Garlic - Processing, properties and toxicity

Garlic

1 Introduction Garlic (Allium sativum L.) is the second most widely cultivated Allium after onion. According to FAO estimates for the year 1999, the world area cultivated is 889,000 ha and production is 8,776,000 mt. China, Korea, India, USA, Spain, Argentina and Egypt are the major garlic growing countries. China ranks first in area (424,000 ha) and production (5,690,000 mt) followed by India in area (113,000 ha) and Korea in production (484,000 mt) (Table 1). In productivity, Egypt tops the list (25,366 kg/ha) followed by USA (16,250 kg/ha), China (13,421 kg/ha) and Korea Republic (11,916 kg/ ha). Korea Republic has the highest per capita availability, i.e. 10.50 kg per year followed by Argentina (5.01 kg) and China (4.53 kg). The world garlic area, production and productivity trends during the past decade show that since 1986 they have improved by about 115.78%, 245.92% and 60.41%, respectively (Table 2). Garlic is in demand almost all the year round all over the world both in fresh form and also in dehydrated form. Today, garlic is used for its flavour, aroma and taste being prepared domestically or forming a raw material for a variety of food manufacturing processes (dehydration and pickling).

Fresh garlic is widely used in cooking. In India and other Asian and Middle East countries, it is used in pickles, curry powder, curried vegetables, meat preparation, tomato ketchup. In the Philippines, Central Eastern Asia and in parts of the tropics, the green tops as well as bulbs of garlic are used. Dehydrated garlic in powdered or granulated form has replaced the use of fresh bulbs for industrial and home use in many countries. Dehydrated products of garlic are common as a condiment and in the food industry. It is reported that in America about 50% of the total production of garlic is dehydrated and sent to food processors.

Garlic is a frost-hardy bulbous perennial erect herb of 30100 cm in height with narrow flat leaves and bears small white flowers and bulbils (Janick, 1979). It is a herbaceous annual for bulb and a biennial for seed production. The shape of garlic is smooth, round and solid for its entire length unlike onion which is hollow. Many cloves of garlic do not produce flowerstalks. The inflorescence may be partially or not at all

Table 1 Area, production, productivity and per capita availability of garlic in different countries during 1999

Country Area Production Productivity Per capita

(000 ha) (000 mt) (kg/ha) availability

(kg/year)

Africa 17 178 10311

Algeria 8 30 3750 1.00

Egypt 5 114 25366 1.73

Morocco 2 7 3829 0.26

N.C. America 25 301 12218

Mexico 9 65 7647 0.68

USA 14 224 16250 0.82

South America

Argentina 15 181 11727 5.01

Brazil 11 63 5563 0.38

Peru 5 30 5883 1.21

Asia 682 7280 10667

China 424 5690 13421 4.53

India 113 452 3996 0.46

Indonesia 11 38 3599 0.18

Korea Rep. 41 484 11916 10.50

Turkey 14 106 7571 1.64

Myanmar 14 53 3782 1.19

Bangladesh 13 40 3022 0.32

Thailand 21 131 6235 2.17

Europe 125 699 5594

Italy 4 34 8550 0.59

France 6 45 7759 0.77

Yugoslavia 12 44 3631 4.14

Russian Fed. 25 161 6381 1.09

Romania 12 58 4833 2.58

Spain 25 170 6827 4.29

Ukraine 21 79 3762 1.55

Oceania 1 7143

New Zealand 1 7143 0.26

World 889 8776 9875 1.49

Source: FAO Quarterly Bulletin of Statistics (3/4) 1999.

exerted, its bulbils forming a swelling somewhere within the false stem a few cm above the bulbs. The bulb consists of 635 smaller bulblets called cloves and is surrounded by a thin white or pinkish papery sheath (Bose and Som, 1986).

Alliums have been cultivated for thousands of years for therapeutic and prophylactic properties, religious significance, and flavour and taste. The Chinese,

Table 2 Trend of area, production and productivity of garlic in the world

World

1986 1999 Increase (%)

Area (million ha) 0.41 0.89 115.78

Production (million ton) 2.54 8.78 245.92

Productivity (ton/ha) 6.16 9.88 60.41

Sources: FAO Quarterly Bulletin of Statistics (3/4) 1999 and FAO Production Year Book 1988.

Sumerians, Indians and Ancient Egyptians are all known to have consumed garlic over

4000 years ago. Among others, Hippocrates (430 BC) and Theophras Tuso (322 BC)

have described the consumption of garlic in the Greek and Roman period

(Rabinowitch and Brewster, 1990). Garlic is thought to have originally come from Central Asia and Southern Europe, especially the Mediterranean region (Thompson and Kelly, 1957). Some authorities consider that Allium longicuspis Regel, which is endemic to Central Asia, is the wild ancestor and spread in ancient times to the Mediterranean region.

Garlic presents an interesting problem of classification and quality given the wide range of cultivars, differing in maturity, bulb size, clove size and number, scale colour, bolting, number and size of inflorescence bulbils and presence or absence of flowers

(Bose and Som, 1986). Taxonomists have recognised at least four botanical varieties within Allium sativum L., namely A. sativum L., Var sativum, A.sativum L. Var., Ophioscorodon (Link) Doll, A. sativum L. Var Pekinense (Prokh) Maekawa and A. sativum L. Var nipponicum Kitamura. Jones and Mann (1963) could not satisfactorily classify many garlic cultivars into clearly defined botanical varieties, because many show combinations of characteristics from two or more varietal groups. It appears, then, that the above botanical classification has little advantage. Following criteria may be of practical use for distinguishing different garlic cultivars (Rabinowitch and Brewster,

1990):

 Morphological characteristics such as: Bolting type, number and size of cloves, number of leaf axils, forming cloves, number of secondary cloves formed in a lateral bud, bulb weight, colour of the outer protective leaf of the cloves, number of protective leaves, width and length of foliage, plant height and tenderness of the green leaves.

 Physiological and ecological characteristics: Time of bulbing and maturity, low temperature and long day requirements for bulb formation, winter hardiness and bulb dormancy.

2 Chemical structure

Garlic or any other Allium is characterised by the remarkable sulphur-containing compound present in it which gives distinctive smell and pungency. Uninjured bulb of garlic contains a colourless, odourless water soluble amino acid Alliin which includes the presence of the volatile flavour compounds. These precursors are of the general name S-alk(en)yl cysteine sulphoxide. The general structure of the flavour precursor is:

O

|

RSCH₂CH (HH₂) COOH

The group to the right of R moiety is the L-cysteine sulphoxide group. The group R can be:

1. CH₃ -called (+)-S-methyl

2. CH₃CH₂CH₂ -called (+)-S-propyl

3. CH₃CH=CH -called trans (+)-S-(1-propenyl)

4. CH₂=CHCH -called (+)-S-(2-propenyl)

When the fresh tissue is damaged, the flavour precursors react under the control of the enzyme alliinase (S-alk(en)yl-L-cysteine sulphoxide Lyase) to release the highly reactive sulphenic acids plus ammonia and pyruvate. Alliin, or S-allyl cysteine sulphoxide, was the first sulphur compound isolated from Allium sativum (garlic). The enzyme alliinase is confined to the cell vacuole, whereas the flavour precursors are confined to the cycloplasm probably within small vesicles associated with their presence in the cell. Hence the enzyme has access to the precursors only when cells are disrupted. In garlic, alliinase catalyses the formation of allicin, which gives fresh garlic its characteristic smell

(Brewster, 1994).

Alliinase (E.C.4.4.1.4., S-alk(en)yl-L-cysteine sulphoxide cyase is the enzyme ultimately responsible for the development of the flavour compounds. The reaction catalysed by the enzyme is a beta elimination of the S-alk(en)yl sulphoxide group from the substrate:

O

||

RSCH₂CHCOOHRS=O: + CH₃CCOOH

| || NH₂ NH

Both products of the reaction are chemically unstable; the Ketimore product spontaneously hydrolyses to pyruvate and ammonia while the reactive sulphur species will combine with any of a number of co-reactants, most often another of the same species, to give a range of flavour components. Thus the reaction is commonly described as:

O O O

|| || ||

2RSCH₂CHCOOH+H₂O>RSSR+2CH₃CCOO+2NH₄

| NH₂

Rabinowitch and Brewster (1990) have elaborated the details about the alliinase solubility and stability, purification, physical, chemical and catalytic properties. They report that alliinase from Alliums is less soluble. It is difficult to maintain this enzyme in aqueous solution. A cosolvent is required to maintain the enzyme activity. It is further reported that alliinase enzyme has a tendency to aggregate and precipitate and gets totally inactivated by freezing.

Alliinase from garlic was first prepared by Stoll and Seebeck in 1947; however, the preparation was relatively crude. A common feature of the preparation is the use of a polyalcohol cosolvent, 10% glycerol in the case of garlic enzyme. The purification of garlic enzyme also required the presence of 10 M pyridoxal phosphate in the buffers. The garlic enzyme has been described as having a molecular weight of 130,000 with two subunits of 65,000. Spectral studies on alliinase from garlic revealed an absorption peak at 420 nm characteristic of pyridoxal phosphate. Garlic alliinase has been reported to contain no carbohydrate. However, the presence of a subunit band giving a periodic acid schiff base stain on SDS gels of the purified enzyme indicates that the enzyme is a glycoprotein.

The general catalytic properties of alliinase were well described long before the enzyme was purified to homogeneity. A broad pH optimum of 58 was described by Stoll and Seebeck (1947) for the garlic enzyme with a temperature optimum under the

condition of assay of 37C. They also made observations on the specification of the reaction: the substrate must be a derivative of (-)L-cysteine, the sulphur atom must be linked to an aliphatic group, the amino group of cysteine must not be substituted and the sulphur atom must be in the sulphoxide form with the + and configurations both being substrate.

3 Processing Raw garlic no doubt has the ideal flavour and provides desirable textural and water retaining properties when incorporated into food products, bulk of raw garlic is water and lacks flavour and aroma. Also handling and storage costs are more when garlic is used in raw form. Dehydration or flavour extraction reduces the quantity which ultimately lowers the transportation and storage costs. Processing, however, may introduce undesirable changes in appearance. It may also modify the natural balanced aroma and flavour.

Many modern automated food processing plants are not able to handle the raw form of garlic as such a range of garlic products are available to meet the particular needs of the market. Garlic flakes, garlic powder, garlic oil and garlic juice are some of the processed forms of garlic which are widely used by the food industry. Garlic capsules and tablets are also prepared and these products have a ready market in view of their high medicinal values.

3.1 Garlic dehydration A schematic representation of the process for garlic dehydration is shown in Fig. 1. The garlic bulb is broken into individual cloves by passing between rubber covered rolls which exert throughpressure to crack the bulb without crushing the cloves. The loose paper shell is removed by screening and aspirating. The cloves are then washed in a flood washer. At this stage the root stubs are floated off. Slicing is then done on a specially designed high-speed cutter. The sliced garlic cloves are then spread on drying trays which in turn are stacked on transfer cans. Tunnel type dehydrators are used for dehydration. The cans are placed in these tunnels and hot air is blown across the trays. The sliced garlic cloves are dried down to about 10% moisture in this operation which takes about

1015 hours at 6065sC. It is, however, preferable to keep the temperature below 57sC

while drying (Pandey, 1997).

Part of the remaining moisture is usually removed in a finishing operation for which garlic cloves are placed in large bins. Warm air is forced upward through the bins. This bin drying and conditioning operation may take about 30 hours. In these drying operations temperature must be controlled so that browning or scorching do not occur. After the slices are dried, they may be screened for removal of large partially dried slabs and fines. The fines may account for one third of the total dehydrated product. Usually fines are ground into powder. The slabs may be broken up and redried. The slices are prepared in the form of flakes. Grinding and packaging of powder is done in low humidity surroundings to avoid gumming and caking. Garlic is commercially dried to

6.5% moisture.

It is reported that dehydrated garlic is available in six forms: powdered, minced, coarse, granulated, chopped and diced. It is generally considered that about 5 kg fresh garlic bulbs from the field make 1 kg of dried product. On the basis of chemical, microbial and sensory studies, it is also reported that 60sC is the maximum temperature

Fig. 1 A schematic representation of the process for garlic dehydration. (From Van Arsdel et al., 1973).

for dehydration of garlic. Hot air drying technique is considered to be the most economical. Storage of garlic powder was best achieved in cans, the use of sealed polyethylene bags being considered unsatisfactory. Odourless garlic powders as health supplements may be produced by inactivating alliinase by contacting garlic with fumaric acid/fumarate or by physically separating garlic powder and alliinase. Freeze drying of the garlic/cyclodextrin (70:30) mixture has also been claimed to produce an odourless product. Dehydrated garlic is prone to discoloration. Darkening is associated with non-

enzymic Maillard reactions and may be prevented by reducing temperature. A green discoloration associated with the maceration of garlic tissue may be minimised if dormant tissue is processed (Rabinowitch and Brewster, 1990).

There are reports that there is adverse effect of processing on quality and intensity of flavour. Dehydration has been associated with loss of more than 90% of the flavour intensity (Pruthi, 1980). This represents both a potential environmental problem and a significant economic loss. The origins of such losses has been examined and it has been concluded that they occur prior to the realisation of a critical moisture content, wherefrom the flavour may be envisaged as being sealed within the product (Mazza and Le Maguer, 1979).

3.2 Garlic oils Volatile oils comprise 0.1 to 0.25% of the fresh weight of garlic. Garlic oil is recovered by steam distillation of freshly ground cloves. It is a reddish brown overpowering liquid. One gram of oil is equivalent in flavouring terms to 900 g fresh garlic or 200 g dehydrated garlic powder. The high pungency of garlic oil makes it difficult to use directly. The oil is commonly diluted in vegetable oil. It is being used in ice-cream, ices, confectionery, baked goods, chewing gum and condiments. The rates of 6 ppm in baked products, 0.010.3 ppm in beverages, 16 ppm in condiments, 12 ppm in chewing gums,

12.9 ppm in confectionery and 40 ppm in ice-cream are being used.

3.3 Other products Garlic is also processed in the form of garlic juice and garlic salt. Garlic oleoresin is a dark viscous liquid, having 12 times the flavour of dehydrated garlic or 50 times that of fresh garlic cloves. Garlic paste is formulated from suitable flavours and viscous edible base.

Garlic salt is comprised of a free flowing, uniformly blended dry mixture of non- iodised salt, the approximate composition of which is shown in Table 3. Garlic salt should not contain more than 2.5% moisture, more than 81% salt and should have moisture-free white garlic powder between 1819%, and 12% calcium stearate. Garlic

Table 3 Approximate composition (100 g) of garlic salt

Garlic salt Water 1.4 g Energy 63 kcal Protein 3.2 g

Fat 0.1 g Carbohydrate 13.8 g Fibre 0.4 g Ash 81.5 g Ca 220 mg Fe 1 mg Mg 11 mg

P 79 mg

K 212 mg Na 31.4 g Zn 1 mg Vitamins

salt has much wider culinary potential than powder and one tablespoon is equivalent to a clove of fresh garlic.

4 Uses Garlic is used practically all over the world for flavouring various dishes. In America about 50% of the entire output of fresh garlic is dehydrated and sold to food processors for use in mayonnaise products, salad dressings, tomato products and in several meat preparations. Raw garlic is used in preparation of garlic powder, garlic salt, garlic vinegar, garlic cheese croutons, potato chips, garlic bread, garlicked meat tit-bits and garlicked bacon, etc., which have been boosted in the American market. Spray-dried garlic products including garlic preparations are also available in the market. In Italy, Europe and Latin America, the spectacular popularity of garlic is shown by the methods by which it is boosted by the garlic growers. In India and other Asian and Middle East countries, garlic is already being used in several food preparations notably in pickles, curry powders, curried vegetables, meat preparations and tomato ketchup, etc. There has been increasing demand for garlic by the food industries for garlic powder as a condiment. Oil of garlic has now been appreciated as a valuable flavouring agent, for use in all kinds of meat preparations, soups, canned foods and sauces (Pruthi, 1987).

5 Functional properties and toxicity Garlic has been considered as a rich source of carbohydrate, proteins and phosphorus. Ascorbic acid content was reported to be very high in green garlic (Prodan et al., 1977). The nutritive composition of fresh and peeled garlic cloves and dehydrated garlic powder as reported by Pruthi (1987) is given in Table 4.

Studies suggest that garlic, which contains more than 200 different compounds, has biological activities that can have medically important effects. Garlic has been used as an excellent carminative, a nerve tonic and an antiseptic agent in Hindu medicine for centuries (Aman, 1969). However, it has been only recently that its medical benefits have been explored in detail. The functional properties of garlic have been reviewed by Agarwal (1996), Koch and Lawson (1996) and Lawson (1998a). These properties include the following:

 cholesterol lowering properties reported by numerous studies (including Reuter and Sendl, 1994; Han et al., 1995; Adler and Holub, 1997). These studies have reported an average 10% reduction in total serum cholesterol. Evidence suggests that these effects are due to allicin or allicin-derived compounds (Yeh and Yeh, 1994; Gebhardt and Beck, 1996). Some recent studies, however, have produced contradictory results

(Berthold and Sudhop, 1998), though this may be due to the composition of the garlic supplements studied (Lawson, 1998b)

 garlic significantly lowers blood pressure (Silagy and Neil, 1994; Das et al., 1995a and

1995b)

 garlic has an influence on platelet aggregation, an important factor in cardiovascular disease (Lawson et al., 1992; Han et al., 1995; Batirel et al., 1996). It also has an effect on blood coagulation and fibrinolytic activity which are factors in the development of thrombosis (Han et al., 1995; Breithaupt-Grogler et al., 1997)