Characterisation of onion (Allium cepa L.) by-products as food ingredients with antioxidant and antibrowning properties

Processing and stabilising onion wastes (residues and surpluses of onion) could solve the environmental problem derived from a great onion wastes disposal. Moreover, obtaining stabilised onion by-products as natural antioxidant food ingredients could be advantageous to food industry, not only to improve the use of onion wastes but also to obtain new natural and functional ingredients. The aim of this study was to characterise onion by-products – juice, paste and bagasse – from two Spanish onion cultivars – ‘Figueres’ and ‘Recas’ – that have been stabilised by thermal treatments – freezing, pasteurisation and sterilisation – in order to evaluate the effect of the processing and stabilisation treatment on the bioactive composition, antioxidant activity and polyphenol oxidase (PPO) enzyme inhibition capacity. The results obtained triggered to choose one onion by-product offering better characteristics for its potential development as a food ingredient: source of antioxidant and antibrowning bioactive compounds. In this study it was shown that processing of ‘Recas’ onion wastes to obtain a paste (mixture content) and applying a mild pasteurisation were the best alternatives to obtain an interesting stabilised onion by-product with good antioxidant properties that made useful its use as functional food ingredient.     

Non-destructive flavour evaluation of red onion (Allium cepa L.) Ecotypes: An electronic-nose-based approach

This work reports preliminary results on the potential of a metal oxide sensor (MOS)-based electronic nose, as a non-destructive method to discriminate three “Tropea Red Onion” PGI ecotypes (TrT, TrMC and TrA) from each other and the common red onion (RO), which is usually used to counterfeit. The signals from the sensor array were processed using a canonical discriminant function analysis (DFA) pattern recognition technique. The DFA on onion samples showed a clear separation among the four onion groups with an overall correct classification rate (CR) of 97.5%.     

Profiling and quantifying quercetin glucosides in onion (Allium cepa L.) varieties using capillary zone electrophoresis and high performance liquid chromatography

There is increasing evidence that flavonols demonstrate beneficial properties for human health. Quercetin is the major flavonol present in onion (Allium cepa cv) and is present predominantly as quercetin 3,4′-diglucoside and quercetin 4′-monoglucoside. These compounds are known to be potent free radical scavengers and antioxidants, and are considered to be protective against cardiovascular disease. Analysis for the presence of these compounds has therefore become more important. Robust capillary zone electrophoresis and high performance liquid chromatography procedures were developed for profiling and quantifying the levels of quercetin 3,4′-diglucoside and quercetin 4′-monoglucoside in 70% methanol/water extracts of six different onion varieties available in Victoria, Australia. Quercetin 3,4′-diglucoside, which is not commercially available as a reference standard, was isolated from freeze-dried onion powder by preparative high performance liquid chromatography and used to quantify the levels in the onion extracts. Significant differences in the levels and ratios of the two compounds were seen between red, brown and white onion varieties (e.g. ‘Redwing’; quercetin 3,4′-diglucoside 191 mg/100 g DW, quercetin 4′-monoglucoside 85 mg/100 g DW; ‘Cream Gold’, quercetin 3,4′-diglucoside 153 mg/100 g DW, quercetin 4′-monoglucoside 58 mg/100 g DW, ‘Spanish white’; quercetin 3,4′-diglucoside <1 mg/100 g DW, quercetin 4′-monoglucoside <1 mg/100 g DW).     

Determination of selenium in garlic (Allium sativum) and onion (Allium cepa) by electro thermal atomic absorption spectrometry

A separation/enrichment procedure has been developed for the determination of selenium in garlic and onion samples by electrothermal atomic absorption spectrometry (ET-AAS) as a slurry sampling after preconcentration with 3,3-diaminobenzidine (DAB) reagent on the activated carbon. The influences of pH, time, amount of carbon and complexing reagent were outlined. The effect of acids used in the digestion of samples was also studied and compared. Selenium level was found to be 0.024 μg g⁻¹ for onion (n = 5; LOD – 0.5 μg L⁻¹; LOQ – 1.7 μg L⁻¹) and 0.015 μg g⁻¹ for garlic (n = 5; LOD – 1.3 μg L⁻¹; LOQ – 3.3 μg L⁻¹). Three different samples of garlic were analyzed by k₀-instrumental neutron activation analysis (k₀-INAA) at the Jozef Stefan Institute (JSI). The data obtained by k₀-INAA show that the content of selenium overlapped the results obtained by ET-AAS.     

Determination of total phenolic content and antioxidant capacity of onion (Allium cepa) and shallot (Allium oschaninii) using infrared spectroscopy

Total phenolic content (TPC) and total antioxidant capacity (TAC) of four onion varieties (red, white, yellow and sweet) and shallot from selected locations (Washington, Idaho, Oregon, Texas and Georgia) were determined using Fourier transform infrared (FT-IR) spectroscopy (4000–400 cm⁻¹). The Folin–Ciocalteu (F–C) assay was used to quantify TPC and three assays were used to determine TAC, including 2,2-diphenyl-picrylhydrazyl (DPPH) assay, Trolox equivalent antioxidant capacity (TEAC) assay and ferric reducing antioxidant power (FRAP) assay. Partial least squares regression (PLSR) with cross-validation (leave-one-out) was conducted on onion and shallot extracts (n = 200) and their corresponding F–C, DPPH, TEAC and FRAP values were employed to obtain four independent calibration models for predicting TPC and TAC for the extracts. Spectra from an extra 19 independent extracts were used as an external validation set for prediction. A correlation of r > 0.95 was obtained between FT-IR predicted and reference values (by F–C, DPPH, TEAC and FRAP assay) with standard errors of calibration (SEC) and standard errors of cross-validation (SECV) less than 2.85, 0.35 and 0.45 μmol Trolox/g FW of extracts for TEAC, FRAP and DPPH assay, respectively; and 0.36 mg gallic acid/g FW of extracts for the F–C assay. In addition, cluster analysis (principal component analysis (PCA)) and discriminant function analysis (DFA) could differentiate varieties of onions and shallot based upon infrared spectral features. Loading plots for the various chemometrics models indicated that hydroxyl and phenolic functional groups were most closely correlated with antioxidant capacity. The use of mid-infrared spectroscopy to predict the total antioxidant capacity of vegetables provides a rapid and precise alternative to traditional wet chemistry analysis.     

Anti-influenza A virus effects of fructan from Welsh onion (Allium fistulosum L.)

A fructan that acts as an anti-influenza A virus substance was isolated from hot water extract of the green leafy part of a Welsh onion (Allium fistulosum L.). The structure of the fructan was characterised and elucidated by chemical and spectroscopic analyses. The fructan was composed of terminal (21.0%) and 2,1-linked β-d-Fruf residues (65.3%) with 1,6-linked β-d-Glcp residues (13.7%). The molecular weight of the polysaccharide and polydispersity was estimated to be 1.5 × 10³ and 1.18, respectively. Although the fructan did not show anti-influenza A virus activity in vitro, it demonstrated an inhibitory effect on virus replication in vivo when it was orally administered to mice. In addition, the polysaccharide enhanced the production of neutralising antibodies against influenza A virus. Therefore, the antiviral mechanism of the polysaccharide seemed to be dependent on the host immune system, i.e., enhancement of the host immune function was achieved by the administration of the polysaccharide. From our observations, the fructan from Welsh onions is suggested to be one of the active principles which exert an anti-influenza virus effect.     

Green pigment in crushed garlic (Allium sativum L.) cloves: Purification and partial characterization

Unknown green pigment, responsible for greening in crushed garlic cloves, was purified and characterized by using a series of column chromatography, liquid chromatography–electrospray ionization (LC–ESI), fast atom bombardment (FAB), matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS), and nuclear magnetic resonance (NMR) spectroscopy. The purified green pigment was highly polar and slightly viscous, with a garlic flavor, and easily turned to a yellow or brown color with exposure to room temperature. The absorption spectrum in methanol showed a crude methanolic green pigment-like profile with two absorbance maxima at 440 and 590 nm.     

Structure of pectic polysaccharides isolated from onion Allium cepa L. using a simulated gastric medium and their effect on intestinal absorption

The polysaccharide fraction extracted with simulated gastric juice from onion bulbs contained a mixture of galactan with short-length sugar chains, pectic polysaccharides and evident content of proteinaceous material. Galacturonan and rhamnogalacturonan were the main constituents of the linear regions of the sugar chains of the pectic polysaccharides. The ramified regions included rhamnogalacturonan-I. NMR data revealed that the side chains of the ramified region contained mainly 1,4-linked β-d-galactopyranose residues and lesser content of 1,3-linked β-d-galactopyranose and 1,5-linked α-l-arabinofuranose residues. Furthermore, the proteinaceous material was determined to be partly linked to the sugar chains.     

Total phenolics,antioxidant properties and quality of fresh-cut onions (Allium cepa L.) treated with mild-heat

This study investigated the effect of mild-heat on fresh-cut onion slices by treating in hot water (50, 60, 70 °C) for 1 min. Total phenolics (TP), antioxidant properties, colour, and weight loss of slices were evaluated during 4 °C storage at 7-day intervals (21 days total). The 60 °C heat treatment resulted in a significant increase in TP, from 44.92 to 52.32 mg GAE/100 g. Except for 50 and 70 °C treatments, TP in control and 60 °C treated fresh-cut onions decreased during storage. The antioxidant properties of fresh-cut onions were 1.31, 0.99, and 62.49 μM TE/g using ABTS, DPPH, and ORAC assays, respectively. The mild-heat treatments did not affect ABTS and DPPH antioxidant activities and the colour of fresh-cut onions. The storage time had mixed effect on the antioxidant properties (ABTS decreased; DPPH and ORAC remained fairly stable). The 50 °C samples exhibited the lowest weight loss during 21-day storage.     

Isolation and chemical evaluation of carob (Ceratonia siliqua L.) seed germ

The present investigation was undertaken in order to study the influence of the dehusking procedure on the germ meal composition of carob (Ceratonia siliqua L.) seeds and also to investigate its detailed composition and nutritive value. Meals of carob seed germ were obtained by acid treatment or boiling water treatment of the whole seeds. These procedures allowed to separation of the tight-fitting brown coat of the seed and the removal of the endosperm. Results indicated that the carob germ meal composition could be affected by the isolation procedures. Small reductions were observed in protein and lipid contents in germ meals from acid extraction. The analysis of the carob germ meal (containing fine fragments of husk and endosperm), which could be really obtained industrially, showed the following composition: moisture 8.3%, ash 6.5%, lipids (neutral and polar) 6.6% containing ∼21% of polar lipids, crude proteins 54.7% and energy value 17.5 kJ/g.     

Distribution of quercetin-3,4′-O-diglucoside,quercetin-4′-O-monoglucoside,and quercetin in different parts of the onion bulb (Allium cepa L.) influenced by genotype

Flavonoids have gained much attention because of their proposed positive effects for human health. Onions are a rich source of flavonoids, consisting mainly of the major flavonols quercetin-3,4′-O-diglucoside (QDG) and quercetin-4′-O-monoglucoside (QMG) in the bulb and the aglycone quercetin in the outer scales. In this study, distribution of these three flavonoids was determined in 16 onion cultivars (Allium cepa) using HPLC–DAD. Three different parts of the onion bulb, the inner layers, the middle layers and the outer scales were analysed. The analysis showed varietal differences in the selected onion cultivars. The cultivars with the highest total flavonoid content were the red skinned ‘Red Baron’ and the yellow skinned cultivars ‘Ailsa Craig’ and ‘Prilep’. The distribution of the total flavonoid content in the different parts of the onion bulb showed the following order: middle layers > outer scales > inner layers. In the inner layers QDG was the major flavonoid, while in the middle layers QDG and QMG were in equal amounts. In the outer scales quercetin was the major flavonoid prior to QMG.     

Amino acid profile of raw and as-eaten products of spinach (Spinacia oleracea L.)

The aim of the work was to evaluate the amino acid composition of fresh spinach and spinach products prepared for consumption. The investigation included fresh and cooked spinach and two kinds of frozen product: one obtained using the traditional method (blanching-freezing-refrigerated storage) and then cooked; and the other, of the ready-to-eat type, obtained using the modified method (cooking-freezing-refrigerated storage) and then prepared for consumption in a microwave oven. In 100 g of edible parts of the as-eaten products, the content of amino acids exceeded that in the raw material from which they were obtained, except for sulphur amino acids and tyrosine. In the as-eaten products, the content of amino acids in protein calculated in 16 g N was similar to that in the raw material, except for lower tyrosine and higher arginine content in the traditional frozen product prepared for consumption. Cystine with methionine was the amino acid limiting the quality of protein in the investigated samples, except for the traditional frozen product prepared for consumption, where the Chemical Score (CS) index was 100.     

Identification of candidate amino acids involved in the formation of pink-red pigments in onion (Allium cepa L.) juice and separation by HPLC

The formation of pink-red pigments ("pinking") by various amino acids was investigated by reacting amino acids with compounds present in onion juice. The unknown pink-red pigments were generated and separated using high-performance liquid chromatography (HPLC) and a diode array detector (DAD) in the range of 200 to 700 nm. To generate pink-red pigments, we developed several reaction systems using garlic alliinase, purified 1-propenyl-L-cysteine sulfoxide (1-PeCSO), onion thiosulfinate, natural onion juice, and 21 free amino acids. The compound 1-PeCSO was a key compound associated with pinking in the presence of both the alliinase and amino acids. Numerous naturally occurring pink-red pigments were detected and separated from pink onion juice using the HPLC-DAD system at 515 nm. Most free amino acids, with the exceptions of histidine, serine, and cysteine, formed various pink-red pigments when reacted with onion thiosulfinate. This observation indicated that onion pinking is caused not by a single pigment, but by many. Furthermore, more than one color compound could be produced from a single amino acid; this explains, in part, why there were many pink-red compound peaks in the chromatogram of discolored natural onion juice. We presumed that the complexity of the pink-red pigments was due to the involvement of more than 21 natural amino acids as well as several derivatives of the color products produced from each amino acid. We observed that the pinking process in onion juice is very similar to that of the greening process in crushed garlic, emphasizing that both thiosulfinate from flavor precursors and free amino acids are absolutely required for the discoloration.     

Determination of falcarinol in carrot (Daucus carota L.) genotypes using liquid chromatography/mass spectrometry

A new analytical method for the determination of falcarinol [(Z)-heptadeca-1,9-diene-4,6-diyn-3-ol] in carrot root samples has been developed and validated. The method consists of accelerated solvent extraction (ASE) of lyophilised carrot root samples with ethyl acetate and LC–MS analysis of the extracts. Falcarinol was determined by extracting the main ion species generated in the ESI positive mode, m/z 268 [M+H–H₂O+MeCN]⁺, from the full MS chromatogram. Quantitation was performed using a falcarinol calibration curve (correlation coefficient 0.9975) as an external standard and pelargonic acid vanillylamide as an internal standard. The method showed good precision with interday and intraday variation of less than 4% and high recovery (average recovery rate 97.9%). LOD (S/N = 3) and LOQ (S/N = 10) were 2.5 and 7 ng, respectively. Using this method, 27 different carrot genotypes grown and harvested under the same conditions were analyzed, and falcarinol contents ranging from 0.70 to 4.06 mg/100 g fresh weight were determined.     

Assessment of bulb pungency level in different Indian cultivars of onion (Allium cepa L.)

Onion (Allium cepa L.) is an important vegetable crop consumed primarily for its ability to enhance the flavor of other foods. The quality of onion depends on its pungency. While highly pungent onions are popular in India, less pungent ones are preferred in other countries. However, the variability in pungency in bulbs of different cultivars however has not been investigated. The present investigation was therefore undertaken to determine the pungency in three popular Indian onion cultivars viz. N-2-4-1, B-780 and Phule Safed. Randomly selected bulb samples of the cultivars were analyzed for the content of pyruvic acid, total soluble solids (TSS) and reducing, non-reducing and total sugars. The red variety N-2-4-1 showed higher level of pungency, while other two varieties were comparatively less pungent.     

Fractionation and biochemical characterization of moth bean (Vigna aconitifolia L.) proteins

Moth bean seeds contained 24 g/100 g protein (micro-Kjeldahl N×6.25) on a dry weight basis. Among the solvents tested, 0.1 mol/l NaOH and 0.1 mol/l sodium phosphate buffer (pH 7.5) were the most effective in solubilizing the seed flour proteins. Native isoelectric focusing of the total soluble proteins indicated moth seed proteins to be acidic (pI range 4.55-<6.55) with a few neutral to alkali proteins (pI range 7.35-9.3). Globulin fraction dominated the seed protein composition accounting for ∼64 g/100 g of the total soluble proteins in the seeds. The globulin fraction was composed of at least three major glycopeptides with an estimated molecular mass range of 45-55 kDa and several additional polypeptides in the 14-32 kDa range. Sulfur amino acids were the first limiting amino acids in the seed flour proteins. Native and heat denatured albumin and glutelin fractions were readily hydrolysed, in 30 min, by pepsin at 37 °C. Although resistant to proteolysis in the native state, heat denatured globulin (100 °C, 30 min, boiling water-bath) was completely digested to <14 kDa polypeptides by pepsin in 30 min at 37 °C. Moth bean was devoid of detectable phytohemagglutinating activity and had low trypsin inhibitory activity when compared with the corresponding activities in soybeans.     

Identification and quantification of phenolic compounds from sunflower (Helianthus annuus L.) kernels and shells by HPLC-DAD/ESI-MS

Phenolic compounds were extracted from defatted sunflower (Helianthus annuus L.) kernels and shells and characterised by HPLC with diode array and electrospray ionisation (ESI) mass spectrometric detection in the negative mode. Quantification of individual compounds was carried out by external calibration. Among the eleven compounds analysed 5-O-caffeoylquinic acid was predominant amounting up to 59.1 mg/100 g in the shells and 3050.5 mg/100 g in the kernels. The specific fragmentation patterns of mono- and dihydroxycinnamoylquinic acids allowed the unambiguous distinction of several stereoisomers which have not been described for sunflower seeds and seed shells so far. The total phenolic content of about 4200 mg/100 g on a dry matter basis revealed defatted sunflower meal to be a promising source of phenolic compounds that might be recovered and used as natural antioxidants. Furthermore, the press residues originating from sunflower oil extraction were shown to be still rich in phenolic antioxidants, thus, providing the opportunity to valorize these by-products in terms of sustainable agricultural production.     

Effect of cooking and sterilisation on the composition of amino acids in immature seeds of flageolet bean (Phaseolus vulgaris L.) cultivars

The aim of the work was to evaluate the amino acid content and protein quality in products of immature bean seeds with a dry matter content of 40%. The investigation included raw seeds, fresh seeds after cooking to consumption consistency and seeds after canning. Raw seeds of the cultivar Alamo contained more amino acids in 100 g fresh matter than the seeds of the cultivars Flaforte and Mona. Cooking and sterilisation brought about decreases in the content of all the amino acids in the investigated cultivars; 1–50% and 9–60% respectively. Higher losses being noted in sterilised seeds. If the results were expressed in 16 g N, the differences in amino acid content were much smaller. The amino acid content of Mona was lower than that of the remaining cultivars. The cooking of seeds did not bring about changes in relation to the raw material (except tyrosine which showed a decrease 35–45%); however, sterilisation affected the content of most amino acids (from −45 to +17%). Compared with the standard (FAO/WHO/UNU, 2007), the limiting amino acid was cystine with methionine (amino acid score 70–83). Of the seeds prepared for consumption, sterilised bean seeds had a higher EAA index value, while of the cultivars, the seeds of Alamo had the highest value (147–162).     

Screening for ligands of human aromatase from mulberry (Mori alba L.) leaf by using high-performance liquid chromatography/tandem mass spectrometry

Aromatase, a cytochrome P450 (CYP19) enzyme that catalyses the conversion of androgens to oestrogens, is an important target for breast cancer therapy. Flavonoid has been proved to be potential therapeutic agent for breast cancer by inhibiting the activity of aromatase and is one of the main active constituents of mulberry leaves. In this study, twelve flavonoids, 3,5,7,3′,4′-pentahydroxyflavone-3-glucosyl-glucoside/3,5,7,3′,4′-pentahydroxyflavone-3,7-diglucoside, mulberroside F, 3,5,7,3′,4′-pentahydroxyflavone-3-(6″-malonyl)-glucoside, 3,5,7,4′-tetrahydroxyflavone-3-(6″-malonyl)-glucoside, 3,5,7,4′-tetrahydroxyflavone-3-O-(6″-acetyl)-glucoside, 3,5,7,3′,4′-pentahydroxyflavone-3-rutinoside, 3,5,7,3′,4′-pentahydroxyflavone-3-glucoside/3,5,7,2′,4′-pentahydroxyflavone-3-glucoside, kuwanon G, 5,7,3′,4′-tetrahydroxyflavone-7-rutinoside/3,5,7,4′-tetrahydroxyflavone-3-rutinoside, 3,5,7,4′-tetrahydroxyflavone-3-glucoside, 3,5,7,3′,4′-pentahydroxyflavone-3-O-(6″-acetyl)-glucoside and chalcomoracin/mongolicin F, and three fatty acids, 9,12,15-octadecatrienoic acid, compounds 12 and 13, were tentatively identified by HPLC-DAD-ESI-MS/MS method. Seven of the compounds, 3,5,7,3′,4′-pentahydroxyflavone-3-rutinoside, 3,5,7,3′,4′-pentahydroxyflavone-3-glucoside/3,5,7,2′,4′-pentahydroxyflavone-3-glucoside, 3,5,7,4′-tetrahydroxyflavone-3-glucoside, chalcomoracin/mongolicin F and 9,12,15-octadecatrienoic acid, were revealed unequal binding capability for aromatase by HPLC-DAD-ESI-MS/MS.     

Fat and fatty acids of white lupin (Lupinus albus L.) in comparison to sesame (Sesamum indicum L.)

The study was undertaken to compare fat and fatty acid profiles in white lupin (Lupinus albus ssp. albus) and sesame (Sesamum indicum L.), representing two different families, Fabaceae and Pedaliaceae. Fat levels were 10.74% and 55.44% in seeds of white lupin and sesame, respectively. The results indicated that oleic, linolenic and arachidic acids in seed fat were higher in white lupin than in sesame cultivars. Oleic acid was the predominant fatty acid in white lupin, whereas linoleic acid was predominant in sesame. Fat content (%) was statistically significantly correlated with linoleic, linolenic and arachidic acids at the genotypic level. The fatty acid composition of white lupin is useful for human consumption. Although oil content of white lupin was lower than that of sesame, white sweet lupin could be improved.