Bay leaves - Cultivation, production and processing, Functional properties

Bay leaves

1 Introduction

The commodity, traded as sweet bay leaf, and true, Roman, or Turkish laurel, is derived from the leaves of Laurus nobilis L. (Family Lauraceae). Because of the similarity in the leaves, several other trees are also variously known as: West Indian bay tree (Pimenta racemosa), Cherry laurel (Prunus laurocerasus), Portugal laurel (Prunus lusitanica), Laurel of the southern states (Prunus caroliniana), the Laurel or Mountain laurel of California (Umbellularia californica). However, the leaves of true L. nobilis must not be confused with other laurels. L. nobilis is a native of the Mediterranean and grows spontaneously in scrubland and woods in Europe and in California. It is widely cultivated in Europe, America and in Arabian countries from Libya to Morocco (Bailey 1963, Anon.

1962).

The flavouring properties of L. nobilis have been known since antiquity. In biblical times, the bay was symbolic of wealth and wickedness, and in the classical world heroes and victors were decorated with a laurel wreath. In addition to being a very well known culinary herb, the leaves and fruits of L. nobilis are used medicinally throughout the world. Infusions or decoctions made from these materials have diaphoretic and carminative effects and also serve as a general gastric secretion stimulant. Laurel oil or butter obtained from the fruits (berries) of L. nobilis is a vital ingredient of laurin ointment, a popular medicine for rheumatism and gout and for the treatment of spleen and liver diseases. It also finds application in veterinary medicine (Anon. 1962; Duke 1989; Wren 1975; Francesco and Francesco 1971).

L. nobilis is an evergreen shrub, or more rarely a tree attaining a height of 1520 m. The smooth bark may be olive green or of reddish hue. The luxurious, evergreen leaves are alternate with short stalks, lanceolate or lanceolate oblong, acuminate, 58 cm or longer and 34 cm wide, coriaceous, pellucid-punctate, and with revolute, entire wavy margins; the upper surface is glabrous and shiny, olive green to brown and the lower surface is dull olive to brown with a prominant rib and veins. The flowers are small, yellow in colour, unisexual and appear in clusters. The fruits (berries) are cherry-like, succulent, purple to black in colour, ovoid, coarsely wrinkled and contain a single seed

with loose kernel. The dried fruits are drupaceous, ovoid, about 15 mm long and 10 mm wide. The outer surface is glabrous, shining, nearly black and is coarsely wrinkled owing to the shrinkage of the narrow succulent region beneath the epidermis. The remains of the style appear as a small point at the apex and a small scar at the base marks the point of attachment of the fruit to the thalamus. The endocarp is thin and woody and the testa is adherent to its inner surface. The entire pericarp is about 0.5 mm thick. The kernel of the seed consists of two large plano-convex cotyledons and small superior radicle; it is brownish-yellow, starchy and oleaginous, with an aromatic odour and aromatic and bitter taste (Bailey 1963; Wallis 1960; Francesco and Francesco 1971).

The cross-section of the leaf shows epidermal cells with thick cuticle; the epidermal cells in surface view are sinous, pitted and thick walled. The lower epidermal walls are more curvilinear and distinctly beaded. The stomata are present only on the lower surface, singly or in pairs. The mesophyll of the leaf is distinctly represented by two layers of parenchymatous palisade cells and a region of spongy parenchyma containing scattered spheroidal oil reservoirs, fibro-vascular and collenchymatous tissues. The leaf has characteristic fragrance when crushed and its taste is bitter and aromatic (Wallis

1960; Bagchi and Srivastava 1993).

2 Cultivation, production and processing

Sweet bay is propagated by seeds or preferably by cuttings. From a well ripened wood, cuttings of about 7.5 to 10 cm length are put in sharp sand either under bell-glasses or in glass cases. The rooted cuttings are placed in small pots containing fairly rich sandy loam with good drainage, and then can be put in a hot bed, with gentle bottom heat where they will make a good strong growth. L. nobilis stem cuttings produce roots better in July/ August, under Mediterranean conditions, than in other seasons, although the optimal rooting period can be extended by bottom heating from May until September (Raviv

1983a). Ligneous, subapical stem cuttings of bay laurel have a higher rooting percentage than herbaceous apical cuttings, probably due to water deficit in the latter, moisture sufficiency may be critical due to the very long rooting period of four to five months

(Raviv 1983b).

Rapid and efficient rooting of L. nobilis occurs at a root medium temperature of 20sC to 30sC, especially during the winter when, if not heated, both the medium and air temperatures are less than 15sC in the Mediterranean region (Raviv and Puticvsky 1983). After that, they may be planted in nursery beds with rich sandy soil and good drainage. In one growing season, the plants may attain a height of 1 to 1.5 m. At the end of the growing season and long before the cold season the young plants together with their stakes are kept in well lit and ventilated sheds, and temperature is kept just above freezing. These plants are kept in close rows and watered once or twice a week. The plants are taken out during the spring season and either potted or plunged in nursery. The rich peaty soil with plenty of water and congenial moist atmosphere near the sea coast are favourable conditions for fast and luxriant growth (Bailey 1963). It also grows well under the partly shaded conditions in gardens or orchards.

The leaves of L. nobilis are plucked and dried under shade for use as a flavouring material in a variety of culinary preparations, especially in French cuisine. The leaves contain an essential oil of aromatic, spicy odour and flavour which can be isolated by steam distillation. The oil is a valuable adjunct in the flavouring of all kinds of food products, particularly meats, sausages, canned soups, baked goods, confectionery, etc.

The oil replaces the dried leaves to great advantage because it can be dosed more exactly and therefore gives more uniform results than the dried leaves (Guenther 1953).

Laurel berries contain about 1% of an aromatic volatile oil and 25 to 30% fat. The separated fat is the Olecum lauri expressum of commerce. The pure fat is of dull green colour, granular and has an aromatic odour. The expressed oil is used in stimulating liniments and in veterinary practice (Wallis 1960).

Currently, two types of essential oils are traded internationally under the name bay oil, although they are entirely unrelated to each other. The West Indian bay oil or bay leaf oil is distilled from the leaves of the tree of Pimenta racemosa, which is found on the various islands of the West Indies, but most particularly in Dominica. The Turkish bay oil or laurel leaf oil is distilled from the leaves of L. nobilis. The sources of the bulk culinary bay leaves are Turkey and the Balkan countries, and in small quantities from France. The annual production level of the genuine L. nobilis oil is only about 2 tons. It is marketed mainly in Western Europe, largely in Germany and the Netherlands (Anon. 1986).

3 Chemical composition

A good deal of work on physico-chemical characterisation and chemical composition of essential oils of different parts of L. nobilis have been reported. The reported values of physico-chemical constants and chemical constituents identified are provided in Table

1. The studies carried out so far on the bay oil indicate the influence of geographical origin of variety and harvest season on the chemical composition. The chemical composition of the flower essential oil is quite different from other parts of the plant, namely leaves, stem bark and stem wood (Fiorini et al. 1997). The earlier studies were mostly carried out by chemical methods (Nigam et al. 1958) but recent GC-MS and GLC analyses has made possible the isolation and characterisation of a number of compounds more accurately and efficiently (Nigam et al. 1992; Fiorini et al. 1997). The chemical structure of some of the important constituents are provided in Figure 1. The presence of 1,8 cineole in appreciable amounts makes the oil of bay leaves an important perfumery item (Pruidze 1971).

4 Functional properties

Although the dried bay leaves and their essential oil are mainly used as a spice and food flavouring agent, the bay oil also finds use in folk or traditional medicines of different countries, for the treatment of a number of diseases. Recent studies have shown that it has the following functional properties:

 antimicrobial and antifungal characteristics

 hypoglycaemic properties (in the control of diabetes)

 antiulcerogenic properties.

The essential oil of L. nobilis has been found to be active against Staphylococcus aureus,

Escherichia coli, Shigella flexnerii and Salmonella typhi, pathogens of the intestinal tract

(Syed et al. 1991). The L. nobilis has also been noted to possess anti-fungal activity (ies),

(Rahari Velomanana et al. 1989; MacGregor et al. 1974).

The hypoglycaemic activity of bay leaf extracts has also been reported (Ashaeva et al.

1984). Bay leaves potentiated the action of insulin in glucose metabolism (Khan et al.

Table 1 Physico-chemical properties and chemical constituents of essential oil extracted from different parts of Laurus nobilis of varying geographical origins

S. No. Geographical Plant part Physical characteristic(s) Chemical constituent(s) identified Reference(s)

origin of the and its determined

resource essential oil

material content

1. NA^a NA Yellowish brown unpleasant odour, -pinene, eugenol, phellandrene Rattu and Maccioni (1952)

d 0.9278 ( )²² d 120s, hd 1.4730, soluble in ethyl alcohol 1:90

2. NA Fruits, 1% pinene, cineole, lauric acid, alcohols and Rattu et al. (1953)

sesquiterpenes

20

²⁰ 0.9218, ( )_D cineole (12.8%), free alcohols 10.7%, Nigam et al. (1958)

18.9s, acid no. 5.92, sap. no. 67.94 esters (chiefly Mecinnamate 17.9%), free cinnamic acid (1.3%), free phenols (2.0%), terpene hydrocarbons (15.4%), and different carbonyl compounds and sesquiterpenes.

4. Idzhevanskii, Leaves d 2.53.3, ( ) 3.83.1. Melkumyan and Khurshundyan

Armenia, (1959) Noemberyam- d 0.9241.4687, n_D 0.94161.4664

skii, Armenia

²⁰ C.9218, ( )_d Carbonyl compounds 11.48%, alkali Nigam et al. (1958)

18.9s. Acid value 5.92, sap. value soluble (by vol) 9%, -pinene, citral

67-94, sap. value (after acetylation) terpineol, Me-cinnamate, cinnamic acid,

99.80 caryophyllene, sesquiterpenes hydrocarbons

NA Information NA -pinene, camphene, sabinene, limonene, Teisserie (1966)

on plant part carene and 1,8-cineole (35%)

not mentioned

2.5%

Table 1 Continued

S. No. Geographical Plant part Physical characteristic(s) Chemical constituent(s) identified Reference(s)

origin of the and its determined

resource essential oil

material content

7. NA NA NA -pinene, camphene, -pinene, sabinene, Teisserie et al. (1966)

3-carene, -phellandrene, -terpinene, myrcene, -limonene, -phellandrene, - terpinene, p-cymine, terpinolene and ocimene

8. Czechoslo- Leaves NA -pinene, camphene, myrcene, limonene, Chow et al. (1965)

vakia p-cymene, -phellandrine, -selinene, and -cadinene

9. Kazakistan Shoot, 0.5% NA -pinene, -pinene, camphene, l-sebinene, Goryaev et al. (1966)

-myrcene, and -phelandrene, 1- limonene, p-cymene, 1-8-cineole, acetic, propionic, butyric, caproic, caprylic, pelargonic and enanthric acid in phenolic in terpens fraction eugenol,

10. Greece Leaves, NA -pinene, camphene, -pinene, sabinine, Giuliana and Stancher (1968)

1.0% myrcene, -phellandrene, d-limonene, cineole, -terpinine, p-cymene,

Turkey Leaves, NA terpinelene, camphor, linalool, -terpineol,

0.8% terpenyl acetate, -selinene, methyl eugenol, terpin-eugenol and acetyl eugenol

11. NA NA NA -pinene, -thujene, camphene, -pinene, Kekelidze et al. (1977)

sabinene, myrcene, -phellandrene, limonene, -phellandrene, 1,8-cineole, - terpinene, p-cymol, linalool, terpinene-4- ol, eugenol, methyl eugenol, trepenyl formate

12. Italy NA NA -thujene (5.9%), -pinene (20.1%) 1,8 Hector and Retamar (1978)

cineole (37.3%) p-cymene (traces), - terpineol (2.2%), terpenyl acetate (10.6%), methyl eugenol (0.3%)

13. Turkey Leaves NA Cis-thujzen-4-ol (a new compound) Novak (1985)

14. Greece Leaves NA 1,8 cineole and -terpenyl acetate (major Tucker et al. (1992)

component) pinocarvone and (E)- pinocarveol (new compounds)

15. Uttarkhand, Fruits, 5% d₃₆ 0.923, n_D ^28s 1,8 cineole (28.4%), methyl cinnamate, Nigam et al. (1992), Appendino

India 5.73s, acid value, 3.34 and ester (20.1%), -phellandrene (10.1%), - et al. (1992)

value, 25.86, ester value after pinene (9.3%), terpenol (5.8%), sabinene

acetylation 54.68 (4.9%), -thujene (3.8%), -humulene

(3.3%), linalool (2.3%), camphor (2.2%), and -gurujunene 2.2%

1 India Petroleum NA 10-hydroxyoctacosanyl tetradicanoate, l- Garg et al. (1992)

ether extract do co sanol tetradecanoate and 11-

of fruits gaveeramanthin, dehydrocostus lactone, costunolide, zalu zanin and sesquiterpene alcohol

17. Toulouse, Flowers, NA (E)-ocimene and sesquiterpenic Fiorini et al. (1997)

France 0.18% compounds -carophyllene, viridio- florene, -clemene, germacrene-D-4-ol and germacrene-D

Leaves, 1,8 cineole, linalool, methyleugenol and -

0.57% terpenyl acetate

Stem bark 1, 8 cineole

0.68%

Stem wood, -terpinyl acetate, methyl eugenol and -

0.07% copaene

a = NA, information, not available,

Fig 1 Structures of some important chemical constituents of essential oil of bay leaves

1990) and reduced glucose transport (Gurman et al. 1992). The administration of 200 and

600 mg/kg doses of the ethanolic extract of leaves of L. nobilis produced a significant decrease in blood glucose levels in diabetic rabbits (Yanardag and Can 1994).

The possible antiulcerogenic activity of L. nobilis seeds was tested on experimentally

(ethanol) induced gastric ulcers in rats. The results indicated antiulcerogenic activity for

20 and 40% aqueous extracts as well as for the oily fraction of the seeds. In acute toxicity studies, the aqueous extract was found safe with LD50 compared to oil LD50 at 0.33 ml/ kg body weight (Afifi et al. 1997).

Bay has also been reported as having a number of other properties. The methanolic extract from the leaves of L. nobilis inhibited the elevation of blood ethanol level in ethanol loaded rats. The bioassay-guided separation resulted in the isolation of costunolide, dehydrocostus lactone, and santamarine as the active constituents. The - methylene- -butyrolactone structure was found to be essential for the preventive effect on ethanol absorption. In addition, the retardation of gastric emptying seemed to be partially involved in the preventive effects (Matsuda et al. 1999).

The effects of aqueous extracts of leaves and flowers of L. nobilis on adult snail and embryo (Biomphalaria glabrata) have been studied. Results obtained have shown a degree of toxicity on the embryos starting at a concentration of 125 ppm. The flower extract appeared to be more effective. Cephalic and shell malformations were found in embryos treated with both leaf (50 ppm) and flower (25 ppm) extracts. The LD90 value on adult snails was estimated as 340 ppm for flower extract and 1900 ppm for leaf extract

(Rey and Kawano 1987). Cockroach repellant activity has also been found in bay leaves

(Verma and Meloan 1981).

The antioxidant properties of bay have been discussed by Lagouri and Bouskou

(1995).

5 Toxicity and allergenicity

Bay leaves and their essential oil do not appear to have any significant toxicity. However, sporadic reports have indicated that bay leaves may cause allergic contact dermatitis

(Asakawa et al. 1974; Cheminat et al. 1984; Goncalo and Goncalo 1991) perhaps induced by one or more sesquiterpene lactone. Certain bay leaf samples of Mexican origin had been detected to be infested with gastrointestinal disease causing Clostridium perfringens spores @ <100 to 450 Cfu/g (Rodriguez-Romo et al. 1998).

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