The Unique Character of New Zealand Lemon Oil and Other Innovative Opportunities from Citrus
Abstract
New Zealand lemon oil (Villafranca on Citrus sinensis rootstock) has unusual characteristics which set it apart from lemon oils produced in other countries. The oil exhibits an optical rotation outside of the range commonly used to define a genuine cold-pressed lemon oil. The New Zealand oil also shows a very high citraldehyde content, a parameter commonly used to indicate the quality of a lemon oil. The meaning of these terms and the implications for the New Zealand citrus industry will be discussed at the conference.
Opportunities are opening up for the production of minimally processed foodstuffs. These products have been identified as the contestants most likely to show the greatest growth in the fruit and vegetable sector over the next five to ten years. Minimally processed citrus fruits have recently been developed in the USA and are now available on that market. Minimal processing will be discussed together with the USA experience in minimally processed citrus fruit and possible applications to New Zealand citrus.
Introduction
Cold-pressed citrus peel oils are valuable by-products obtained during or after citrus juice extraction. In particular, lemon oil is of great importance to the food and beverage industry in addition to finding application in the production of lemon-flavoured detergents and shampoos. Production of lemon oil came to a halt in New Zealand in the mid 1980s when Oasis Industries ceased processing of lemons into juice concentrate and oil in Te Puke. This paper presents some background information on the processing of lemons into juice and oil and emphasises some of the unique and valuable characteristics of New Zealand lemon oil. Finally, opportunities in minimally processed citrus products are presented and a couple of ideas with regard to the Asian market for citrus are considered.
Extraction of juice and oil from fresh lemons
Oil exists in the yellow layer of the peel of the lemon which is known as the "flavedo". It is found in balloon-like cells called "oil glands". The oil is held in place by "turgor pressure" and to release the oil from the gland a high pressure must be applied which overcomes the turgor pressure of the cell. Under such conditions, the oil is expelled from the cell and forms an oil droplet on the surface of the flavedo. Immediately the pressure on the peel is released, the oil is sucked back into the gland.
The FMC in-line juice extractor is one of a number of juice extractors used to extract juice from citrus fruit. The FMC extractor also has the ability to simultaneously extract oil from citrus at the same time as extracting juice. The principle of this extractor is shown in Fig. 1. The fruit is graded for size and fed to an extractor which can handle a set range of graded fruit. Inter-meshing jaws encompass the fruit and draw together crushing the fruit between them. The juice exits through a mesh screen which penetrates the centre of the fruit and the juice is thus separated from the peel, pith and seeds.
This crushing action is sufficient to force the oil out of the glands onto the surface of the flavedo. If the pressure were to be relaxed at this point, the oil would merely suck back into the gland and be irrecoverable. However, the FMC machine operates a water spray onto the surface of the fruit at the very instant that the oil is found sitting on the surface. The oil is carried away as an oil-in-water emulsion; oil recovery is achieved by passing the mixture through a number of high-G centrifuges. The recovered oil is known as polished, cold-pressed citrus peel oil.(not to be confused with steam-distilled oil).
New Zealand lemon oil
In the 1980s, when oil was extracted in New Zealand from lemons, the cultivar selected was Villafranca grown on sweet orange (Citrus sinensis) rootstocks. There are a number of parameters used to identify quality and authenticity of lemon oil. Two of the most commonly used parameters are aldehyde value and optical rotation.
Aldehyde value
The quality of lemon oil is commonly determined by measuring aldehyde content. Values in the range of 2.2 to 3.8% are indicative of good quality (highly aromatic oil). The aldehyde content of New Zealand lemon oil, calculated as citraldehyde, averaged 4.23%. This was higher than the highest level reported for Florida lemon oil of 3.61%; the minimum level for Florida lemon oil was reported as 2.15%.
Optical rotation
In order to appreciate the term optical rotation as applied to lemon oil, it is necessary to understand something of the nature of light. A narrow beam of light actually vibrates in many directions as it proceeds through the air. If a polarised film is inserted in the path of the beam (eg polarised sun-glasses), all vibrations except in one plane are eliminated. (This is why polarised sun-glasses reduce glare so effectively when viewing waves with the sun shining on them). The light emerging after passing through the polarised film is know as plane-polarised light - it is only vibrating in one plane, all other vibrations having been halted at the polarising film.
Optical activity is the name given to the phenomenon exhibited by compounds which, when placed in the path of a beam of polarised light, are capable of rotating the plane of polarisation to the left or right. Such compounds are said to be optically active. In the laboratory we can measure the angle through which a pure organic compound (or a solution of the compound) rotates a beam of polarised light and this value is a property of the compound. Lemon oil contains a mixture of optically active organic compounds. The contribution of these compounds to the optical rotation shown by lemon oil results in an overall rotation to the right. Lemon oil is said to be dextro rotatory.
The United States Pharmacopeia (USP) (1975) and the Food Chemicals Codex (1981) use the optical rotation range of +57 to +65.6° as a criterion of purity of cold-pressed lemon oil. The optical rotation of +53.3° for New Zealand lemon oil is outside these two designations of purity and therefore our oil would, using this criterion, not be recognised as a pure, genuine lemon oil. In fact this aspect of authenticity for a lemon oil has been taken up by the New Zealand Food Regulations (1984). Under Section 203, Culinary essences, culinary extracts, and culinary flavourings, oil of lemon is defined by a number of parameters which include: "A 10 cm layer shall have an optical rotation at 20°C of not less than +57° and not more than +65° (using light with a wavelength of the D line of sodium)". Therefore, although New Zealand lemon oil complies with all the other criteria for authenticity (weight per mL, refractive index, aldehyde content), it fails on account of its optical rotation value. In theory, the food industry would be acting illegally if it presented our genuine New Zealand lemon oil on the local market as "oil of lemon".
New Zealand trade in lemon juice and oil (Source: Statistics of New Zealand)
In the year July 1994 to June 1995, New Zealand exported 1,400 litres of lemon juice returning NZ$5,942 fob. By comparison, imported lemon juice in bulk containers was high at 234,711 litres valued at NZ$786,018 cif. A further quantity of lemon juice was imported in "containers other than bulk containers" amounting to 30,699 litres and valued at NZ$108,562 cif.
As lemon oil is not now produced in New Zealand, there were no exports of the oil during the 12 months period. Imports of lemon oil amounted to 5,533 kg at a cost of NZ$88,680 cif.
In New Zealand we have the ability to produce a high value, high quality lemon oil due presumably to our unusual local combination of scion, rootstock, soil and climatic conditions. At the same time we could be producing lemon juice and/or lemon juice concentrate for domestic and industry use. Perhaps it is time to re-address the possibility of lemon growing in New Zealand.
Minimally processed citrus
"Minimal processing" is a term with which those involved in horticulture will become increasingly familiar in the years ahead. Minimally processed fruits and vegetables can be defined as those fruits and vegetables that may have been cleaned, peeled, cut, sliced, packaged or processed by any means short of killing the tissues of the fruits or vegetables. Minimslly processed fruits, however, retain their flavour, aroma, and nutrition better than conventionally processed fruit.
Although minimal processing of fruit is not new, the expectation of longer shelf-life for these products has risen. The demand for minimally processed fruits is the result of the twin desires of consumers for convenience and quality. It has been predicted that by the year 2000, the market value for minimally processed fruits and vegetables will grow to US$4000-8000 million a year.
It is only in recent years that minimal processing has caught the interest of the food scientist. More research is needed to improve thetechnology for producing high quality, minimally processed fruit. In the USA, Sunkist has recently undertaken research into minimally processed citrus and has already achieved market success with minimally processed orange products.
Over the last 30 years Sunkist has investigated economical methods of peeling externally blemished citrus fruit in order to provide a high quality value added product. In the mid ‘80s, Sunkist observed a levelling off and then slight decline in their sales of fresh citrus fruit. They studied the market and decided that the potential for growth could lie in the production of pre-peeled fruit, whole or sliced. In late 1987 Sunkist contacted the USDA in Winterhaven, Florida with the goal of developing a commercial enzyme peeling process.
As development proceeded, Sunkist examined over fifty different enzymes in an effort to discover which produced the most desirable results: ease of peeling, amount of albedo removal, minimal cell damage to the surface of the fruit (leakage), and best surface textureal quality. The company established and patented specific time/temperature combinations to produce whole peeled citrus fruit. The peeled product shelf life was tested at several different temperatures. Maximum shelf life was 16-18 days at 1-2°C and 10-12 days at 4-5°C. Equipment was designed and patented to assist in the treatment and removal of the peel.
In 1989 Sunkist began producing product in its Freshly Peeled Plant in Ontario, California. Recent years brought changes and additions to the process. These changes improved the quality of the product, increased shelf life and productivity of the plant. The company has examined several methods of extending shelf life in order that the product quality can be maintained throughout the retail, food service and industrial distribution chains. These included; barrier packaging, irradiation, UV light treatment, ozone, natural microbicides, and preservatives. Sunkist’s product line has increased in both size and variety of products available as they continue testing new markets including retail, fast food, vending, club store, food service and industrial. Sunkist has experienced rapid growth in the peeled orange market where products are packaged in 22 lb bulk packs for institutional use and 3-4 per pack for consumer sales. The declining sales have been arrested and turned around through the creation of high quality, convenient minimally processed citrus fruits.
Health foods and nutraceuticals
The well-developed citrus industry in the USA boasts a plethora of products extracted from citrus and research into new possibilities continues. These are mainly achieved by economies of size and New Zealand would not be well-placed to competitively produce and market many of these products even for domestic use. For example, a range of different pectins is available extracted from citrus for use as gelling agents in processed foodstuffs. These pectins are able to produce gels and low or zero sugar content..
In recent years, we have become increasingly aware that what we eat affects our health. This has led to the development and sale of the so-called "health foods" which convey positive benefits. One item of interest is the range of "low-sugar jams" for inclusion in slimming diets or simply to avoid high intake of sugars. Citrus lends itself nicely to this range and low-sugar citrus jams can be produced by incorporating low-esterified pectins in the formulation. Low-esterified pectins are prepared from citrus pectin by either acid hydrolysis or by treatment with ammonia. In New Zealand we can produce these jams from our local fruit but include modified citrus pectin from overseas.
A new dimension has recently been added to our concept of food and some foods are now marketed as having something extra, a specific physiological function in addition to their basic nutritional value. These "physiologically functional foods" are promoted as having a variety of effects. Some reduce stress, lower choloesterol, combat memory loss or help to prevent cancer. A sub-class have probiotic (the opposite of antibiotic) properties because they contain live organisms that are known to be beneficial to humans. More recently, the term "nutraceutical" has been used to describe a physiologically functional food. The definition of nutraceutical is "any substance that can be considered a food or part of a food and provides medical or health benefits, including the prevention and treatment of disease". The distinction between nutraceuticals and the pharmaceutical sector is becoming less and less clear. We should address these developments with citrus in mind and look to research bridging the gap between human nutrition and health, and food science and molecular biology.
Finally, we should remain aware of products based on citrus which are "new" to our culture but are familiar items in other cultures. Some of these could include increased sale of fresh fruit in cultures where gift-giving or consumption of fresh fruit cause huge increases during certain festivals or celebrations. Chinese New Year (Chun Jie) is one festival when the consumption rises phenomenonally. It takes place on the day that the first new moon enters Aquarius; this can only happen not earlier than 21 January and not later than 20 February. The festival lasts for 15 days.
The presentation of oranges during Chun Jie is basically a southern Chinese ritual, arising from the fact that the Cantonese pronunciation of oranges is "Gam" which has the same name as gold. When visiting one’s relatives and friends it is a must to bring along two or even four oranges as gifts.
In South Korea a tea is made from the skin of citrus. It is sold in a similar manner to black tea in Western countries and is known locally as "Youja". Lateral thinking in terms of the market and the consumer combined with unique characteristics of some New Zealand citrus should enable greater penetration of the export market in the future.
Acknowledgement
I wish to express my appreciation to Scott Witters, Market Analyst, HortResearch, for researching statistics on the New Zealand trade in lemon oil and juice.