MilliCurie for 10 min) enhanced three isolates of A. niger, investigated to produce more biomass and polygalactronase, pectinmethylglacturonase, cellulase and protease. Haggag and Mohamed (2002) indicated that Trichoderma harzianum, Trichoderma viride and Trichoderma koningii irradiated with 0.5 kGy dosage resulted in the highest percentage of pathogen growth reduction by producing highly active exo-enzymes.
Afify, Abo El-Seoud, Ibrahim, and Bassam (2013) indicated that the biomass of Trichoderma spp. was increased and reached its maximum at 250 Gy and as a general trends, the gamma radiation over than 0.25 KGy reduce the growth of Trichoderma spp.
The present study is aimed at evaluating some economical wastes as grape-must, banana-must and sugarcane molasses as a sole source of carbon in the fermentation process by using some gamma-irradiated fungal species for the gluconic acid production.
2. Materials and methods
2.1. Isolation and identification of organisms
Different fungal isolates were obtained from cultivated soil samples and waste materials of sugarcane processing from Hawamedia Distilleries Factories. Also some other organisms were isolated from wastes of the grape-must and banana-must collected from the fruit local market in 6 October City, Cairo. The dilution plate method described by Johnson, Curt, Bond, and Fribourgy (1959) and Czapek's Doxs Agar medium (Oxoid Limited, 1982) supplemented with rose Bengal (1/15,000, W/V) as bacteriostatic agents (Smith & Dawson, 1994) was used for isolation of fungi. For the isolation, plates were incubated at 28 ± 2 °C for 7 days and developing fungi were purified and identified by macro and microscopic characteristics using the following references (Barron, 1998, Carmichael et al., 1980, Domsch et al., 1980, Gilman, 1957, Nelson et al., 1983, Paper and Fennell, 1977). Isolated fungi were maintained on potato dextrose agar (PDA) slants and incubated at 30 °C for 7 days. The slants were stored at 4 °C and sub-cultured every month. The spore suspension was prepared by suspending the spores on the slant in 10 mL of sterilized saline solution.
2.2. Fermentation technique
Gluconic acid fermentation was carried out by submerged fermentation in 250 mL cotton wool plugged Erlenmeyer flasks with 50 mL of fermentation media of Czapek's Dox Broth consisted of (g/L) sucrose 30.0, NaNO3 3.0, KH2PO4 1.0, MgSO4·7H2O 0.5, KCl 0.5 and FeSO4·7H2O 0.01 having pH 6.0. The medium was modified by substituting sucrose with 120 g/L glucose from each previously diluted substrate type, i.e. grape-must, banana-must and crude molasses.
2.3. Preparation and purification of grape-must
Market-refused red grapes (100% ripened) that did not meet with the quality norms were used in fermentation reaction for gluconic acid production. Clarification of grape-must was followed as described by Grassim and Fauquembergue (1996) with slight modifications. Briefly decomposed and market-refused grapes were collected (1 kg) and mixed with eleven double, distilled water.
These were then steamed, crushed and heated at 80 °C for 30 min for release the red color from the grape skin and to inactivate the endogenous polyphenol oxidase. Material that obtained was filtered through muslin cloth and the juice that emerged was considered as grape-must, which has then diluted to give 10–12% sugar concentration and used for gluconic acid fermentation.
2.4. Preparation and purification of banana-must
Market-rejected yellow rotten banana that did not meet quality norms for consumption was utilized as the substrate for gluconic acid fermentation. Preparation and clarification of banana-must was followed as described by Grassim and Fauquembergue (1996). Briefly, the rotten bananas (1 kg) were peeled, ground and blanched in double distilled water. The obtained slurry was heated at 85 °C for 2–3 min to inhibit polyphenol oxidase.
Potassium metabisulphite (100 μM) was then added to prevent browning. The slurry was subjected to vacuum filtration and the free run juice thus collected was referred to as banana-must.
2.5. Clarification of molasses
Crude molasses was found to contain high concentrations of heavy metals and other compounds that inhibited gluconic acid fermentation, hence it was treated with hexacyanoferrate (HCF) prior to use. The crude sugarcane molasses (1 kg), obtained was diluted 4–5 times with deionized water and passed through, a bed activated charcoal for decolourization. HCF (3.8 mM) was added to the decolorized molasses at pH 4.0–4.5, followed by heating at 70–90 °C for 15 min. The precipitate formed containing metallic complex was removed by filtration, and the filtrate was referred as treated sugarcane molasses. The pH of clarified molasses was adjusted to 4.5 before its use for gluconic acid fermentation.
2.6. Dry biomass estimation
The content of each flask was filtered and the mycelial residues were washed with distilled water. These mycelial residues were dried in an oven for 24 h at 60 °C till their weight are to be constant and the dry biomass was calculated in g/L of fermentation medium (Singh et al., 1999). The culture broth was pooled and the volume measured. The aliquots were also used for biochemical analysis.
2.7. Gamma irradiation
Slants of the tested fungal species were exposed for doses of 0.1, 0.2, 0.3, 0.4 and 0.5 kGy of gamma rays using 60Co gamma cell at the Nuclear Research Center, Inshas, Egypt. The dose rate at the time of radiation treatment was 3.116 kGy/h.
2.8. Biochemical analysis
Assay of total acidity (T.A.): The total acidity of the culture filtrates was determined by titration against standard alkaline solution (El-Ktatney, 1978, Peppler, 1967) using phenolphthalein as an indicator.
2.9. Detection of gluconic acid
The gluconic acid produced was determined qualitatively and quantitatively by chromatographic analysis (Koepsell et al., 1952, Singh et al., 1999).
2.10. Statistical analysis
Results were expressed as the mean ± standard deviation (SD). Statistical significance was evaluated using analysis of variance (ANOVA, SPSS software version 22) test followed by the least significant difference (LSD) test at 0.05 level.
3. Results and discussion
3.1. Screening fungal isolates for gluconic acid production
Data represented in Table 1 showed that 21 fungal isolates were identified and tested for its ability to produce gluconic acid on Czapek's Dox broth medium. Most of the used isolates have been recorded as acid producers by the aid of compendium of soil fungi (Domsch et al., 1980), in accordance with our screening, several species of Aspergillus and Penicillium, in addition to some Mucorales were exhibited acid production by alkaline titration ranged from (20–2210 mL NaOH/L medium) and dry biomass ranged from (0.95–14.08 g/L medium). Ten selected fungal species showed positive results after paper chromatography analysis of their culture filtrates, while the other eleven fungal species were exhibited negative results for gluconic acid production on Czapek's Dox broth. A similar system to screen diverse fungi for their metabolic activities and in accordance with our results was carried out by Temash and Olama (1999), considered A. niger as a very strong producer of gluconic acid in addition to other known Aspergillus spp. and Penicillium spp. Also in agreement with our results Roukas (2000), Madhavi et al., 1999, Sankpal et al., 2001, El-Enshasy, 2003 and Shindia, El-Sherbeny, El-Esawy, and Sherin (2006) reported that among the different fungal genera that can accumulate a large amounts of gluconic acid are restricted to many species of Aspergillus, especially A. niger which considered as the most industrially important gluconic acid producer in fermentation industry.
Table 1. The potentiality of he tested fungi for producing gluconic acid.
Fungi species Total acidity (mL NaOH/L medium) Gluconic acid Dry biomass (g/L)
Aspergillus flavus 1020 − 6.84 ± 0.2f
A. fumigatus 920 + 5.42 ± 0.1g
A. niger 2210 + 14.08 ± 0.6a
A. ochraceous 205 − 10.07 ± 0.7c
A. terreus 1770 + 9.17 ± 0.2d
A. versicolor 300 − 4.25 ± 0.3h
Penicillium frequentans 3110 + 10.21 ± 0.5c
P. litacinum 620 + 8.02 ± 0.5e
P. purpurogenum 840 + 6.86 ± 0.3f
P. puberulum 3680 + 12.89 ± 0.9b
Cladosporium and Herbarum 20 − 3.24 ± 0.3i
C. cladosporioides 70 − 2.75 ± 0.7j
Trichoderma viride 180 − 5.85 ± 0.6g
T. koningii 250 − 6.90 ± 0.1f
Mucor racemosus 720 + 5.20 ± 0.8g
Alternaria alternata 60 − 4.84 ± 0.4h
A. citri 40 − 0.95 ± 0.1k
Fusarium oxysporum 440 − 3.17 ± 0.3i
F. moniliforme 500 + 3.76 ± 0.6i
F. solani 780 − 2.90 ± 0.5j
Rhizopus oryzae 1120 + 4.65 ± 0.6h
(+) Indicates gluconic acid production. (−) Indicates no gluconic acid production.
Calculated mean is for triplicate measurements from two independent experiments ± SD. Means with different superscripts in the same column are considered statistically different (LSD test, P ≥ 0.05).
3.2. Effect of gamma irradiation
The ten positive isolates for gluconic acid production were exposed to different doses of gamma radiation (0.0–0.5 kGy) and incubated at 28 °C for 7 days after which the survival rate and gluconic acid were measured (Table 2). Results showed that A. niger exhibit the highest level of gluconic acid (69.35 g/L) at radiation dose (0.1 kGy) followed by Penicillium frequentans (40.31 g/L) at the same dose, while Aspergillus terreus and Fusarium moniliforme gave the lowest levels (4.67 and 5.69 g/L) respectively. Gamma-rays (from 0.2 to 0.5 kGy) have inversely proportion with gluconic acid production until reached to 0.0 g/L at 0.5 kGy and 18.78 g/L in case of A. niger, similar to Botros, Ahmed, Farag, and Hassan (2012) whereas they reported that 0.1 kGy exposure dose for Saccharomyces cerevisiae exhibit the maximum ethanol production. Gamma radiation, as a physical method, is known to cause injury to microorganisms and has been widely used for creating mutagenesis (Abosereh et al., 2006, Parviz et al., 2011, Ismaiel et al., 2014). The highest protease activity was achieved by Streptomyces spp. at 0.3 kGy exposure dose (Ahmed & Botros, 2011).
Table 2. Effect of gamma irradiation on gluconic acid (g/L) and survival rate.
Fungal isolates Irradiation doses (kGy) and survival rate (%)
0.0 % 0.1 % 0.2 % 0.3 % 0.4 % 0.5 %
Aspergillus niger 62.17 ± 2.2c 100 69.35 ± 3.5a 87.3 68.75 ± 1.9b 66.4 56.38 ± 3.4d 42.4 33.12 ± 2.9f 30.8 18.78 ± 2.4g 22.5
A. fumigatus 10.24 ± 1.3b 100 11.09 ± 2.5a 95.3 7.40 ± 1.0c 76.3 5.85 ± 1.2d 53.2 3.80 ± 2.6e 43.2 0.70 ± 0.03f 27.9
A. terreus 4.21 ± 0.7a 100 4.67 ± 0.4a 83.2 2.80 ± 0.4b 62.8 1.75 ± 0.c4 47.9 0.90 ± 0.03d 35.7 0.0 ± 0e 19.5
P. frequentans 39.69 ± 2.5ab 100 40.31 ± 11.3a 88.0 32.25 ± 11.2b 73.4 20.9 ± 1.8c 55.6 13.17 ± 0.7d 37.7 4.42 ± 0.3e 15.8
P. purpurogenum 15.27 ± 2.5ab 100 15.70 ± 1.6a 92.2 14.75 ± 0.8b 71.8 12.05 ± 0.9c 61.8 7.90 ± 0.5d 42.9 3.85 ± 0.6e 20.0
P. litacinum 13.75 ± 4.2a 100 10.90 ± 0.8b 97.3 7.80 ± 1.2c 66.6 6.48 ± 0.5d 57.3 3.49 ± 0.3e 41.8 0.75 ± 0.02f 21.7
P. puberulum 56.25 ± 1.9c 100 60.17 ± 12.7b 89.2 62.75 ± 11.8a 62.7 50.90 ± 1.2d 48.2 30.75 ± 7.2e 37.8 16.90 ± 2.1f 18.7
Mucor racemosus 19.28 ± 2.7a 100 17.75 ± 0.9b 87.4 14.18 ± 0.8c 59.2 9.17 ± 0.7d 45.6 4.69 ± 0.5e 29.9 0.81 ± 0.03f 13.8
Fusarium moniliforme 7.82 ± 0.9a 100 5.69 ± 0.4b 92.9 5.02 ± 0.4c 67.7 4.17 ± 0.2d 51.2 2.0 ± 0.1e 41.7 0.0 ± 0f 11.8
Rhizopus oryzae 12.35 ± 2.2a 100 10.21 ± 2.6b 88.8 9.16 ± 1.2c 77.1 8.09 ± 0.8d 66.2 7.18 ± 0.3e 37.9 3.01 ± 0.3f 10.4
Calculated mean is for triplicate measurements from two independent experiments ± SD. Means with different superscripts in the same row are considered statistically different (LSD test, P ≥ 0.05).
3.3. Effect of incubation temperature on gluconic acid production
Table 3 showed the effect of incubation temperature (25–40 °C) on gluconic acid production and dry biomass using A. niger, Penicillium puberulum and P. frequentans. 30 °C was found to be the optimum temperature for efficient fermentation for highest gluconic acid production (70.75 g/L) and dry weight biomass (16.58 g/L medium) by A. niger, (58.18 g/L and 13.42 g/L medium) by P. puberulum and (42.90 g/L and 11.37 g/L medium) by P. frequentans, respectively. Above or below this degree the gluconic acid and dry biomass were decreased. 30 °C was also reported as optimal for maximal gluconic acid production by Moresi et al., 1991, Subba-Rao et al., 1994, and Singh, Sharma, and Singh (2001b). At 40 °C, the production of gluconic acid was negligible.
Table 3. Effect of temperature on the gluconic acid production and dry weight of gamma irradiated (at 0.1 kGy) A. niger, P. puberulum and P. frequentans in submerged culture for 7-days incubated at 28 ± 1 °C and pH 6.
Temperature (°C) A. niger P. puberulum P. frequentans
Dry biomass (g/L) Gluconic acid (g/L) Dry biomass (g/L) Gluconic acid (g/L) Dry biomass (g/L) Gluconic acid (g/L)
25 12.02 ± 0.5b 51.14 ± 0.9b 10.91 ± 0.2b 45.06 ± 1.2b 8.81 ± 0.2b 32.13 ± 2.3b
30 16.58 ± 0.8a 70.75 ± 0.8a 13.42 ± 0.7a 58.18 ± 0.4a 11.37 ± 1.1a 42.90 ± 3.1a
35 10.22 ± 0.4c 38.75 ± 0.7c 8.05 ± 0.4c 31.17 ± 0.4c 8.04 ± 1.3b 27.15 ± 1.3c
40 2.01 ± 0.1d 0.0 ± 0d 1.19 ± 0.3d 0.0 ± 0d 1.05 ± 0.09c 0.0 ± 0d
Calculated mean is for triplicate measurements from two independent experiments ± SD. Means with different superscripts in the same column are considered statistically different (LSD test, P ≥ 0.05).
3.4. Effect of initial pH on the gluconic acid production
The pH value is one of the most critical factors affecting the fungal growth as well as the formation of organic acids. The quantities of gluconic acid and dry biomass respect to initial pH of the fermentation media are shown in Table 4. Results showed the effect of the initial pH on the gluconic acid production and the growth of (0.1 kGy) irradiated A. niger, P. puberulum and P. frequentans in submerged culture at 30 ± 1 °C for 7-days, the initial pH ranged from 4.0 to 8.0. Gluconic acid production by (0.1 kGy) irradiated A. niger, P. puberulum and P. frequentans at initial pH 6 and 30 °C for 7 days was 71.85, 58.41 and 44.16 g/L media, respectively. Changing the pH value decreases the gluconic acid production by the tested fungi and also the dry biomass. So pH 6 is considered as the optimum value for the growth of these fungi and their gluconic acid production. These results are in agreement to Botros et al. (2012) whereas they showed that the ethanol production by yeast cells irradiated at 0.1 kGy were highly affected by the change of the pH value of the fermentation medium. Also, the obtained results are similar to data showed by Ganguly et al., 2010, Rapeanu et al., 2009, Willaert and Viktor, 2006 and Singh and Singh (2006).
Table 4. Effect of initial pH on the gluconic acid production and growth of gamma irradiated (at 0.1 kGy) A. niger, P. puberulum and P. frequentans in submerged culture for 7-days incubated at 30 ± 1 °C.
pH-values A. niger P. puberulum P. frequentans
Dry biomass (g/L) Gluconic acid (g/L) Dry biomass (g/L) Gluconic acid (g/L) Dry biomass (g/L) Gluconic acid (g/L)
4 8.18 ± 0.6d 58.15 ± 2.8d 9.19 ± 0.8b 32.39 ± 1.5d 6.48 ± 0.5d 26.04 ± 2.2d
5 12.07 ± 0.2bc 64.31 ± 1.8b 12.81 ± 0.1ab 40.17 ± 2.3c 11.71 ± 0.4ab 38.12 ± 1.8b
6 17.36 ± 0.9a 71.85 ± 1.1a 14.19 ± 0.5a 58.41 ± 2.8a 13.05 ± 0.8a 44.16 ± 2.1a
7 11.21 ± 0.3c 60.14 ± 0.9c 12.82 ± 0.2ab 42.50 ± 1.3b 10.03 ± 0.5b 28.97 ± 1.8cd
8 7.92 ± 0.1e 44.15 ± 2.7e 6.27 ± 0.8c 30.07 ± 2.1e 7.00 ± 0.3c 20.14 ± 1.3e
Calculated mean is for triplicate measurements from two independent experiments ± SD. Means with different superscripts in the same column are considered statistically different (LSD test, P ≥ 0.05).
3.5. Effect of incubation period on gluconic acid production
Table 5 shows the effect of different incubation periods on the growth and gluconic acid production by the three (0.1 kGy) irradiated fungi A. niger, P. puberulum and P. frequentans. Data showed that the maximum gluconic acid production was 70.75 g/L produced by A. niger, followed by 59.31 and 42.91 produced by P. puberulum and P. frequentans, respectively. The dry biomass and the gluconic acid production of the tested fungi are affected with the change of the incubation period whereas there is a decrease in both the dry biomass and gluconic acid production by increasing or decreasing the incubation period for all the tested fungi. These data are similar to data showed by Botros et al. (2012) whereas, they indicated that the maximum ethanol production was obtained after 120 h of fermentation of S. cerevisiae cells exposed to 0.1 kGy dose and this production was decreased by increasing or decreasing the incubation period. Also, our results are in agreement with data showed by (Ganguly et al., 2010, Singh and Singh, 2006).
Table 5. Effect of incubation period on the gluconic acid production and growth of gamma irradiated (at 0.1 kGy) A. niger, P. puberulum and P. frequentans in submerged culture at 30 ± 1 °C and pH 6.
Incubation period (days) A. niger P. puberulum P. frequentans
Dry biomass (g/L) Gluconic acid (g/L) Dry biomass (g/L) Gluconic acid (g/L) Dry biomass (g/L) Gluconic acid (g/L)
3 4.71 ± 0.1d 19.33 ± 0.8c 2.91 ± 0.2d 12.70 ± 4.1c 2.31 ± 0.3c 11.73 ± 0.9d
5 10.31 ± 0.5c 51.13 ± 2.9b 8.35 ± 0.7c 31.17 ± 0.8b 7.81 ± 0.6b 26.23 ± 1.1c
7 16.58 ± 0.3a 70.75 ± 2.1a 14.23 ± 0.3a 59.31 ± 0.9a 12.73 ± 0.2a 42.91 ± 1.6a
9 16.07 ± 0.8a 69.83 ± 1.8ab 14.18 ± 0.5a 58.47 ± 1.6ab 12.18 ± 0.8a 42.11 ± 0.9ab
11 15.23 ± 0.2b 69.11 ± 1.5ab 13.88 ± 0.7b 58.11 ± 1.7ab 12.02 ± 0.5a 41.76 ± 2.4b
Calculated mean is for triplicate measurements from two independent experiments ± SD. Means with different superscripts in the same column are considered statistically different (LSD test, P ≥ 0.05).
3.6. Effect of using different wastes as sole carbon source
Table 6 showed that utilization of sugarcane molasses, banana-must or grape-must as a carbon sole source in gluconic acid production under submerged fermentation by the three potent (A. niger, P. puberulum and P. frequentans) under optimal fermentation conditions (0.1 kGy, pH 6, 30 °C for 7-days incubation) caused some increasing in the isolates productivity of gluconic acid.
Table 6. Utilization of grape-must, banana-must and sugarcane molasses as a sole source of carbon for gluconic acid production by gamma irradiated (at 0.1 kGy) A. niger, P. puberulum and P. frequentans in submerged culture at 30 ± 1 °C and pH 6 incubated for 7 days.
Carbon source Gluconic acid (g/L)
A. niger P. puberulum P. frequentans
Grape-must 54.25 ± 1.2c 52.75 ± 3.2c 44.75 ± 0.9c
Banana-must 61.28 ± 0.6b 56.37 ± 1.5bc 47.15 ± 1.3b
Sugarcane molasses 69.87 ± 1.6a 63.14 ± 2.3a 51.18 ± 2.3a
Calculated mean is for triplicate measurements from two independent experiments ± SD. Means with different superscripts in the same column are considered statistically different (LSD test, P ≥ 0.05).
Results illustrated that of sugarcane molasses exhibited the maximum production of gluconic acid (69.87, 63.14 and 51.18 g/L) by A. niger, P. puberulum and P. frequentans, respectively while, the utilization of banana-must exhibited moderate production of gluconic acid (61.28, 56.37 and 47.15 g/L). The lowest gluconic acid production (54.25, 52.75 and 44.75 g/L) was obtained by the same isolates respectively, in case of utilizing grape-must.
So it was obvious that A. niger is the most potent isolate in gluconic acid production followed by P. puberulum and P. frequentans in the three tested media. This data was in accordance with (Singh, Kapur, & Singh, 2005) whereas they indicated that there is an abundant growth of A. niger was observed with crude grape and banana-must. So they reported that grape-must and banana-must were utilized as the sole sources for gluconic acid production. Also the results are in agreement with (Shindia et al., 2006, Singh et al., 2003, Singh et al., 2001a, Singh et al., 2001b).
4. Conclusion
This study concluded that sugarcane molasses and fruit wastes which can be either as decomposed fruit pulps during storage and processing the fruit material in horticulture industries or as market rejected fruit, have high sugar content and are also low priced, so these materials are cost-effective and are easily available, therefore they are promising substrates for economical production of gluconic acid by using A. niger.
Acknowledgment
We are thankful to Dr. Ahmed A. Ismail, Assistant Professor of Microbiology, Dept. of Botany and Microbiology, Faculty of Science, Zagazig University, Egypt and Mr. El-Sayed R. El-Sayed, M.Sc., Plant Research Dep., Nuclear Research Center, Atomic Energy Authority of Egypt, Cairo, Egypt, for their critical comments.
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