Differential hydrolysis of alk(en)yl cysteine sulphoxides by alliinase in onion macerates: flavour implications

Intact cells of Allium spp have no odour, but when cells are disrupted, the enzyme alliinase (EC 4.4.1.4) hydrolyses the S-alk(en)yl sulphoxides (ACSOs) to produce pyruvate, ammonia and the many volatile sulphur compounds associated with flavour and odour. In onions, there are three main sulphoxides: methyl, propyl and 1-propenyl, which gives rise to onion's tear-producing effect. The relative proportions of these are a factor in determining the subsequent flavour. γ-Glutamyl peptides, which are biosynthetic intermediates to ACSOs, are also present in significant amounts, but their contribution to flavour is not known. There is little work on the reaction of alliinase in vivo. The hydrolysis of ACSOs by alliinase was studied in rapidly macerated bulbs at intervals between 5 s and 2 h. ACSO and γ-glutamyl peptide levels were measured by HPLC, and pyruvate levels were measured spectrophotometrically. Although the hydrolysis of propenyl cysteine sulphoxide (PrenCSO) was immediate and almost 100% between 5 and 20 s after bulb maceration, the hydrolysis of propyl cysteine sulphoxide (PCSO) and methyl cysteine sulphoxide (MCSO) was incomplete. After 5 s maceration, about 50% PCSO and MCSO remained and thereafter no further hydrolysis occurred. The levels of ACSOs and the extent to which they were hydrolysed were dependent on the cultivar and sulphur environment in which it was grown. The addition of pyridoxal phosphate cofactor enhanced the extent of MCSO and PCSO hydrolysis. Additional purified alliinase per se had no effect. Substantial hydrolysis of γ-glutamyl peptides also occurred after maceration, and this was unexpected. Levels of pyruvate product were between 15 and 25% of the expected amount from ACSO hydrolysis assuming a stoichiometric relationship. The incomplete hydrolysis of MCSO and PCSO, enhancement of activity by additional pyridoxal phosphate and non-stoichiometric production of pyruvate are evidenced that reaction inhibition of alliinase may be occurring in onion macerates. © 1998 Society of Chemical Industry.     

Ammonium nitrate fertility levels influence flavour development in hydroponically grown ‘Granex 33’ onion

High levels of nitrogen (N) fertility have been shown to influence bulb flavour characteristics in onion (Allium cepa L). To test the effects of lower levels of N fertility on onion bulb flavour, ‘Granex 33’ onions were grown hydroponically in a greenhouse with varying solution N levels. Eleven levels were tested by increasing the concentration of NH₄NO₃ in solutions from 20 to 140 mg l^?1 N. Mature plants were harvested and evaluated for plant leaf and bulb fresh weights (FWs), bulb soluble solids content (SSC), bulb total pyruvic acid, bulb total sulphur (S), and bulb sulphate (SO₄^2?). To determine the effect of N on the flavour biosynthetic pathway of onion, total and individual S‐alk(en)yl cysteine sulphoxides and related peptide intermediates were also tested. Leaf and bulb FWs responded quadratically to N concentration, as did total bulb S. Bulb SO₄^2? and SSC, though significantly influenced by N concentration, did not respond with a meaningful trend. Bulb pyruvic acid increased linearly with N level increases, as did (+)‐S‐propyl‐L ‐cysteine sulphoxide. Total precursors, (+)‐S‐methyl‐L ‐cysteine sulphoxide, and trans‐(+)‐S‐1‐propenyl‐L ‐cysteine sulphoxide responded quadratically to N levels. At lower N levels, trans‐(+)‐S‐1‐propenyl‐L ‐cysteine sulphoxide content was highest relative to the other precursors. However, at elevated N levels, (+)‐S‐methyl‐L ‐cysteine sulphoxide accumulated in the highest concentrations. Peptide intermediates 2‐carboxypropyl glutathione and γ‐glutamyl propenyl cysteine sulphoxide responded linearly and quadratically respectively to increasing N fertility levels. Nitrogen fertility levels can influence flavour intensity and quality and should be considered when growing onions for flavour attributes. Copyright © 2003 Society of Chemical Industry     

Investigations on the conversion of onion aroma precursors S‐alk(en)yl‐L‐cysteine sulphoxides in onion juice production

Onion flavour develops when the cells are disrupted, allowing the enzyme alliinase (EC 4.4.1.4) to act upon the aroma precursors S‐alk(en)yl‐L ‐cysteine sulphoxides (ACSOs). For maximum flavour release, optimal cell disintegration and complete enzymatic conversion of ACSOs are desired. To assess an industrial scale onion juice extraction in regard to the extent of cell disintegration and the rate of enzymatic ACSO conversion, samples from all mass flows (ie onion, macerate, juice and pomace) were subjected to ACSO analysis. Applying a modified HPLC method, which allows baseline separation of the aroma precursors, three ACSOs, namely methyl‐, propyl‐ and 1‐propenyl‐L ‐cysteine sulphoxides (MCSO, PCSO and 1‐PeCSO), were determined. Initial ACSO ratios changed in the course of increasing degree of comminution from 17:83 to 100:0 (MCSO/1‐PeCSO). Only negligible amounts of PCSO were detected (<9.2 nmol g^?1 fresh weight (fw)), contributing less than 0.2% to the total ACSO content. For evaluation of the disintegration efficiency a chart was established consisting of an abscissa representing a percentage scale of ruptured cells and ordinates for the development of the marker parameters ACSOs and reaction product pyruvate relative to their initial contents. From this correlation a cell disintegration percentage after grinding of about 70% was deduced according to the determined ACSO contents. On a molar basis, macerate, juice and pomace showed 15, 0.6–2.4 and 0.7–1.0% of initial ACSO contents respectively. Only trace amounts of 1‐PeCSO were found in the juice (<16 nmol g^?1 fw). MCSO was converted to almost 90%, indicating that enzymatic transformation, in a practical sense, goes to completion. Copyright © 2004 Society of Chemical Industry     

The use of synthetic (±)-s-1-propyl-l-cysteine sulphoxide and of alliinase preparations in studies of flavour changes resulting from processing of onion (allium cepa l.)

Large losses of onion flavour components result from certain post-harvest processes such as cooking by boiling, dehydration, freezing etc. The mechanisms by which these losses occur have been investigated on the basis of the alliinase-precursor system and shown to include: (a) complete or partial enzyme destruction; (b) partial non-enzymic destruction of precursors; and (c) partial enzymic hydrolysis of precursors with loss of volatile reaction products. A stable, freeze-dried alliinase preparation and synthetic (±)-S-1-propyl-L-cysteine sulphoxide ((±)-PCSO) were prepared by known methods. Addition of an excess of enzyme to the processed product under optimal conditions was used in the determination of residual native substrate in the processed products and of an excess of the synthetic substrate for assay of enzymic activity. As compared with fresh onion as 100, enzymic activities of the processed products were as follows: freeze-dried 45, laboratory-frozen 18, hot-air dried 8.8–10, commercially-frozen 6.6, boiled 5.2 and pickled 0.01. Non-enzymic destruction of substrate (precursor) was greatest in the hot-air dried product (ca 80%) and least in freeze-dried onion (2.0%) Model experiments based on the alliinase preparation, (±)-PCSO and cis-S-1-propenyl-L-cysteine sulphoxide (the major component of the native onion precursors is the trans-isomer) exaggerated destruction of enzyme at 60 and 100 °C as compared with the relevant processed products, yet underestimated the substrate losses at these temperatures. It was concluded that factors other than those embodied in the model conditions were also involved.     

Quantitative analysis of the S-alk(en)yl-L-cysteine sulphoxides in onion (Allium cepa L.)

Quantitative analysis of each of the S-alk(en)yl-L-cysteine sulphoxides was achieved by the following procedure. A methanol: chloroform: water extract of onion tissue was purified by electrophoresis. Alk(en)yl-L-cysteine sulphoxides were separated by thin-layer chromatography (t.l.c.) on commercial silica-gel plates and were reacted with ninhydrin, the spot intensity being measured by an integrating densitometer. The synthetic analogue (±)-S-1-butyl-L-cysteine sulphoxide was used as an internal standard in each extract. In white bulb onion (cv. Southport White Globe), (±)-S-1-propyl-L-cysteine sulphoxide is the predominant flavour precursor at a concentration of 2.9 ± 0.4 mg g^?1 fresh weight. ±-S-1-methyl-L-cysteine sulphoxide and (±) trans-S-1-propenyl-L-cysteine sulphoxide are present at the lower concentrations of 0.9 ± 0.25 mg g^?1 fresh weight and 0.6 ± 0.15 mg g^?1 fresh weight respectively. Stable n-butyl, trifluoroacetyl derivatives of standards and of the alk(en)yl-L-cysteine sulphoxides in extracts, were formed. Subsequent g.c.-m.s. enabled parent ions and fragmentation patterns to be recorded and allowed the conclusive identification of the endogenous alk(en)yl-L-cysteine sulphoxides.     

A rapid spectrophotometric method of determination of thiopropanal s-oxide (lachrymator) in onion (allium cepa l.) and its significance in flavour studies

The method depends upon rapid extraction of the lachrymator (thiopropanal S-oxide) into hexane at 0 °C and observation of the absorbance maximum at 254 nm as compared with that of the synthetic compound as a standard. Thiopropanal S-oxide was synthesised by dehydrochlorination of 1-propanesulphinyl chloride by a published method. Evidence that the observed absorbance was due to the presence of thiopropanal S-oxide was based mainly on agreement of the properties of the synthetic compound with those of hexane extracts of fresh onion juice with respect to λ_max values, reaction with L-cysteine reagent, and thin-layer chromatography. Further confirmation was afforded by the properties of alliinase fission products of synthetic (±)-S-l-propenyl-L-cysteine sulphoxide. A series of applications of the method in determination of onion flavour intensity, including non-destructive sampling, is described.     

Assessment of the flavour of fresh uncooked onions by taste‐panels and analysis of flavour precursors,pyruvate and sugars

Flavour in fresh onions is dominated by volatile sulphenic and thiosulphenic acids that are liberated once alk(en)yl cysteine sulphoxide (ACSO) flavour precursors are cleaved by the enzyme alliinase after tissue disruption. The levels of pyruvate and ACSOs in over 100 samples of onions marketed in the UK were measured, and compared with assessment by taste‐panels. There was a linear relationship between the content of ACSOs and pyruvate. Measurements of pyruvate indicated that the marketing classification of some types of onion did not correspond to their pyruvate levels. A significant linear relationship was found between a sensory measure of strength and pyruvate over the range 1.2–9.3 µmol pyruvate g^?1 fresh weight. In most cases, when a flavour classification of sweet, mild or strong was applied to a sample of onions based on pyruvate content, the taste‐panels agreed with the categorization. The taste‐panels were unable to identify a sweet flavour in onions, except at low levels of pyruvate. Taste‐panels were able to define a likeability character (attractiveness of flavour) for onions, which correlated with the level of pyruvate. However, for some varieties, the flavour classification or likeability did not correspond to predictions based on pyruvate levels alone. Pyruvate measurements were seen as a suitable method for routine quality control once the characteristics of a variety of onion had been established, but initial evaluations should include well‐designed taste‐panel assessments. Copyright © 2004 Society of Chemical Industry     

Analysis of the Major Flavonol Glycosides Present in Four Varieties of Onion (Allium cepa) and Changes in Composition Resulting from Autolysis

The major flavonoids of mature onion bulb were confirmed as the 3,4′-O-diglucoside (Qdg) and 4′-O-monoglucoside (Qmg) of quercetin using a combination of chromatographic comparisons, mass spectrometry and nuclear magnetic resonance spectroscopy. These two components account for over 85% of the total flavonoids in three varieties of onion with Qdg as the main component. Quercetin is detected in these long stored onions but only at low levels of less than 2% of the total. The remaining flavonoid fraction (approx 15%) comprises upto 17 different components of which quercetin-3-O-glucoside and isorhamnetin glucoside are prominent members although each contribute less than 1% of the total flavonoid fraction. There are significant differences in the levels of Qdg and Qmg between the four different onion varieties analysed; Qdg varying from 50–1300 mg kg^-1 fresh onion tissue and Qmg from 36–394 mg kg^-1. Maceration of the tissue for the three varieties tested led to a loss of Qdg and the appearance of Qmg and free quercetin. In the variety Rijnsburger 50% of the Qdg was degraded in 5 h and had completely disappeared after 24 h. These changes in Qdg can be quantitatively explained by increases in Qmg and free quercetin. The possible significance of quercetin glycosides in the diet is discussed. © 1997 SCI     

Effect of sulphate nutrition on flavour components of onion (Allium cepa)

It is known that the volatile components of raw and cooked onions contain mono-, di- and tri-sulphides and other flavour-contributing sulphur compounds. The relationship between the sulphate nutrition of spring (salad) onion (Allium cepa) and its flavour strength determined by sensory, biochemical and chromatographic methods has been investigated. The plants were grown in sand culture in a glasshouse and the nutrient solutions contained five levels of sulphate from zero to an adequate amount. In their growth response, total sulphur content and visual symptoms, the plants showed characteristic essential nutrient deficiency. The sensory tests, and instrumental and biochemical assays showed that flavour strength increased approximately in proportion to sulphate concentration in the medium; lachrymatory potency also increased. There were significant coefficients of correlation between sulphate concentration of the nutrient medium and some aspects of flavour strength. The correlation coefficients between total sulphur content (% of the edible portion) and total peak area, n-propyl disulphide peak area and total pyruvate were also high. These observations suggest the possibility of control of onion flavour strength and lachrymatory potency, under field conditions, by control of sulphate nutrition. The question of loss of maximum potential onion flavour as a consequence of sulphur deficiency in soils is discussed. A headspace method for sampling onion flavour volatiles and for their introduction into gas chromatographs is described.     

Characterization of Shortday Onion Cultivars of 3 Pungency Levels with Flavor Precursor, Free Amino Acid, Sulfur, and Sugar Contents

ABSTRACT:  This study was conducted to characterize shortday onions of 3 pungency levels with regard to the composition of flavor related compounds. A total of 9 onion breeding lines/cultivars were selected, per each of low, medium, and high pungency level, with pyruvic acid contents of 1.9 to 2.8, 4.8 to 5.4, and 7.2 to 8.3 μmoles/mL, respectively. The contents of flavor precursors (S-1-propenyl-L-cysteine sulfoxide [1-PeCSO] and S-methyl-L-cysteine-sulfoxide [MCSO]), free amino acids, free sugars, soluble solids (SSC), and total sulfur (S) in onion bulbs were measured. The flavor precursor contents ranged from 0.03 to 0.16 mg/g fresh weight (FW) for MCSO, 0.07 to 0.65 mg for 1-PeCSO, and 0.12 to 0.77 mg in total, and precursor contents increased with the pungency levels. Onions of different pungency levels did not differ in the contents of individual or total free amino acids, and the most abundant amino acids were glutamine and arginine. The total sugar contents ranged from 50 to 75 mg/g FW, and total S contents (3.5 to 5.1 mg/g dry weight) were not correlated with the pungency levels. However, pungency levels were correlated inversely with bulb weight and positively with SSC, presumably by the effect of dilution. This study indicates that onion pungency is primarily determined by the content of flavor precursor compounds and not by total S, total sugars, or individual/total free amino acids in shortday bulbs.     

Non-structural carbohydrates in processed soft fried onion (<Emphasis Type="Italic">Allium cepa</Emphasis> L.)

The effects of curing, removal of top and root, peeling, storage of raw and peeled onion on the content of non-structural carbohydrates and the ability of onion to develop a brown colour were studied. Glucose, fructose, sucrose, 1-kestose, neokestose, nystose, 1^F--d-fructofuranosylnystose and four unknown oligofructosides were quantified using high performance anion exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD). Field drying affected the concentration of fructose only. Glucose, fructose and an unknown fructan decreased in concentration while the concentration of, e.g. 1-kestose, neokestose and nystose increased by removal of root and top of the onions when stored at 15 °C for one week. Peeling had minor influence on changes in non-structural carbohydrates. Storage of non-peeled (raw) onions at 5 °C and storage of mechanically damaged and peeled onion at 20 °C promoted fructan hydrolysis and browning of soft fried onion. Similar changes were produced by long-term storage of cultivars of common onion. Dry matter and fructose increased during storage of peeled onions at both 5 and 20 °C. The increase in fructose content during storage was due to a significantly decrease in the content of 1-kestose, neokestose, 1^F--d-fructofuranosylnystose, an unknown fructan and total fructans. It was concluded that the missing ability of industrially processed onions to develop a brown colour may be overcome by storage of the peeled onions for a few hours or even overnight at 5 °C.     

A survey of volatile components of some allium species in terms of s-alk(en)yl-l-cysteine sulphoxides present as flavour precursors

The volatile flavour components of 27 Allium species and cultivars, mostly edible but including some decorative species for comparison, have been investigated by a series of gas and thin-layer chromatographic and u.v. spectrophotometric methods. By means of simulation experiments with synthetic precursors and intermediates, the data have been interpreted in terms of the amino acid precursors present in the intact tissues and the species classified as containing (a) S-1-propenyl-, (b) S-2-propenyl- and (c) S-methyl-L-cysteine sulphoxides as their principal flavour precursors. Characteristic examples of the three types are A. cepa L. (onion), A. sativum L. (garlic) and A. aflatunense B. Fedtschenko, respectively. An object of the work has been to demonstrate broad chemical similarities between species as well as their differences, which are emphasised in keys for classification on the basis of morphological characters. In addition to onion, leek, shallot, garlic and chives, species such as A. chinense, A. fistulosum and A. tuberosum have widespread use as food, particularly in the Far East. The literature on this aspect has been briefly summarised.     

Effects of cultivar,lifting time and nitrogen fertiliser level on quercetin content in onion (Allium cepa L.) at lifting

Commercial cultivars of onion (Allium cepa L.) were grown at Torslunda research station, Sweden with different levels of nitrogen fertiliser and lifted at different growth stages. Soon after lifting, before any drying or curing, tissue from the fleshy edible part of onions was extracted in ethanol and the raw extracts were analysed by high‐performance liquid chromatography without any previous hydrolysis. It was confirmed that the main flavonoid compounds were quercetin monoglucoside and quercetin diglucoside, whereas only trace amounts of quercetin aglycone and other flavonoids were found. The greatest variation in quercetin content, in the range 100–500 mg kg^?1 fresh weight, occurred between years. The concentration of quercetin glucosides in the onions was strongly correlated (R² = 0.98) with the total amount of global radiation in August. Individual onions with fallen leaves had significantly higher concentrations of quercetin glucosides than individuals in the same row with erect leaves. Only minor differences were found between the three cultivars analysed. Higher levels of nitrogen fertiliser had only minor effects on onion yield and size and resulted in lower or equal amounts of quercetin glucosides. Nitrogen leakage from the soil, a potential source of environmental problems, could therefore be minimised by avoiding high nitrogen fertiliser levels with almost no effect on onion flavonol content. Late lifting of onions (80% fallen leaves) resulted in up to 45% higher concentrations of quercetin glucosides compared with early lifting (50% fallen leaves). Copyright © 2006 Society of Chemical Industry     

Formation of dimethyl sulphide from S-methylmethionine in onion seedlings (Allium cepa)

Homogenates of onion seedlings catalyse the decomposition of S-methyl-methionine into dimethyl sulphide and homoserine. The reaction is rather slow and seems not to be of primary importance in onion flavour formation or S-methylmethionine metabolism. Extracts of scarlet runner bean seedlings are also active, whereas dormant onion bulb and other seedlings tested showed no or very low activity.     

Protective effects of fermented onion juice containing higher amount of querectin aglycone against oxidative stress by 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH) treatment in Sprague–Dawley rats

It has been reported on beneficial effects of onion extracts containing a large amount of quercetin glucosides for several decades. This study was performed to investigate antioxidative and hepatoprotective effects of fermented onion (Allium cepa L.) extracts containing higher amount of quercetin aglycone (FOQ) against oxidative stress by 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH) treatment in rats. RP-HPLC analysis elucidated that the fermented onions contain larger amount of quercetin aglycone than that of fresh onions, and the content of quercetin aglycone in 1.0 kg of fresh and fermented onion pickles was 261.34 ± 19.47 and 360.25 ± 27.32 mg, respectively. In vitro free radical scavenging assay showed potently radical-quenching ability of FOQ against reactive oxygen species (ROS) such as hydroxyl, superoxide, and carbon-centered radical using electron spin resonance (ESR) spectroscopy. Biological parameters in plasma and liver tissue were evaluated with 36 healthy Sprague–Dawley rats in the experimental groups (n = 6); control (basal diet), AAPH treated, quercetin aglycone treated, ascorbate-AAPH treated, quercetin aglycone-AAPH treated and FOQ-AAPH treated group. The results revealed that quercetin aglycone or FOQ intakes could increase the level of glutathione (GSH), enzymatic activity of glutathione reductase (GSSG-R), and catalase (CAT), but could decrease the level of glutamate oxaloacetic transaminase (GOT), glutamate pyruvate transaminase (GPT), thiobarbituric acid reactive substance (TBARS) production with the abnormal levels caused by AAPH induced oxidative stress. Moreover, histological study for central vein (CV) area of hepatic lobule was illustrated that cytoplasmic phase near the CV in the qurecetin and FOQ pretreated before AAPH injection could be maintained with normal morphology like that of the control. The present study illustrated that the fermented onion extracts containing larger amount of quercetin aglycone (FOQ) have antioxidative activities against AAPH-induced carbon-centered radical, and its activity can present the protective effect against hepatic damage by oxidative stress similar with the ability of ascorbate (vitamin C).     

Amount of characteristic compounds in 15 cultivars of onion (Allium cepa L.) in controlled field trials

BACKGROUND: In order to develop the production of sweet onions (Allium cepa L.) and onions with a high content of flavonols and fibre in Norway, it was necessary to evaluate available cultivars with respect to their content of selected chemical compounds. A field trial with 15 different cultivars was conducted at Landvik in southern Norway. Compounds that reflect onion pungency and sweetness (pyruvate, sugars) and parameters known to have a beneficial impact on health (fructo‐oligosaccharides (FOS), flavonols, antioxidant capacity) were analysed. RESULTS: The sweetest onions with respect to low content of pyruvate and high amount of fructose and glucose ranged in pyruvate content from 2.7 to 3.6 µmol mL^?1. These cultivars were juicy and contained low amounts of FOS and total flavonols. The most pungent onions contained the highest amounts of FOS among the cultivars: 4.53, 3.80 and 5.81 g per 100 g fresh weight (FW) respectively. They also had a high content of flavonols (86.6, 159.2 and 97.6 mg per 100 g FW) and high dry weight content (14.6, 13.5 and 15.6 g per 100 g). CONCLUSION: The sweetest onion cultivars were found to be ‘NUN 9746’, ‘Musica’, ‘Domenica Supersweet’, ‘Colossus’ and ‘Cronus’. The most pungent cultivars were ‘Powell Brown’ and ‘Tilbury’, which also contained the highest amounts of flavonols and gave the highest Trolox equivalent antioxidant capacity (TEAC) values among the 15 cultivars. Copyright © 2007 Society of Chemical Industry     

Distribution of flavour components in onion (Allium cepa L.), leek (Allium porrum) and garlic (Allium sativum)

Dry bulb onions and leeks, at the marketable stage of development, were dissected into nine and ten fractions respectively, comprising the stem and leaves in succession from the oldest outer layers. Each fraction was separately analysed for flavour components by determination of thiosulphinate and pyruvate. Similarly, garlic cloves were divided into five fractions. There was a consistent pattern of flavour distribution in the seven onion varieties studied. The dried outer scales were virtually free from flavour components, the concentrations of which (on a fresh weight basis) increased progressively from a minimum value in the outer leaf to the innermost tissue and the stem. The latter often contained more than twice the concentration present in the outer leaf. The pattern in the roots and edible portions of leek differed in detail from that in onion. Flavour intensity increased progressively from the outer leaf to the inner tissues and the concentrations in both the roots and the stem were approximately equivalent to that of the fourth leaf numbered from the outer leaf. The pattern of flavour distribution in garlic cloves was simpler than that of onion and leek. The storage leaf accounted for the bulk of the weight of the clove as well as its flavour content. Some aspects of knowledge of the metabolism of inorganic nitrogen and sulphur compounds in roots and other tissues and transport of the corresponding metabolites have been briefly reviewed as the basis of a preliminary interpretation of the above observations. Attention has been drawn to the bearing of the results on preparation of onion for cooking and processing and on sampling for analysis.     

DIMETHYL TRISULPHIDE,ITS MECHANISM OF FORMATION IN HOP OIL AND EFFECT ON BEER FLAVOUR

The precursor of dimethyl trisulphide (DMTS) in hops is destroyed by sulphur dioxide during kilning, but then re-forms slowly during storage. A redox mechanism is suggested and S-methyl-cysteine sulphoxide is postulated as the DMTS-precursor. The effect of DMTS on beer flavour is described. The flavour threshold value for DMTS in a commercial beer has been found to be 0·10μg/litre.     

Nutritional Composition of Processed Onion: S-Alk(en)yl-l-cysteine Sulfoxides, Organic Acids, Sugars, Minerals, and Vitamin C

The aim of this work was to study the effect of onion high-pressure processing (HPP) and HPP combined with freeze-drying and pulverization, focusing on onion nutritional attributes in terms of S-alk(en)yl-l-cysteine sulfoxides, organic acid, sugar, mineral, and vitamin C contents. Allium cepa L. var. cepa, ‘Recas’ was treated at T1 (200 MPa/25 °C/5 min), T2 (400 MPa/25 °C/5 min), and T3 (600 MPa/25 °C/5 min). After treatment, high-pressure (HP)-treated (T1, T2, and T3) and untreated or non-HP-treated (T0) samples were frozen (diced onion). Subsequently, part of the diced samples was freeze-dried and pulverized (pulverized onion). In general, the freeze-drying process of fresh onion maintained the concentration of S-alk(en)yl-l-cysteine sulfoxides (ACSOs), organic acids, total sugars, total protein, and minerals, and significantly increased vitamin C concentration. Regarding HPP, the concentration of ACSOs, total sugars, and vitamin C of fresh onion was preserved at the three HPP assayed (200, 400, 600 MPa for 5 min at 25 °C); however, for some organic acids and minerals, small decreases were found. In HP-treated onion (especially for treatment at 400 MPa), freeze-drying retained the levels of the major organic acids, total sugars, total protein, and minerals (Ca, Fe, Mg, K, Na, Zn, Mn, and Se), and increased the content of ascorbic acid and total vitamin C.     

Antimicrobial and antioxidant activity of crude onion (Allium cepa,L.) extracts

Antioxidant and antimicrobial activity of the ethyl acetate and water subfractions of methanolic extracts of three Spanish onion varieties were assayed. Flavonoids were mainly present in ethyl acetate subfraction being 34.92 ± 0.75, 7.95 ± 0.16, 0.38 ± 0.01 μmol of rutin eq. g^?1 D.W. and its antioxidant capacity was 74.86 ± 1.77, 24.59 ± 0.67, 4.55 ± 0.44 μmol Trolox g^?1 D.W. of Grano de Oro, Fuentes de Ebro and Calçot de Valls varieties, respectively. The antimicrobial activity of flavonol standards and onion extracts was evaluated against some food spoiler microorganisms. Quercetin and kaempferol were inhibitory against gram positive bacteria such as Bacillus cereus, Staphylococcus aureus, Microcroccus luteus and Listeria monocytogenes. Gram negative bacteria Escherichia coli and Pseudomonas aeruginosa were less sensible to the antimicrobial effect of both flavonol standards and Candida albicans was totally resistant. Among the onion extracts tested only ethyl acetate subfraction showed antimicrobial inhibition.