Lemons (Citrus limon) are one of those fruits that epitomises the Citrus genus. It has a characteristic zesty, acidic and aromatic flavour but this varies with variety, state of ripeness and country of origin.
The supply of lemons is mainly from the Mediterranean region with a significant supply chain in California, USA. Italy, especially the Amalfi Coast and the island of Sicily have been the key suppliers of culinary lemon for centuries. Sicily in particular has provided much of the fruit. Lemon concentrate for use in beverages has been a key commodity. It has also been bedevilled by adulteration and the trust in the market for Sicilian products has been compromised over many years.
Growth
The lemon fruit excels in growing in semi-arid irrigated areas and coastal areas of the world. It is nowadays grown around the world and there are many countries that rely on it for all sorts of culinary purposes. In the humid tropics, lemon trees generate a fruit with coarser peels. A Mediterranean-climate is ideal for growing lemons which is why they are so popular around Spain, Italy, Israel and northern African countries. Lemons are also suited to growing in sub-Himalayan climates too such as India.
The fruit of the lemon tree varies greatly in size, shape, colour, rind texture and juice content. The fruits of the Assam lemon and the Verna (Vernia or Berna of Spain) are long, obovate to oblong in shape, and medium to large in size.
Varieties:
Assam lemon (Nepali oblong): a fruit that matures in December to January, nearly seedless but not quite. Blooms throughout the year, oblong to obovate fruit, 10-15cm long and 3-4cm wide, apex nippled. The fruit rind is medium thick with a central axis that is partially filled or hollow.
Baramasi lemon: a seedless variety with medium-sized, medium-thick rind, round fruit. Along with ‘Galgal’, avriety grown in the foothills and sub-mountainous regions of Himachal Pradesh, Punjab and Uttar Pradesh.
Eureka: The most important cultivar in the world. The fruit are small to medium-sized. This cultivar sets multple crops which can be harvested during three periods.
Lisbon – a heavy yielding variety grown in Australia, California, Argentina nd Portugal from whence its name comes. A seedy fruit which is similar to Eureka but with a pronounced mamilla. The Lisbon lemon trees form a dense canopy. This means the tree is less susceptible to frost damage.
The Seedless lemon: known entirely for having seed-free fruit. The shape is ellipsoid to ovate with a nipple at the apex. The base is round with smooth peel which is thin, the axis is hollow. Makes 10 to 13 segments. The pulp is light yellow and very juicy. The fruit matures in November to January.
Villa Franca : A smooth skinned variety similar to Eureka. Has thick rind, medium-large fruit, ovate-oblong, bright lemon yellow when mature, quite seedy, juicy. A variety grown commercially in Australia.
Harvesting
All lemons are susceptible to winter-chill injury. Lemons generally are difficult to store in commercial cold stores which are always operated at low temperatures.
Storage Conditions
Lemons suffer from the disappearance of chlorophyll in the rind. It is markedly affected by lower oxygen levels of between 5 and 10 per cent. The time required for green lemons to turn yellow is about 16 weeks in an atmosphere of 10 per cent oxygen content.
Lemon Essential Oil
The essential oil from lemon is a prized ingredient in citrus based perfumes and fragrances as well as bringing out the lemon citrus notes in foods and beverages. The best quality lemon oil comes from cold-pressing the peels as is the case with all citrus peel products.
The first published studies on the composition of lemon oil come from Morton in 1929 who reviewed the absorption spectra of essential oils and perfume chemicals in the visible and UV regions at the University of Liverpool (UK). Sale (1953) then published a comprehensive survey on lemon oil composition which has helped analysts and adulterers alike.
The principal components in lemon peel oil are γ-terpinene, terpinolene, d-limonene and citral. As with most citrus fruit, the production of volatiles is greater with a degree of ripening. So it is with lemon, where lemon yellow fruit produces more volatiles than unripe green lemons.
Compared to other common citrus fruit such as oranges, grapefruit and mandarins, lemon oil has lower proportions of monoterpenes. There is a higher content of beta-pinene and gamma-terpinene. It is the terpinene which contributes most flavour loss and deterioration because it degrades to p-cymene. The main hydrocarbons are tetradecane and pentadecane.
The main non-volatiles in lemon oil are coumarins (Stanley & Vannier, 1957) and psoralens (furocoumarins). We’ll see later why they are so important in characterising lemon oil and beating the adulterators. They established the presence of 5-geranoxypsoralen, 5, 7-dimethoxycoumarin, 5-geranoxy-7-methoxycoumarin and byakangelicin. These could of course be artefacts but their presence is significant.
Lemon oils change immediately following extraction. In freshly extracted oils there is some loss of aldehydes and esters along with oxidation and isomerization of terpenes. Storage produces further significant changes.
Adulteration of Lemon Essential Oil
A significant challenge for genuine suppliers of lemon oil is the damaging impact of adulteration. The cold-pressed lemon oil is cut with much lower quality steam-stripped oil. It is still a considerable issue and the masterminds of this activity regularly try to find ways to circumvent those checking for adulteration. The conventional method to check for adulteration of lemon oil is to compare the ultraviolet absorption characteristics of pure lemon oil with the sample under investigation. The UV profile is contributed to by coumarins and psoralen which are naturally present in cold-pressed lemon oil. Extracts of these are added to lemon oil to mask adulteration. One particular study found ethyl p-dimethylaminobenzoate and 7-geranyloxycoumarin (auraptene) being routinely used in some lemon essential oils. Likewise, 7-methoxycoumarin (herniarin) and 5,7-dipropyloxy-4-methylcoumarin have also been found in some commercial oils and these are not natural compounds in lemon oil. There are however some novel psoralens in the authentic cold-pressed oil which are hard to find in other sources and could be used effectively to determine if the essential oil is authentic or not. These compounds are 5,8-disubstituted psoralens, in particular 5-isopent-2′-enyloxy-8-(2′,3′-epoxyisopentyloxy)psoralen (McHale & Sheridan, 1988).
Medicinal Properties
Lemons have a high nutritional and medicinal value. The fruits contain key natural chemical components, such as citric acid, ascorbic acid, minerals, flavonoids, VC, VA, VB1, VB2 and various trace elements, such as Ca, P, Fe, Zn, and Mg (Del Rio et al., 2004).
Lemon Essential Oil Products
References
, , & (2004). Citrus limon: A source of flavonoids of pharmaceutical interest. Food Chemistry, 84, pp. 457–461 (Article)
McHale, D., & Sheridan, J. B. (1988). Detection of adulteration of cold‐pressed lemon oil. Flavour and fragrance Journal, 3(3), pp. 127-133.
Sale, J. W. (1953). Analysis of lemon oils. Journal of Association of Official Agricultural Chemists, 36(1), pp. 112-119 (Article)
Stanley, W. L., & Vannier, S. H. (1957). Chemical Composition of Lemon Oil. I. Isolation of a Series of Substituted Coumarins. Journal of the American Chemical Society, 79(13), pp. 3488-3491 (Article)
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