JOURNAL OF BIOSCIENCE AND BIOENGINEERING Vol. 88, No. 1, 107-109. 1999

New Vinegar Production from Onions JUN-ICHI HORIUCHI,* TOHRU KANNO, AND MASAYOSHI KOBAYASHI

Department of Chemical System Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido, 0904507, Japan

Received 25 January 1999/Accepted 26 March 1999

The possibility of producing a new type of vinegar from worthless onions, which fail to meet the quality standards required for marketing, was investigated. Several kinds of onion were initially tested as raw material for vinegar production, and vinegar was successfully produced from the juice of a red onion, the cultivar Kurenai, by batch culture using yeast and Acetobacter aceti. Nutritional analysis revealed that the potassium content of onion vinegar was extremely high, while the amount of sodium was lower than that in conventional vinegars. It was also shown that the total amino acid and total organic acid contents of the onion vinegar were respectively 1.6-6.9 times and 3.5-11.5 times those in other kinds of vinegars.

[Key words: vinegar, onion, acetic acid fermentation, agricultural waste]

The Okhotsk area of Hokkaido produces more onions than anywhere else in Japan. New products derived from onions are desired, not only for value addition to the vegetable but also to allow the effective utilization of worthless onions that fail to meet the quality standards required for marketing to the customer, which amount to about 10% of all onions harvested and are usually dis- posed of as agricultural waste.

Vinegar, known worldwide as a traditional acidic con- diment (1, 2), is widely produced from rice, malt, ap- ples, wine and various other agricultural materials (3-S). Onions are considered to be a promising source of vine- gar because they are rich in sugar and various nutrients (6). Vinegar made from onions is also of interest in terms of its potential as a new functional condiment with physiological characteristics derived from minerals and polyphenol. However, to our knowledge, there are no published reports on vinegar production from on- ions. In the present study, we experimentally examined the possibility of using onions to produce vinegar.

The six kinds of onion listed in Table 1 were tested as raw materials for the experiments. Onion juice for vine- gar fermentation was obtained as follows. Onions were pressed in a mechanical juicer and the extract was promptly autoclaved for 30min at 120°C. The extract was then filtered twice, using 6.0 pm pore-size membrane to remove coarse particles and a 0.4pm pore-size mem- brane to remove microbes. By this procedure, 60 wt% of the onions processed was recovered as onion juice. The filtered juice was stored in a refrigerator at 4.O”C and used as needed. There is no problem in using discarded onions for vinegar production because they were rejected mainly on account of their shape or size; there is no difference in terms of sugar and nutrients contents be- tween onions that meet the quality standards and those that do not.

Saccharomyces cerevisiae AHU3532 was used for alco- hol fermentation. Acetobacter aceti TUA549B, which was kindly provided by Tokyo University of Agriculture, was used for acetic acid fermentation. Batch experiments were carried out in flasks fitted with aeration equipment. Alcohol fermentation was first performed at 25°C with CO2 bubbling in order to maintain an anaerobic condi-

* Corresponding author.

tion. After the completion of alcohol fermentation, ace- tic acid fermentation was started at 30°C under the aero- bic condition by seeding with A. aceti. Sample taken from the flasks were filtered by a 0.45 pm pore-size mem- brane and used for analysis of the sugars, ethanol, and organic acids by HPLC. Free amino acids were analyzed by the NBD-F (4-Fluoro-7-nitrobenzofurazan) method in combination with HPLC. Several minerals were deter- mined by inductively coupled plasma atomic emission spectroscopy (ICP: Hitachi-3.6).

Table 1 summarizes the contents of several saccharides in juices obtained from various kinds of onions. Su- crose, glucose, and fructose were the main sugars in the onion juices. The total sugar concentration varied from 46.9 to 79.6g/l. The red onion Kurenai, which had the highest sugar concentration among the cultivars tested, was selected for use in this study. A sugar content of 79.6 g/l in the onion juice corresponds stoichiometorically to 40.7 g-ethanol/l and 53.1 g-acetic acid/l. The sugar content was theoretically enough to produce vinegar meeting the acidity requirement (4%) specified in the Japan Agricultural Standard (JAS). The soluble solid (Brix) and acidity of the onion juice were 10.8 and 0.15% respectively.

Table 2 compares the main mineral contents of the onion juice with those of malt and rice vinegars and the average contents of various special vinegars (8, 9). The onion juice was shown to have an extremely high potas- sium content, while the amount of sodium was rather low in comparison with the malt and rice vinegars. This sug- gests the onion vinegar could be effective in helping pre- vent hypertension (10). The calcium and magnesium con- tents were also higher than in the other conventional vinegars.

TABLE 1. Sugar concentrations in onion juice obtained from various onions (g/f)

Cultivars Sucrose Glucose Fructose Total Hokkai no. 1 9.9 20.9 16.1 46.9 Arch 8.5 25.6 16.5 50.6 Haruhiguma 13.3 29.3 21.7 64.3 Okhotsk H86 13.7 30.6 23.7 68.0 Kitamomiji 20.1 28.0 21.8 69.9 Kurenai 23.2 29.6 26.8 79.6

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TABLE 2. Mineral concentrations in onion juice and other vinegars (mg/l) ___ .-

Na Ca Fe K Mg Mn Zn Ref. Onion juice 20 41.2 0.38 1960 78.3 0.20 0.12 This work Malt vinegar 3100 20 1.0 80 10 - 0.6 9 Rice vinegar 2900 20 1.0 60 50 - 1.9 9 Various vinegars” 26.8 3.3 0.96 12.5 8.0 - - 8 a Average values calculated from data in ref. (8).

TABLE 3. Results of alcohol and acetic acid fermentation of onion juice (g/i)

Onion juice Onion alcohol Onion vinegar Total sugar 64.0 Ethanol 0.0 Acetic acid 0.0

2.8 Trace 27.1 2.0 0.0 29.4

Sulfur containing amino acid alliin is usually present in onions. Alliin is converted to allysine compounds (pro- pylallysine, methyl propylallysine) by cellular alliinase during processing (6). Allysine compounds have a keen, stimulating smell and an anti-bacterial function, that in- hibits the growth of yeast. In preliminary experiments, dif- ficulty was experienced in fermenting the raw onion using yeast. However, since the allysine compounds are vola- tile and they can be removed by autoclaving.

Alcohol and acetic acid fermentation using the onion juice were performed by batch culture in that order. Only onion juice was used for the experiments; no other nutrients and minerals were added. Table 3 summarizes the total sugar, ethanol, and acetic acid concentrations in onion juice, onion alcohol, and onion vinegar. Alco- hol and acetic acid fermentations were successfully com- pleted using autoclaved onion juice. Most of the total sugar was converted to acetic acid via ethanol. About 40 and 200 h were respectively required to complete alcohol and acetic acid fermentation.

Though there was no difficulty in carrying out the alco- hol and acetic acid fermentations using onion juice in batch culture, the final acetic acid concentration of 3Og/l was a little low compared with the theoretical value calculated from the sugar content of the onion juice. The acetic acid concentration was lowered by evaporation loss due to the long fermentation time. In addition, the sugar concentration in onion juice fluctuat- ed because the sugar content in onions gradually de- creased as the longer they are stored. Evaporation loss can be decreased by shortening the fermentation time using an efficient fermentation system, such as bioreactor (7), while fluctuations in onion juice quality will be minimized by producing the juice at the time of harvest- ing. However a certain degree of quality variation in onion vinegar will inevitably occur because it is produced from natural materials. If the final acetic acid concentra-

tion in the onion vinegar does not meet the JAS require- ment, the sugar can be supplemented to the juice to maintain an appropriate concentration of acetic acid in the vinegar (more than about 4Og/l). It was found by adding glucose to the onion juice, the final acetic acid concentration could be increased to about 5Og/l (data not shown). The taste of the filtered vinegar was fruity and no unpleasant smell was observed.

Various organic acids in vinegar are important in im- parting a suitable taste and flavor. In Table 4, the con- tents of various organic acids in the onion juice and the onion vinegar are compared with those in other kinds of vinegar (1). The total organic acid concentration of the onion vinegar was 283 mg/lOOml, which is 3.5-11.5 times the concentrations in other kinds of vinegar. The amount of citric, malic, and pyroglutamic acids were particularly high. During the fermentation for vinegar production, the total organic acids in the onion juice decreased from 350 to 283 mg/lOOml, mainly due to reductions in the amounts of tartaric and pyroglutamic acids. However, the citric acid content doubled. The lactic acid concentration in the onion juice was low and it remained relatively low in the vinegar.

The amino acid content also influences the taste of vinegar. Table 5 summarizes the concentrations of vari- ous amino acids concentration in the onion juice, the onion vinegar and other kinds of vinegar. The concentra- tion of total amino acids in the onion vinegars was 210mg/100ml, which is 1.6-6.9 times the concentrations in other kinds of vinegar. The arginine concentration was extremely high in the onion vinegar. About 45% of the total amino acids in the onion juice were consumed during the fermentation for vinegar production, and except for arginine and glycine, the concentrations of in- dividual amino acids decreased during the fermentation.

Onion vinegar has a red color which arises from the presence of anthocyanin, a polyphenol pigment in the on- ion juice. The anthocyanin content of the onion vinegar was about 50 mg/l; this is considerably lower than that in red wine, which is generally about 2OOOmg/l (11).

The higher contents of organic acids, amino acids and minerals in onion vinegar than in conventional vinegars reflects rich nutritional content of the onion itself. These favorable features are expected to lead to the develop- ment of a high-value vinegar or drinks.

TABLE 4. Comparison of organic acid contents in onion juice, onion vinegar, and various other kinds of vinegar (mg/lOO ml)

Organic acid Onion juice Onion vinegar Rice vinegar Alcohol vinegar Apple vinegar Wine vinegar Malt vinegar Tartaric acid 32.9 6.4 - - - 27.1 - Citric acid 43.7 98.7 2.09 3.1 2.64 20.3 0.69 Malic acid 76.6 47.5 0.52 6.41 6.53 20.3 0.06 Pyroglutamic acid 161.2 100.5 1.27 35 - - - Lactic acid 2.0 6.0 20.3 20.1 3.6 5.5 40.3 Succinic acid 33.4 24.1 14.1 11.9 8.1 2.85 Total 350 283 38.3 64.6 24.7 81.9 43.9

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TABLE 5. Comparison of amino acid contents in onion juice, onion vinegar, and various other kinds of vinegar (mg/lOO ml)

Amino acid Onion juice Onion vinegar Rice vinegar Alcohol vinegar Apple vinegar Wine vinegar Malt vinegar Asp 21.2 0.12 5.4 3.3 5.4 4 1.2 Gill 48.9 10.9 50.1 17.7 3.0 9.7 2.1 Ser 62.7 1.91 4.2 1.8 3.2 4 2.8 GlY 3.37 3.61 12 7.2 1.0 1.4 3.0 His 13.7 9.58 2.1 1.0 0.9 1.8 0.4 Thr 18.5 5.61 3.9 2.2 1.4 3 2.2 Ala 12.0 11.5 14.6 3.4 3.4 6.2 7.9 Arg 115 144 5.8 2.6 1.5 18.9 1.3 Pro 8.25 5.78 3.3 1.5 1.7 10.5 11.8 Val 10.8 2.99 6.3 3.0 1.9 3.1 3.5 CYS trace trace 0.3 0.02 trace trace trace Met 5.07 2.38 1.1 0.5 0.3 0.5 0.8 Be 5.94 0.68 4.1 2.2 1.2 1 2.3 Leu 15.2 0.77 7.7 3.5 1.7 3.3 4.0 Phe 14.4 2.85 4.2 1.9 0.8 2.5 1.9 LYS 7.52 3.19 3 3.6 2.4 5.2 1.7 ‘br 14.4 4.05 4.9 2.1 0.8 1.9 1.2

Total 377 210 133 57.5 30.6 77.0 48.1

The vinegar fermentation by batch culture used in the present study was rather inefficient in terms of productiv- ity due to its long duration (7). To overcome this draw- back, the development of a continuous onion vinegar fermentation system is ongoing.

In conclusion, it was found that vinegar could be suc- cessfully produced from the juice extracted from onions using yeast and Acetobacter. Although the sugar con- tent in onions was slightly low for the production of nor- mal vinegar, it could easily be supplemented by adding sugar to the onion juice. The nutrition analyses revealed that onion vinegar has high contents of organic acids, amino acids, and minerals. This new type of vinegar is expected to become a worthy condiment produced from worthless onions.

The authors thank Mr. H. Kobayashi, Mr. H. Tomita, Miss F. Kubota, Mr. K. Yamaguchi, and Mr. A. Takahashi for their lab- oratory contributions.

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