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Onion - influences on flavour and Production


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1       Introduction Onion is a famous spice commodity grown all over the world and consumed in various forms. It has been in cultivation for more than 4000 years. The maximum diversity of Allium species is found in a belt from the Mediterranean basin to Iran and Afghanistan, i.e. Iran, north Iraq, Afghanistan, Soviet middle-Asia and west Pakistan,1  indicating the primary centre of origin. The earliest record comes from Egypt where onions appear as carvings on pyramid walls and in tombs from the third and fourth dynasties (2700 BC).2  It is thought that Romans took the onion north of the Alps. The onion was among the first cultivated plants taken to the Americas from Europe. Europeans took the species to East Asia during the last century.

The distinctive  flavour of alliums have established plants as an essential  part of the cuisine of the world. It is used as immature, mature bulbs as vegetable and spice as well as food for poultry and non-milking cattle. In India and China, onion is the basis of many dishes. Nearly every Indian recipe starts with the same procedure. Fry chopped or sliced onion slowly, add spices (frequently fresh garlic and ginger and dried spices like cumin, turmeric, chillies) and  fry until  the  onion turns golden. The mixture (wet masala) may afterwards be pure΄ed, simmered with tomatoes or yogurt or just added to boiled or fresh vegetables, meat, chicken and fish to form curry. The onion rings can be fried and used as fast  food. Onion pakodas made out of sliced  onion  and  chickpea  flour is an important snack  item  in  Indian  hotels.  Onion  can  be  eaten  raw  or  cooked;  mild  flavoured  or coloured bulbs are often chosen for salad.

Alliums are also used widely in food processing. However, handling of bulk of onion and storage of bulbs for 6 months is cumbersome. Losses of fresh onion in storage have been  reported  to  be  about  25  to  30%.3   Value  addition  of  raw  onion  in  the  form  of processed products is the most practical solution. Whilst onion is traded in its raw form, the international  market is increasingly focused on dehydrated products such as flakes, rings,  granules,  powder,  etc.,  and  processed  onion  as  frozen  or  canned,  or  onions  in vinegar and onions in brine.

Allium cepa is cultivated mainly as a biennial, but some types are treated as perennials. It is propagated by seeds, bulbs or sets (small bulbs). Very wide variation in shape, size

and  colour  is  observed  in  this  species.  Intensive  selection  during  domestication  and natural  hybridization  might  have  created  variability.  There  is  a  lot  of  controversy regarding  the  taxonomic  position  of  Allium  and  related  genera.  In  early  classification onion was placed in the Liliaceae. Some British and American botanists included onion in Amaryllidaceae. In a more recent taxonomic treatment of monocotyledons, Allium and its   close   relatives   are   recognized   as   the   distinct   family   Alliaceae,  close   to   Amaryllidaceae. Robinowitch and Brewster1  suggested the following hierarchy:

1.   Class:                 Monocotyledones

2.   Super order:      Liliiflorae

3.   Order:                Asparagales

4.   Family:              Alliaceae

5.   Tribe:                 Allieae

6.   Genus:               Allium

Jones  and  Mann4   gave  simple  classification  of  genus  Allium  cepa  for  the  use  of horticulturists as under:

   a. Common Onion Group (Allium cepa L. var. Cepa; Allium cepa L. ssp. Cepa, and ssp. Australe Trofim): Bulbs are large, normally single. Plants produce from seeds or from  seed  grown sets.  The  majority  of cultivars  grown for  dry  bulbs  belong  to  this group. This is the most important group of trade grown all over the world. It includes hundreds   of   open   pollinated   varieties,   land   races   and   commercial   F1    hybrids. Maximum diversity exists in southwest Asia comprising countries like India, Pakistan, the former Soviet Republics and also Mediterranean countries.

   b.  Aggregatum  Group  or  Shallots  (Allium  ascalonicum  auct.  Non  strand;  Allium cepa  L. ssp orientale kazak): Bulbs are smaller than common onions and several  to many form an aggregated cluster. Reproduction is almost exclusively vegetative via daughter bulbs. Occasionally scapes are developed and in some types, seed production is possible. This group is of minor importance. Locally  adapted  cultivars are grown mainly in home gardens. Cultivars are more suited to humid tropical regions of Asia, West Africa and the Caribbean area.

   c.  Ever-Ready  Onion  (Allium  cepa  L.  var.  perutile  stearn):  Bulbs  are  narrow  with shorter flower stalks and smaller umbel. Bulbs or leaves can be gathered at all times of the year. It is used mainly as salad onion. This group is again sub-divided into potato or multiplier onion and shallots.

(i)   Potato or Multiplier Onion: The bulbs divide into between 3 and 20 bulb sets that are wider than they are long. They are covered by outer dry skins.

(ii)  Shallot: Shallots form cultures of narrow, separate bulbs. The leaves and flowers are usually smaller than common onion. Shallots are suitable for high latitude and short season regions.

2       Chemical structure and influences on flavour

The  importance  of  onion  lies  in  the  flavour  that  it  imparts  to  various  other  dishes.  A

common onion contains the following nutrient components:5

Moisture            88.6–92.8% Protein      0.9–1.6% Fat   Trace–0.2%

Carbohydrates   5.2–9.0% Ash    0.6%

Energy              23–38 cal 100g  1

Elements           mg 100g  1  fresh weight

Ca                     190–540

P                       200–430

K                       80–110

Na                     31–50

Mg                    81–150

Al                      0.5–1

Ba                     0.1–1

Fe                      1.8–2.6

Sr                      0.8–7

B                       0.6–1

Cu                     0.05–0.64

Zn                     1.5–2.8

Mn                    0.5–1.0

S                       50–51

Vitamins           100g-1

Vitamin D         0.3 mg Riboflavin          0.05 mg Nicotinic acid                          0.2 m Vitamin C            10.0 mg Folic acid                          16.0  g Biotin     0.9       g Pantothenic acid                          0.14 mg

Among various  free amino  acids  which  vary  greatly,  glutanic  acid and  orginine are abundant in onion. The concentration of these amino acids is higher at the centre of the bulb and decreases towards the outer scales.

Onion is characterized by its distinctive flavour and pungency which is due to sulphur containing  compounds  available  in  the  scales  of  bulbs.  The  sulphur  is  in  the  form  of various   non-protein  amino   acids   which   include   the   precursors  of  volatile   flavour compounds. These precursors are odourless, non-volatile amino acids of general name S- alk(en)yl cysteine sulphoxides.6  The precursors occur naturally in four types:


1.   CH3—S—CH2—CH(NH2) COOH O

2.   CH3—CH2—CH2—S—CH2—CH(NH2) COOH O

3.   CH3—CH = CH—S—CH2—CH(NH2) COOH— O

4.   CH2—CH—CH2—S—CH2—CH(NH2) COOH

Intact onion cells have no odour, but when cells are disrupted the enzyme alliinase is released.  This  enzyme  hydrolyses  the  S-alk(en)yl  cysteine  sulphoxides  to  produce pyruvate,  ammonia  and  many  volatile  sulphur  compounds  associated  with  flavour  and odour.7   The  enzyme  is  confined  to  cell  vacuole,  whereas  the  flavour  precursors  are confined  to  cytoplasm.  The  enzyme  has  access  to  precursors  only  when  cells  are disrupted.  Boiled  onion  does  not  produce  flavour  as  the  enzyme  is  destroyed  before having access to the flavour precursors. The tear-producing character of onion on cutting is  known  as  the  lacrimatory  factor.  Lacrimator  is  formed  enzymatically  during  the hydrolysis  of  S-propenyl  cysteine  sulphoxide.8   Thus  alliums  with  S-propenyl  cysteine sulphoxide  have  a  tear-producing  effect  and  alliums  with  S-allyl  cysteine  sulphoxide resemble the taste of garlic.

Besides the bulb, the flavour precursors are also available in leaf blades, base plate, and roots of onion. However, they are absent in seeds.9  The taste of onion differs from variety to  variety  and  within  varieties  grown  under  different  conditions  and  different  growth stages. Flavour precursors are synthesized in leaf blades and transported to the scales of bulb  where  they  are  stored.10   Younger  blades  are  more  productive  than  older  ones. Precursor content increases during bulbing and then gradually decreases towards maturity. During  storage  of  bulbs,  the  flavouring  compounds  increase;  although  the  bulb  is  in  a resting stage, it is still metabolically active. The increase in sulphur content is maximum until  the  sprouting  of  bulbs.  The  level  of  sulphur  suddenly  drops  after  sprouting.  The reduction  of  flavour  at  the  end  of  storage  period  may  be  due  to  metabolism  and translocation  of  the  flavour  precursors  themselves  for  nutrients  for  developing  roots.11

Kopsell  et  al.12  observed  increase  in  trans-(+)-S-(1-propenyl)-1-cysteine  sulphoxide  in seven cultivars under study. There was decrease in S-methyl-L-cysteine sulphoxide.

As well as genotype and stage of growth, environmental factors exercise great effect on  flavour  strength.  The  bulbing  response  in  onion  is  a  function  of  temperature  and photoperiod. Plants grown at higher temperature under optimum photoperiod bulb more rapidly than those grown at lower temperatures. A threefold increase in volatile sulphur at higher  temperature  was  observed  by  Platenius.13   The  varieties  grown  in  India  and Pakistan produce bulbs with higher pungency as they mature during temperature range between 25–35sC. Similarly high amount of watering produce bulbs with less flavour.13

Soil  fertility  status  and  high  amount  of  sulphur  containing  fertilizers  alter  the  flavour strength.  More  pungent  bulbs  were  produced  by  Platenius  on  peat  than  sandy  soil.13

Addition of sulphur in major nutrients was found to increase pungency in onion bulbs.14

3       Production

A global review of area and production of onion shows that it is grown in 126 countries over an area of 2.3 million hectares producing 40.0 million tons of dry onion. Sixty-two per cent of the world’s production is from Asiatic countries. Among them, China (19.3%) and India (17.8%) are the giants by area as well as by total production. The world’s onion productivity  is  17.16  tons/hectare.15    About  90%  of  onion  is  consumed  within  the countries of production. Total import of dry onion in the world is worth $118 million, while export is $910.5 million.16

Onion  cultivation  and  processing  is  at  its  most  advanced  in  countries  such  as  the United States. Systematic breeding programmes for the development of high TSS white onion  varieties  launched  by  various  research  organizations  and  seed  companies  in  the United States has been the backbone of the processing industry. Development of suitable

varieties  in  short-day  onion  grown  in  Asiatic  countries  holds  good  promise  with  the available input. India exports dehydrated onion (4124 tons) and processed onion (9095 tons) worth Rs.178.11 million and 191.85 million, respectively.17

3.1       Dehydrated onion Onion  is  dehydrated  in  the  form  of  flakes,  rings,  kibbles  and  powder.  A  large  part  of dehydrated onion production is used as seasoning in production of catsup, chilli sauce and meat  casseroles  as  well  as  cold  cuts,  sauces,  soup,  mayonnaise,  salad  dressing,  sweet pickles, dog food, potato chips, crackers and other snack items. Food service outlets also use  dehydrated  onion  because  of  its  convenience  in  storage,  preparation  and  use. Processing units require white onion varieties with important attributes:

   white onion with globose shape, free from greening

   high total soluble solids (>20%)

   high degree of pungency

   high insoluble solids

   low reducing to non-reducing sugars ratio to avoid caramelization

   high yields

   good storage quality

Important   cultivars   for   dehydration   include   white   Creole,   Southport   White, Dehydrator  No.  8,  Dehydrator  No.  14,  VH-12,  etc.  Indian  varieties  are  of  short-day type  and  do  not  possess  total  soluble  solids  more  than  15%.  However,  due  to  high pungency they make good quality dehydrated flakes and granules. The coloured varieties like dark red, red and yellow are also used for dehydration purposes but the quality of dehydrated produce is inferior to white varieties.

Traditionally  onion dehydration  is performed by sun drying in India,  on a domestic scale. Various types of solar driers have also been designed for dehydration purposes.18

However,  controlled  drying  under  optimum  temperature  and  time  gives  good  quality product.  Cabinet  drying  at  55–60sC  for  10–15  hours  gives  a  better  quality  dehydrated product  than  sun  drying  and  drying  in  solar  huts.19, 20   Commercial  dehydration  is achieved by forced hot air with the total process divided into three stages: drying at 75, 65 and  55–60sC,  the  conditions  of  dehydration  becoming  milder  as  the  moisture  content falls.5  Van Arsdel et al. 21  have given a schematic representation of the process for onion dehydration as shown in Fig. 1.

The process is completed by placing  the  dehydrated  onion pieces in bins where the final  moisture  content  (~4%)  is achieved  via  the  circulation  of warm  air  currents.  The approximate  composition  (100 g  1)  of  dehydrated  onion  should  be  as  shown  in  Table


3.2       Onion oils and other onion products

There are a number of other onion products.

   Onion oils: Concentrated oils extracted from onion can be used to impart the flavour of onion to processed food without the difficulties of handling a large bulk of fresh bulbs.6  Onion  oil  is  obtained  by  the  distillation  of  minced  onions  which  have  been allowed  to stand for a number of hours before  distillation.  The oil is of dark amber

Fig. 1    Onion dehydration process.

coloured  liquid.  The  yield  of  oil  varies  from  0.002  to  0.03%.  One  gram  of  oil  is equivalent to 4.4 kg fresh onions or 500 g onion powder.22  Use of onion oil is very safe from a microbiological contamination point of view, but there is the problem of flavour  being  lost.  Onion  oil  is  also  used  in  non-alcoholic  beverages,  ice  creams, confectionery, baked goods, condiments, meats and pickles.5

   Onion juice: Onion juice  with low flavour component is another processed product. Massarated pulp of onion is flash heated (140–160sC) and then cooled at 40sC. The product is evaporated to 72–75% solids to facilitate preservation. During processing aromatic components may be removed so that the product has a low flavour profile.22

   Onion salt: Onion salt  is prepared by mixing 19–20%  onion powder with  78% free flowing pulverized refined table salt and 1–2% anti-caking agent which prevents water absorption, and caking, etc.23

Table 1    Composition of dehydrated onion22

Water                              5.0 g Energy           347 Kcal Protein          10.1 g Fat                                1.1 g Carbohydrates           80.7 g Fibre     5.7 g

Ash                                 3.2 g Ca      363 mg Fe        3 mg Mg          122 mg P                                340 mg K   943 mg Na       54 mg Zn         2 mg Vitamins (ascorbic acid)   15 mg

   Onion  pickles:  Pickled  onions  are  eaten  in  large  quantities  in  many  European countries. Onion pickles are prepared out of two types, namely (i) brown or dark red onion 28–45 mm diameter and white or silver skin (pearl or cocktail) onions between

10 and 28 mm in diameter.5  These  onions are  produced by planting  with high plant density. The onions are first peeled and allowed to ferment in 10% brine solution for

24–96 hours. During fermentation sugars from the bulbs are converted to lactic acid and  a  small  amount  of  acetic  acid  and  alcohol.  The  fermentation  is  controlled  by adding  small  quantities  of  lactic  acid.  The  pickled  bulbs  are  bottled  in  vinegar, possibly darkened with caramel and pasteurized at 80sC.6

   Vinegar from onion: A new type of vinegar can be produced from onions that have been  rejected  for other  conventional  purposes because  of low quality.24  Horiuchi  et al.24  tested several types of onion as raw material for vinegar production. Vinegar was produced successfully from juice of red onion cv. Kurenai by batch culture using yeast and  Acetobacter  aceti.  The  vinegar  produced  from  onion  had  a  higher  potassium content, while sodium was lower than in conventional vinegars. The total amino acid and  organic  acid  contents of onion  vinegar  was much  higher  than  in  other  kinds  of vinegars. The commercial feasibility of this new type of product needs to be assessed. In a country like India a colossal quantity of red onions go to waste during the lean season and these can best be utilized for value addition.

4       Functional properties

Besides use as a condiment and spice for flavouring and enriching various cuisines, onion has been known for its high medicinal properties for thousands of years. Chinese, Indians and  Egyptians  have  known  about  its  various  medicinal  properties  since  antiquity  and these  have  been  well  documented.  Charak  Samhita,  an  ancient  Indian  medical treatise describes many curative uses of onion. Augusti25  listed various traditional uses of onion, including:

   It acts as stimulant, diuretic and expectorant and mixed with vinegar, it is useful in the case of sore throat.

   Essential  oil  from  onion  contains  a  heart  stimulant,  increases  pulse  volume  and frequency of systolic pressure and coronary flow.

   Onion consumption lowers blood sugar, lipids and cholesterol.

   Fresh onion juice  has antibacterial  properties due to allicin,  disulphide  and cysteine compounds and their interactions.

   Antiplatelet  aggregation effect in human and animal blood has been reported due to regular consumption of onion.26, 27

The antioxidant activity of onion (Allium cepa) and onion scales has been studied in lipid oxidation models28–33  and in radical scavenging assays.34, 35  Both yellow and red onion were  poor  antioxidants  towards  oxidation  on  methyl  linoleate33   contradictory  to  high antioxidant activity towards oxidation of LDL.35  Onion had also a poor antioxidant score in the ORAC activity test while garlic (Allium sativum L) expressed a score four times higher.34   Yin  and  Cheng36   reported  that  the  presence  of  garlic  bulb,  garlic  greens, Chinese leek, scallion, onion bulb, and shallot bulb significantly delayed lipid oxidation of  phosphatidylcholine  liposomes.  While  allicin  4  is  responsible  for  the  antioxidant activity  of garlic  bulb,37  compounds  other  than  allicin  are  involved  in  determining  the antioxidant effect of other Allium members. According to Velioglu et al.38  anthocyanin- rich  vegatables  including  red  onion  scales  generally  showed  very  strong  activities towards oxidation of   -carotene linoleic acid model systems. Similarly, green onion tops were reported twice as active as green onions with quercetin 5 included in the antioxidant substances.28, 29

5       Quality issues

5.1       Dry onions

For export from India the following specifications have been defined by the Agricultural

Processed Products Export Development Authority (APEDA).39

A.   General big onions:

1.   6 cm bulb diameter, light red to dark red colour, globular, pungent – onions are suitable for Asian markets and Arab countries.

2.   4  cm  bulb  diameter,  globular,  pungent  and  light  red  colour  –  onions  are suitable for Bangladesh market.

3.   8 cm bulb diameter, globular and oval round shape, yellow or brown colour –

onions are suitable for European and Japanese markets.

B.    Small onions:

3 cm bulb diameter, dark red to violet red, globular shape – onions are suitable for

Malaysia and Singapore markets.

C.   Multiplier onions:

2.5–3.5 cm bulb diameter, dark red colour, bigger size bulblets – onions are suitable for Malaysia, Singapore, and Sri Lanka markets.

As  per  international  quality  standards40   dry  onions  should  be  intact,  sound,  clean, sufficiently  dry  for  intended  use,  free  from  abnormal  external  moisture,  free  from  off odours and the stem must be twisted or clean cut. Shape and colour should be typical to the variety. The size should be uniform with minimum variation in the group. The size of group can be of 10–20 mm, 15–25 mm, 20–40 mm, 40–70 mm and 70 mm plus. They should be free from abnormal swelling, doubles, sprouting and saprophytic fungus. The bulbs should be packed in sacks of jute or nylon nets with appropriate capacity varying from 25–40 kg. Consumer prepacks of 2 kg capacity, such as nets, plastic film bags or stretch-wrapped trays can be used.

Table  2    American  Dehydrated  Onion  and  Garlic  Association  quality  standards  and  grade specifications

Products                          Colour (optical index)     Bulk index (ml/100 g)

Sliced                                90                                  400

Large chopped                  90                                  300

Minced                            150                                  180

Granulated                      150                                  140

Agglomerated – coarse    150                                  140

Agglomerated – fine        150                                  140

Powdered products         150                                  140

5.2       Dehydrated products The American Dehydrated Onion and Garlic Association have standardized quality and grade specifications for dehydrated onion products.41  Based on particle size the products are classified as shown in Table 2.

In all products moisture should be 5.0%. The products should be free from black or dark  brown  pieces,  seed  stems,  sediment  or  sediment  attached  to  onion,  extraneous vegetable matter (such as tops, rootlets, and other harmless vegetable matter), outer roots, metallic particles, hair, etc. The material should be packed in moisture barrier material like multi-walled polythene bags, fibre drums and stored under cool and dry conditions. Exposure to high temperature and light reduces the colour quality of dehydrated products. Freshly harvested as well as stored bulbs are used for dehydration. During harvesting, handling and storage, the bulbs carry a heavy load of harmful bacteria, fungi yeast and mould.  There  is  every  possibility  of  passing  these  microbes  to  the  final  product.  The

count of various microbes should be at tolerable levels as follows: Aerobic plate count    < 500.000/g

Yeast and mould      < 5000/g Coliforms            < 200/g Salmonella                                  Absent/25g E. Coli            Absent/g

5.3       Other onion products Onion is processed in the form of pickled onion, as onion in brine and onion in acetic acid. For processing, bright white onions with globose shape, fully cured, free from rots, mould, fungus, external damage, sprouting and greening are used. Bulbs of 16–25, 25–45 and  45–70  mm  diameter  grade  are  used  for  processing.  Smaller  grade  fetches  better

Table 3    Chemical composition of onions in brine and acetic acid42

Composition                    Onions in brine               Onions in acetic acid

Salinity as NaCl              16% ± 0.5%                     5% ± 0.5% Acidity as acetic acid           0.3% ± 0.5%                               4% ± 0.2% CaCl2                           0.5%       0.5%

SO2                                                                    250 ppm max                  250 ppm max pH           below 3.5        below 3.5

prices. The chemical composition of onions in brine and acetic acid should be as shown in  Table  3.42  The  processed  onions  are  packed  in  food  grade  HMHDPE  barrels  of

220–240 litre capacity.

6       References

1.  ROBINOWITCH, H.D.  and BREWSTER, J.L.  1990. Onions and Allied Crops, Vol. I. CRC Press, Boca Raton, Florida.

2.  TACKHOLM, V. and DRAR, M. 1954. Flora of Egypt, Vol. 3, 94, Cairo University Press, Cairo.

3.  CHADHA,  M.C.  and  SIDHUS,  A.S.  1990.  Studies  of  the  storage  life  of  onion  under ambient conditions.  Proc. of National  Symposium on Onion and Garlic, 3 June,

1990, pp. 187–95.

4.  JONES,  H.A.  and  MANN,  L.K.  1963.  Onions  and  their  Allies.  Chapter  2  and  3,  New


5.  FENWICK, G.R.  and HANLEY, A.B.  1990. Chemical composition. Chapter 2 of Onions and  Allied  Crops,  Vol.  III  (eds  J.L.  Brewster  and  H.D.  Robinowitch).  CRC  Press, Boca Raton, Florida.

6.  BREWSTER,  J.L.  1994.  The  biochemistry  and  food  science  of  alliums.  Chapter  9  of

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