BITTERNESS IN CITRUS JUICES (3) – OIL
Bitterness in citrus juices, other than grapefruit, is perceived as having a negative influence on taste.
In general, there are three naturally occurring components in citrus that contribute to bitterness:-
- The flavonoid ‘naringin’ that occurs predominantly in grapefruit and provides the expected bitter note.
- The limonoid ‘limonin’ that exists in a tasteless pre-cursor form in all citrus and develops during or after processing. The bitter taste is largely unpleasant and is sometimes described as ‘dirty’ or ‘earthy’ with a lingering aftertaste.
- The terpene ‘d-Limonene’ in citrus oil, which if present in excess amounts through a high oil content produces a harsh, bitter like taste sensation.
This article describes the occurrence and control of citrus oils in juices.
Citrus oils occur in two locations in the fruit; by far the larger amount is found in the oil glands or vesicles of the outer coloured layer, the flavedo. The lesser amount exists in the juice sacs of segments in the main body of the fruit. Both oils comprise predominantly the terpene d-limonene at around 90%, depending on fruit type.
In general for oranges, peel oil comprises around 0.41 – 0.96% v/w of the fruit depending on size, variety and seasonality. For tangerine/mandarin varieties figures are 0.40 – 1.02% v/w and for grapefruits 0.28 – 0.46% v/w respectively.
Essence oil is present in much smaller amounts at around 0.005% v/v in citrus juices and rather than existing as discrete droplets, they are found absorbed on to the pulp.
Both types of oil have powerful, but differing flavour properties and in juice contribute to the expected fruity and juicy character. However, the level of oil in juice needs to be controlled within narrow limits to avoid either a bland and insipid character or an overpowering sensation described variously as ‘oil burn’, ‘flavour burn’ or ‘bitterness’ on the tongue.
The major component of citrus oils is the hydrocarbon d-limonene, often accounting for over 90% of the total. Limonene itself has very little taste, but in excess amounts gives rise to the adverse ‘bitterness’ described above. It is the minor oxygenated components of the oils that are responsible for the flavour character. Typical d-Limonene of citrus essential oils are shown below.
|Oil Type||Limonene %v/v|
|Orange, tangerine, tangelo.||>95|
|Lemon||75 – 80|
|Lime (Mexican, Persian)||50 – 55|
As the oil content of juice is important as regards to flavour quality, maximum levels of recoverable oil have been set by various bodies, such as the United States Department of Agriculture (USDA) and the European Fruit Juice Association (AIJN). For most citrus juices, the maximum oil content at defined single strengths is established as 0.035%v/v (USDA) and 0.030%v/v (AIJN). The full list is shown in Table 1 of Appendix 1.
In all forms of physical juice extraction from the fruit, it is inevitable that some peel oil becomes mixed in with the juice stream. Processors take care to minimise oil inclusion as far as is possible, particularly when producing freshly squeezed or single strength juices, often adopting ‘soft squeeze’ conditions. The choice of juice extractor type also has a strong influence on peel oil inclusion with specially designed available machinery for the ‘freshly squeezed’ juices. The use of centrifugation has been used to remove surplus oil directly from NFC juice (resulting also in juice oil with better top notes than when recovered by vacuum stripping).
For pasteurised juices excess oil can be removed in a de-oiler, which can also act as the pasteurisation and enzyme inactivation stage. In a de-oiler the juice is heated and flashed into a vacuum chamber, where a portion is evaporated off as a vapour which is subsequently condensed back to a liquid. The recovered liquid is decanted or centrifuged to remove the oil phase and then usually added back to the juice. Typically, 3 – 6% of the juice is evaporated under conditions of 87.5°C at 480mm Hg vacuum or 54.4°C at 115mm Hg vacuum which can remove around 80% of the oil. The process also acts to deaerate the juice.
For concentrated juices the resulting final oil content is often too low, resulting in a poor flavour character. It is usual to add back a small amount of recovered peel oil, either singly or in combination with recovered essence (or juice) oil to achieve an improved flavour.
Cold pressed peel oil is obtained from the fruit by the action of rasping the flavedo to release the oil, which is held under pressure in the oil glands, capturing the oil in a water spray to form a crude emulsion which is then centrifuged to recover the oil.
Essence oil is obtained from fractionation of the first and second effects of a multistage evaporator.
Juice oil results from the centrifugation of the condensate from juice de-oiling, or directly from the juice.
Each oil has differing composition and flavour quality so that various combinations can be made for add-back to a juice concentrate.
The total oil content of citrus fruit varieties ranges on average from 2.8 to 7.75 Kg/Metric Tonne, as shown in Table 2 of Appendix 1. Commercial recovery of oil can be around 65 – 70% of the total available.
The traditional official analysis to determine the oil content of citrus juices is by the Clevenger steam distillation method. The procedure involves boiling a relatively large quantity of single strength juice and recovering the distillate via a condenser. The distillate is collected in a graduated U-tube, where the included oil droplets coalesce and rise to the surface. After allowing the contents of the U-tube to cool, the volume of oil is read from the scale. The method can take between 1 and 3 hours to achieve a quantitative recovery of the oil. Even with 500 -1000ml of juice, the volume of recovered oil will be only 0.15 to 0.3ml for a juice with the maximum expected level of oil.
A more rapid determination is by the Scott method of bromide/bromate titration where a smaller quantity of juice, 50 – 100ml, is co-distilled with isopropyl alcohol. The distillate is acidified and titrated with standardised potassium bromide – bromate solution, where typically titres of 10 – 25ml are obtained for an oil content of 0.025%. The method takes only minutes to perform while also offering better accuracy and precision than that of Clevenger.
It is generally accepted that for good quality juice for direct consumption, oil contents should be in the range 0.015 to 0.030%v/v, to ensure adequate juiciness without ‘burn’ or ‘bitterness’
Table 1. Maximum Oil Contents for Juices at Single Strength.
|FRUIT||JUICE TYPE||CODE||OIL %v/v MAX|
|Grade A||Grade B|
|ORANGE||Canned Orange Juice||CSSOJ||0.035||0.055|
|Frozen Concentrated Orange Juice||FCOJ||0.035||0.040|
|Reduced Acid Frozen Concentrated Orange Juice||0.035||0.040|
|Concentrated Orange Juice for Manufacturing||COJFM||–||–|
|Canned Concentrated Orange Juice||0.035||0.040|
|Dehydrated Orange Juice||0.035||0.045|
|Pasteurised Orange Juice||POJ||0.035||0.045|
|Orange Juice from Concentrate||OJFC||0.035||0.045|
|Grapefruit Juice from Concentrate||GJFC||0.020||0.025|
|Frozen Concentrated Grapefruit Juice||FCGJ||0.025||0.025|
|Concentrated Grapefruit Juice for Manufacturing||–||–|
|Dehydrated Grapefruit Juice||0.020||0.025|
|TANGERINE||Canned Tangerine Juice||0.025?||0.035?|
|Concentrated Tangerine Juice for Manufacturing||–||–|
Table 2. Peel Oil Content of the Major Citrus Cultivars.
|Fruit||Lb Oil / Ton Fruit
|Kg Oil / MT Fruit
Sources of information.
- B. Sinclair. The Orange, it’s Biochemistry and Physiology 1961. J. G. Kirchner, Oils in Peel, Juice Sac and Seed, 291. University of California. Division of Agricultural Science. The Regents of the University of California.
Steven Nagy, Philip E Shaw, Matthew K Veldhuis. Citrus Science and Technology. Volume 2. Fruit Production, Processing Practices, Desired Products and Personnel Management. 1977. Robert E Berry, Matthew K Veldhuis. 200 – 201.
Citrus Nutrition and Quality. 1980. Eds S. Nagy and J. A. Attaway. 299-301. ACS Symposium Series 143. ISBN 01-8412-0595-7.
- V. Ting, Russell L. Rouseff. Citrus Fruits and Their Products, Analysis and Technology. 1986. 5.48 – 49. Marcel Dekker Inc. 1986. ISBN 0-8247-7414-0.
Florida’s Citrus Juice Standards for Grades and Their Differences from United States Standards for Grades and United Sates Food and Drug Administration Standards of Identity. P. J. Fellers. 1990. Proc. Fla. State Hort. Soc. 103:260-265.
Dan A. Kimball. Citrus Processing. Quality Control and Technology. 1991. 74-89. An AVI Book. ISBN 0-442-006 15-2
Steven Nagy, Chin Su Chen, Philip E. Shaw. Fruit Processing. ISBN 0-471-19024-1 (Cloth : Alk paper).Technology. 1993. James B. Redd, Charles M. Hendrix, Jnr. 84-97. Chen Su Chen, Philip E. Shaw, Mickey E. Parish.154-155. Philip E. Shaw, Steven 182-184 Nagy. A. G. Science Inc. ISBN 0-9631397-1-1.
James B. Redd, Philp E. Shaw, Charles M. Hendrix, Jnr. Donald L. Hendrix. Quality Control Manual for Citrus Processing Plants. Volume 111. 1996. Charles M. Hendrix, Jnr. 259. AG Science Inc. ISBN 0-09631397-1-6(v.3).
Robert J. Braddock. Handbook of Citrus By-Products and Processing Technology.1999. 149-174. John Wiley & Sons Inc. (Florida Agricultural Experimental Station Journal Series No. R-067467). ISBN 0-471-19024-1. (Cloth : Alk paper).
(Note: Tangerine oil not listed in USDA standard below)
https://www.flrules.org/gateway/ChapterHome.asp?Chapter=20-64 (Access list for all Florida Standards, now aligned with USDA, effective 19/06/12, except Canned Tangerine Juice 28/0/97)