Biological Approach for Effective Utilization of Worthless Onions Vinegar Production and Composting

Resources, Conservation and Recycling 40 (2004) 97–109

Biological approach for effective utilizationof worthless onions—vinegar production

and composting

Jun-ichi Horiuchia,∗, Kiyoshi Tadaa, Masayoshi Kobayashia,Tohru Kannob, Kunio Ebiec

a Department of Chemical System Engineering, Kitami Institute of Technology, Koen-cho 165,Kitami-shi, Hokkaido 090-8507, Japan

b International Students Center, Kitami Institute of Technology, Koen-cho 165, Kitami-shi,Hokkaido 090-8507, Japan

c Department of Civil Engineering, Kitami Institute of Technology, Koen-cho 165, Kitami-shi,Hokkaido 090-8507, Japan

Received 26 August 2002; accepted 31 January 2003

Abstract

About 10% of all the onions harvested are disposed of or cheaply treated in a market as worthlessonions every year in Japan because they failed to meet the quality standards required for marketing tothe customer. Therefore, in order to allow both value additions and the effective reuses of worthlessonions, a biological approach combining vinegar fermentation and composting was investigated.Onions were pressed in a mechanical juicer and separated to give onion juice (60 wt.%) and onionresidues (40 wt.%). The juice was converted to onion vinegar by a two-step fermentation system.Nutritional analysis revealed that the onion vinegar contained the various minerals, amino acids andorganic acids, which is expected to be a new valuable product from onions. The onion residues werethen utilized as a source of composting, and found to be successfully composted within a week. Basedon these investigations, it became possible to establish the scheme so that the worthless onions canbe totally recycled as onion vinegar and fertilizer (compost).© 2003 Elsevier B.V. All rights reserved.

Keywords: Onion; Vinegar; Composting; Alcohol; Acetic acid; Flocculating yeast; Recycle

∗ Corresponding author. Tel.:+81-157-26-9415; fax:+81-157-24-7719.E-mail address: [email protected] (J.-i. Horiuchi).

0921-3449/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-3449(03)00036-3

98 J.-i. Horiuchi et al. / Resources, Conservation and Recycling 40 (2004) 97–109

1. Introduction

Onion, known as a nutritious vegetable, is widely produced in the world. In Japan, morethan 1 200 000 tons of onions are produced per year: however, about 10% of all the onionsharvested failed to meet the quality standards required for marketing to the customer (TheMinistry of Agriculture, Forestry and Fishery of Japan, 2001). Subsequently, they are dis-posed of as agricultural wastes or cheaply traded in the market. Furthermore, a lot of onionshave been recently imported from other Asian countries at low prices which drasticallyreduce the market price of onions, leading to the decline of domestic agriculture. Therefore,it is strongly required to utilize the worthless onions in order to allow both value additionsand the effective reuses of them. To realize this, it is important to establish a scheme so asto minimize the emission or the environmental impact as well as the successful utilizationof worthless onions.

Onions are considered to be a favorable source for biological utilization because theycontain much sugar and various nutrients. In our previous studies, we reported that a newtype of vinegar with a higher content of minerals, amino acids and organic acids could beproduced from the worthless onions by a two-step fermentation system (Horiuchi et al.,1999, 2000c). On the other hand, the onion residues, which are generated from onion juiceproduction process, still remain unutilized. As an approach for the utilization of onionresides, composting, which is a biological process which produces fertilizer from variousorganic solids by aerobic or anaerobic microorganisms (Nakasaki et al., 1985; Horiuchiet al., 2003), is considered to be practical. It also should be noted that biological processes,such as vinegar fermentation and composting, are environmentally friendly processes whichoccur under ambient temperature and normal pressure without consuming excessive fuels.However, to our knowledge, there are few studies on total utilization of worthless vegetablesbased on biological processes.

In this study, therefore, a biological approach combining effective vinegar fermentationof onion juice and composting of onion residues was investigated to establish a scheme sothat the worthless onions can be totally recycled as onion vinegar and fertilizer (compost).

2. Materials and methods

2.1. Materials and strains

The red onion,cultivar R-3 was used as the raw material for the experiments. Onionswere processed as follows. Onions were cut into pieces and pressed in a mechanical juicer.Onions were fractionated to onion juice (60 wt.%) and onion residues (40 wt.%). The extract(onion juice) was packed in a heat-sealed pouch (2.5 l) and the packed juice was promptlycooked for 30 mm in 90◦C hot water. The cooked juice was then cooled, stored at−20◦C.Prior to the experiments, the defrosted juice was filtered using filter paper (2.0�m poresize) and then autoclaved at 120◦C for 20 mm.Table 1shows the main components ofonion juice. There is no problem in using discarded onions for vinegar production becausethey were rejected mainly on account of their shape or size. The onion residues was alsostored at−20◦C and used for the composting experiments. Water content and pH of onionresidues was 85.7% and 5.55, respectively.

J.-i. Horiuchi et al. / Resources, Conservation and Recycling 40 (2004) 97–109 99

Table 1Composition of onion juice produced from red onion R-3

Glucose (g/1) 20.6Fructose (g/1) 20.6Sucrose (g/1) 26.3Total sugar (g/1) 67.5

Ca (mg/l) 80.8K (mg/l) 1965Na (mg/l) 9Mg (mg/l) 82.3P (mg/l) 442Fe (mg/l) 0.78Mn (mg/l) 1.65Zn (mg/l) 1.45

Total amino acids (mg/l) 3850Total organic acids (mg/l) 4100

pH 5.4

Flocculating yeastSaccharomyces cerevisiae IR-2, which was isolated from Indonesianfermented food and well-characterized (Kuriyama et al., 1985), was used for the alcohol fer-mentation.Acetobacter pasteurianus no. 1, which was kindly provided by the Hokkaido. In-dustrial Technology Center, was used for the acetic acid fermentation (Miyazaki et al., 1996).

2.2. Analytical procedures

Onion juice and culture broth were centrifuged at 4◦C at 7000× g for 6 min. Thesupernatant was used for the determination of the sugars, ethanol and acetic acid by HPLC(Tosoh, 8020 series, Tokyo). Several minerals were determined by inductively coupledplasma atomic emission spectroscopy (ICP: HITACHI-3.6, Tokyo). Total sugar concentra-tion was calculated as the sum of the sucrose, fructose and glucose concentrations. Thecell concentration was measured by the dry cell method for yeast and the OD660 using aspectrophotometer (Shimadzu, Kyoto; UV- 1600PC) for acetic acid bacteria. One unit ofOD660 corresponded to about 0.4 g-dry cells/l in this culture. Free amino acids were ana-lyzed by the NBD-F (4-fluoro-7-nitrobenzoftirazan) method in combination with HPLC.pH was measured by a pH meter.

The CO2 content in the exhaust gas from compost was analyzed by a gas chromatograph(Hitachi 164, Hitachi Co., Tokyo, Japan) equipped with a thermal conductivity detectorusing a stainless steel column packed with Porapak Q (Waters Corp. MA, USA). Watercontent in the compost sample was measured according to the standard method (105◦C,24 h). pH of the compost sample was estimated by analyzing the mixed liqueur formed bysuspending 30 g of compost sample into 70 g of distilled water.

2.3. Vinegar production

Fig. 1shows the schematic representation of the experimental apparatus for onion vinegarproduction employed in this study. A jar fermentor (working volume 1.5 l, Biott Corp.,


Fig. 1. Schematic diagram of the experimental apparatus for onion vinegar production. (1) Medium reservoir; (2)peristaltic pump; (3) jar-fermentor; (4) onion alcohol broth reservoir; (5) charcoal pellet bioreactor; (6) vinegarbroth reservoir; (7) settler for cell recycling; (8) aseptic air.

Japan) with a glass settler (diameter 65 mm, working volume 300 ml) for cell recycle forethanol fermentation and a vertical charcoal pellet bioreactor for acetic acid fermentationwas serially linked via a reservoir 4. Onion juice was continuously fed to the completemixing jar fermentor by a peristaltic pump. Agitation was controlled at 200 rpm during thefermentation. Air was not supplied in order to minimize the evaporative loss of ethanol andthe culture pH was not controlled during continuous onion alcohol production. The culturebroth was overflowed into a settler, where yeast cells were sedimented and separated fromthe broth. The supernatant was flowed to the reservoir and the concentrated cells wererecycled to the fermentor. The re-circulation ratio (flow rate of cell recycle over substratefeed rate) was five during cultivation. Onion alcohol produced in the jar fermentor wasonce stored in the onion alcohol broth reservoir and then continuously fed to the charcoalpellets bioreactor by a peristaltic pump. Suspended yeast cells, inevitably present in thesupernatant from the jar fermentor, were not removed. A charcoal pellets bioreactor, 400 mmin height and 50 mm in diameter (Tokyo Rika Kikai Co., Tokyo), was used for acetic acidfermentation. The reactor was filled with 0.5 l charcoal pellets and the working volume wasevaluated as 0.5 l. The void fraction of the packed pellets was 0.43. The pellets used in thisstudy were produced from waste mushroom medium and were characterized by their highspecific surface area (200 m2/g) with numerous micropores (2–10�m) and their bacterialaffinity. The detailed characterization of the pellets and the characteristics of the bioreactorwere previously reported (Horiuchi et al., 2000a). The bioreactor filled with the pelletswas autoclaved for 30 mm at 120◦C and then employed for the experiments. The culturetemperature was maintained at 30◦C and the alcohol broth produced from onion juice wascontinuously fed from the bottom at various feed rates. Aseptic air was also supplied fromthe bottom; the supply rate was modified in proportion to the medium feed rate so as tomaintain an aerobic condition. Productivity, (PD, g/l h) was the volumetric production rateof ethanol or acetic acid. The ethanol and acetic acid yield, (Ye andYa) was defined as theratio of the total alcohol or acetic acid production to the theoretical ethanol or acetic acidproduction, which was stoichiometrically calculated from the consumed sugars or ethanol.


Fig. 2. Schematic diagram of the experimental apparatus for laboratory scale composting. (1) Air inlet; (2) compostreactor; (3) internal punch tray; (4) compost material; (5) thermowell; (6) sampling hole; (7) exhaust gas; (8)Styrofoam container.

2.4. Composting

Fig. 2 shows the experimental apparatus for the laboratory scale composting experi-ments. A plastic composting reactor (6 l, polypropylene) with an internal punched trayas a composting material support was used for the experiments. The reactor was locatedin a box of expanded polystyrene used for thermal insulation. Air was supplied from thebottom through the tray at various rates. The CO2 and O2 content of the exhaust gas andcomposting temperature were monitored during composting. The compost temperature wasmeasured by a thermocouple buried in the composting material. The onion residues weremixed with bark chips (water content 10%, 5–10 mm size, pH 6.75) to adjust the watercontent of material, and used for the composting. The bark chips do not influence thecharacteristics of primary composting of onion residues in terms of CO2 evolution andtemperature increase because they were not easily degraded. No inoculum was employed.Prior to sampling, the composting material was well mixed in order to minimize samplingerror.

3. Results and discussion

3.1. Vinegar production

Vinegar fermentation is essentially a two-step process comprising the anaerobic conver-sion of sugars to ethanol and the aerobic oxidation of ethanol to acetic acid. Therefore, in


Fig. 3. Steady state results of productivity (�), total sugar (�), ethanol (�) and cell (�) concentrations for onionalcohol production at various dilution rates.

order to improve the total performance of vinegar production, both the ethanol fermentationand the successive acetic acid fermentation process have to be optimized.

Onion alcohol production by continuous fermentation with cell recycle was examinedfirst. The cultivation was commenced by inoculating 600 ml of a preculture of yeast cellsat a dilution rate of 0.2/h, and confirmed as steady state by checking the operating param-eters such as the cell, sugar and ethanol concentration in the effluent. Continuous alcoholproduction with cell recycle was successfully carried out under various dilution rates. Mostof the total sugar was smoothly converted to about 33 g/l ethanol; the residual total sugarconcentration was around 3.5 g/l. The high flocculating activity of cells was maintained,resulting in no difficulty in operating the continuous process throughout the experiments.

Fig. 3andTable 2show the steady state analysis of the effluent from continuous alcoholproduction process using IR-2 at various dilution rates. At the dilution rates of 0.112 and0.37/h, most of the total sugar was converted to around 33 g/l ethanol. Cell concentration wasaround 30 g/l between the dilution rates of 0.112 and 0.57/h. By increasing the dilution rate,

Table 2Steady-state analysis of continuous alcohol production with cell recycle under various dilution rates

D (/h) RT (h) R (–) Cell (g/l) Outlet PDe (g/l h)

Sugar (g/l) Ethanol (g/l)

0.112 8.9 5 28.0 2.4 33.1 3.710.37 2.7 5 31.1 3.5 33.9 12.50.57 1.75 5 26.7 6.9 30.9 17.60.86 1.16 5 21.3 19.1 24.6 21.21.12 0.89 5 8.4 44.9 8.4 13.8

D, dilution rate; RT, retention time; R, cell recycle ratio; PDe, ethanol productivity.


Fig. 4. Steady state results of productivity (�), ethanol (�) and acetic acid (�) concentrations for onion vinegarproduction at various dilution rates.

ethanol and cell concentrations decreased while total sugar concentration was increased.Maximum productivity reached about 21.2 g/l h at the dilution rate of 0.86/h, which wasabout three times as compared with 6.6 g/l h of the repeated batch operation using the sameonion juice and yeast strain in our previous study (Horiuchi et al., 2000b). There was noinhibitory effect of using onion juice on the continuous alcohol fermentation by the IR-2.

Then, continuous acetic acid fermentation by the charcoal pellet bioreactor was examined.By inoculating 300 ml of a preculture of free cells at a low dilution using synthetic mediumand after confirming that the acetic acid bacteria successfully grew on the surface of thepellets, the onion alcohol was fed. The operation of the reactor was continued for about 50d. After confirming that the establishment of a steady state, the dilution rate was stepwiselyincreased. With each increase in the dilution rate, the acetic acid concentration fell whilethe ethanol concentration rose. Throughout the experimental run, the operation was stableand no breakthrough or process malfunction was observed.

Fig. 4andTable 3show the steady state analysis of the effluent from the charcoal pelletbioreactor at various dilution rates. At the dilution rate of 0.058/h, ethanol was completely

Table 3Steady-state analysis of continuous onion vinegar production in the charcoal pellet bioreactor under various dilutionrates

D (/h) RT (h) PO2 (atm) Aerationrate (vvm)

Outlet pH (–) PDa (g/l/h)

Ethanol (g/l) Acetic acid (g/l)

0.058 17.1 0.21 1.1 0.4 37.9 3.67 2.20.089 11.2 0.21 1.4 5.0 32.5 3.70 2.90.124 8.1 0.21 1.4 9.1 26.4 3.74 3.30.144 7.0 0.21 1.5 13.3 20.7 3.89 3.0

D, dilution rate; RT, retention time;PO2, oxygen content; PDa, acetic acid productivity.


converted to acetic acid. By increasing the dilution rate, acetic acid concentration decreasedwhile ethanol concentration was increased. Maximum productivity reached about 3.3 g/l hat the dilution rate of 0.124/h. Characteristics of acetic acid fermentation by the charcoalpellet bioreactor were almost similar to the results in the previous report using a syntheticmedium (Horiuchi et al., 2000a). Acetic acid yield over consumed ethanol,Ya was around0.88–0.92 during operation which is slightly higher than the yield obtained in the previousstudy using a synthetic medium (Horiuchi et al., 2000b). This indicates the utilization ofethanol by yeast cells present in onion alcohol under aerobic condition did not occur in thecharcoal pellet bioreactor because the activity of yeast cells was inhibited by high aceticacid concentration and low pH (3.6–3.8).

In the continuous two-step system, the operating result of the first step usually influencesthe operation of the second step, which may make the stable operation of the two-stepsystem difficult. However, since the high cell concentration could be kept both in a con-tinuous alcohol production fermentor and a packed bed bioreactor during operation in oursystem, there was no problem of operational stability. The employment of the continuousprocess with cell recycle using a flocculating yeast for the alcohol fermentation improvedthe operational flexibility of the total system because alcohol production rate could be easilycontrolled in response to the change in operational conditions of the successive charcoalpellet bioreactor.

Thus, the two-step fermentation system was successfully operated for vinegar productionusing onion juice. Total fermentation time required to produce onion vinegar was around15–20 h, which is sufficiently competitive because the conventional vinegar production pro-cess using static culture requires more than 2 weeks for completion of vinegar fermentation.

Table 4summarizes the main components of the onion juice, onion alcohol and onionvinegar produced by the two-step system. Totally, 37.9 g/l acetic acid was obtained from67.3 g/l total sugar by employing the two-step fermentation system. The taste of the filteredvinegar was fruity and no unpleasant smell was observed.

In Table 5, the contents of several main minerals, total organic acids except acetic acid andtotal amino acids in the onion vinegar are compared with those in other kinds of vinegars.The analysis of mineral content gave an extremely high potassium content of the onionvinegar, while the sodium content was rather low. Mg and Ca contents were relativelyhigh. This suggests that the onion vinegar could be applied as a nutraceutical food for theprevention of hypertension disease. Various organic acids and amino acids also in vinegarare important in imparting a suitable taste and flavor as well as nutrition. The concentrationof total amino acids in the onion vinegar was 2100 mg/l, was also higher than those in

Table 4Composition of onion juice, onion alcohol, and onion vinegar produced by the two-step fermentation system

Concentrations (g/l)

Onion juice Onion alcohol Onion vinegar

Total sugar 67.5 6.9 2.5Ethanol 0.0 30.9 0.4Acetic acid 0.0 0.0 37.9


Table 5Minerals, total amino acids and total organic acids concentrations in onion vinegar and other vinegars (mg/l)

Na Ca Mg K Total aminoacids

Total organicacids

Reference

Onion vinegar 9.0 80.8 82.3 1965 2100 2830 This workMalt vinegar 3100a 20a 10a 80a 481 439 Itoh (1978)Rice vinegar 2900a 20a 50a 60a 1330 383 Itoh (1978)

a Data from ‘Standard Composition of Japanese Foods’, ver. 4. Japanese Science and Technology Agency,Tokyo, 1995.

other kinds of vinegar. However, about 45% of the total amino acids in the onion juice(Table 1) was consumed during the fermentation for vinegar production. The total organicacid concentration of the onion vinegar was 2830 mg/l, which is 6–7 times of those inother kinds of vinegar (Itoh, 1978). Onion vinegar has a red color which arises from thepresence of anthocyanin, a polyphenol pigment in the onion juice. The anthocyanin contentof the onion vinegar was about 50 mg/l. These favorable features are expected to lead to thedevelopment of a high-value vinegar or drinks.

3.2. Composting of onion residues

Then, the possibility of composting of onion residues was experimentally examined. It isimportant for successful composting to optimize operating conditions, particularly, watercontent (Fujita, 1993). In order to maintain proper oxygen transfer in composting material,which is strongly influenced by water content in the compost, its water content has to beproperly determined. The water content of onion residues is 85.7%, which is rather high anddifficult to compost successfully alone. Therefore, the water content in composting materialwas adjusted by modifying the ratio of onion residue (water content: 85.7%) and bark chips(water content: 10%).Fig. 5shows the time courses of CO2 evolution rate and temperature incompost under various water contents. As can be seen, the progress of primary compostingwas influenced by water content, the optimal water content was 61.1%. Primary compostingrate was delayed as the water content increased due to the decrease of the oxygen transferrate. It should be noted that the slight difference in water content between 60 and 65% willseriously influence the progress of primary composting of onion residues.

Fig. 6(a and b) shows the typical results from the composting of onion residues. Theprimary composting started smoothly, resulting in a rapid increase of the compost tem-perature and the CO2 evolution rate. The compost temperature reached 52◦C at 36 h. TheCO2 evolution rate also peaked at 36 h and then drastically decreased due to the smoothprogress of the primary fermentation. Arrows indicate the re-mixing of the compost sample.Slight increase in temperature and CO2 evolution was observed after the first mixing (72 h);however, no increase was observed after the second mixing (132 h). The CO2 evolutionrate became nearly zero at 144 h. These results suggested that easily degradable materi-als in onion residues were mostly converted to CO2, H2O and microbial biomass withina week. From the carbon balance around the composting apparatus, it was estimated thatabout 90% of the carbon in the onion residues was released as CO2 in the exhaust gas.Water content before and after primary composting was 61.1 and 61.8%, respectively, and


Fig. 5. Time courses of composting experiments using laboratory scale compost reactor under various water con-tents. (a) Time courses of CO2 evolution rate, (b) compost temperature and external temperature (�). Experimentalconditions; water content, (�), 61.1% (2.3 kg onion residues and 1.15 kg of bark chips), (�), 64.3% (2.5 kg ofonion residues and 1.0 kg of bark chips) (�), 73.4% (4.0 kg of onion residues and 1.0 kg of bark chips); aerationrate, 0.33 l/mm/kg-compost.

no serious change was observed. There was no unpleasant smell in the compost and the pHwas around 7.0. Naturally, the compost contains various minerals including high potassiumand nitrogen sources originated from onions. The compost is safe in its components, suchas heavy metals, compared with the compost produced from sewage sludge or other indus-trial wastes because the component of materials can be clearly identified. The bark chips,which are difficult to decompose easily, were used as additives to onion residues in thisstudy. Therefore, the compost obtained here requires secondary composting (maturing) todecompose the bark chips before applying them to farmland.

Thus, it was found that the onion residues could be successfully composted and that theproduced compost will be a favorable fertilizer for agricultural use.

3.3. Total recycle scheme of worthless onions

Based on these investigations, material balance (weight base) for effective utilization ofworthless onions is summarized inFig. 7. Worthless onions, 100 kg, was fractionated toonion juice, 60 kg, and onion residues, 40 kg. Onion juice, 60 kg, was fermented to onionvinegar, 55 kg. About 5 kg of production loss including evaporation, cell withdrawal andstart-up loss inevitably occurred during vinegar production. Onion residues, 40 kg, weresuccessfully composted by mixing with 20 kg of bark chips. Thus, the worthless onions,


Fig. 6. Time courses of composting experiment using laboratory scale compost reactor. (a) Time courses of CO2

evolution rate (�), (b) compost temperature (�), and external temperature (�). Experimental conditions; aerationrate, 0.33 l/mm/kg-compost; water content, 61.1%; composting material, mixture of onion residues (2.3 kg) andbark chip (1.15 kg).

100 kg, were totally reused as a substrate for biological processes; worthy products, onionvinegar, 55 kg, and compost, 60 kg, could be successfully produced.

In Japan, the selling price of rice vinegar is around 4–8 US dollars per liter, therefore,55 kg onion vinegar corresponds to 220–440 US dollars. Since the current price of 100 kgof worthless onion is around 20–40 US dollars in Japan, vinegar production is considered

Fig. 7. Material balance of the scheme for biological utilization of worthless onions combining vinegar productionand composting.


to be a promising way for value addition to worthless onions. Because the onion vinegarhas favorable features in its constituents originating from onions as a healthy product, it isexpected to be commercially competitive. This will also contribute to enhance the feasibilityof the scheme proposed.

It is known that the cultivation of onions requires much fertilizer compared with othervegetables; a large amount of chemically synthesized fertilizers are imported into Japan forthis purpose. If the compost produced from onion residues is used for the cultivation ofonions, it will contribute to the development of a sustainable agriculture system as well asthe reduction of chemical fertilizer consumption.

Another advantage in the scheme proposed here is the minimization of emission andenvironmental impact. Worthless onions have been incinerated using fuels or land filled inJapan, which causes environmental pollution, release of CO2 and additional fuel consump-tion. On the other hand, the scheme inFig. 7generates only 5% of production loss, and theother 95% can be consumed or recycled in the natural material circulation flow.

4. Conclusions

We found that the worthless onions could be biologically converted to the worthy prod-ucts, onion vinegar and compost. The onion juice was successfully converted to onionvinegar by a two-step fermentation system. Nutritional analysis revealed that the onionvinegar contained various minerals, amino acids and organic acids, which is expected to bea new valuable product from onions. The onion residues were then utilized as a source forcomposting and were successfully composted within a week. Thus, the worthless onionswere totally reused as a substrate for biological processes. Due to marketing failures, worth-less fruits and vegetables are inevitably generated and usually disposed of as agriculturalwastes. The scheme using biological processes combining vinegar production and com-posting would be one of the ways for the effective utilization of such agricultural wastes.

Acknowledgements

This work was partially supported by a research grant from Kitami-city public utilitybureau. Authors are grateful for Mr G. Sakurada and S. Araki of Kitami-city sewage centerfor their assistance.

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Biological Approach for Effective Utilization of Worthless Onions Vinegar Production and Composting