The History of Citrus: You Crossed a What with a What?!

Posted by Ron Bakus on Nov 8, 2018 12:30:48 PM

The tartness of a lemon, the sweetness of an orange. It’s hard to believe that these two contrasts come from plants that are so closely related, that if they were people, they’d be step-siblings. The story of citrus begins in the Miocene epoch, which was 23.3 - 5.3 million years ago and continues through today, where citrus is regarded as a staple all around the world, and in China, is still given as gifts. Dive with us into the world of citrus which is full of limonin, hybrids, and a history that dates back to a time where the gomphotheres roamed.

An ancient past

Fossil records suggest that the Citrus Genus dates back to 7 million years ago, and originated in the Himalayas before spreading throughout south-east Asia. There are five ancient species of citrus from which all modern varieties come from: the small flower papeda (Citrus micrantha), citron (Citrus medica), pomelo (Citrus maxima), mandarin (Citrus reticulata), and kumquat (Citrus japonica).

The five ancient varieties are all unique with their own taste and consistency, but have similar enough genomes, allowing them to easily crossbreed. Additionally, these hybrids are fertile, allowing them to continue to populate, unlike seedless bananas or watermelons. This cross-pollination began in nature, but once humans got ahold of citrus, they did what they do best: tinkering with different blends of citrus until we got an orange that was perfectly sweet, a lemon that was beautifully tart, and so on.

Citrus flourishes in the Mediterranean or semi-tropical climates. Of course, there are many varieties in its home continent of Asia, but in the United States, citrus is most commonly grown in Florida and California.

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So, what makes a citrus a citrus?

Citrus trees can be large shrubs or small-to-moderate-sized trees, with five-petaled, 2-4 cm flowers that are generally strong in scent. The fruit itself is a hesperidium (a berry with a leathery rind) with a pericarp (its peel) concealing the fruit. They have zest (the outermost layer of the peel), pith (middle layer), and pulp and seeds (the center layer) (UCLA).

One fact almost everyone knows about citrus is that they have a high vitamin C (ascorbic acid) content. The amount of vitamin C in each type of fruit varies on the species, variety, and, of course, the method of cultivation of the fruit. Being a vitamin, ascorbic acid is essential to human nutrition, and a deficiency in vitamin C causes scurvy.

James Lind, a Scottish physician, ran one of the first ever medical trials in 1747 to see if it was possible to prevent scurvy. He did this by experimenting with sailors in the Royal Navy who had spent two months on the high seas and had contracted scurvy. By giving one group of the sailors two oranges and one lemon to eat daily (the other groups, which showed no improvement, received cider, dilute sulfuric acid, vinegar, seawater, and barley water), he was able to show that the vitamin C in citrus was able to cure and prevent the affliction of scurvy. He then wrote about this discovery in his book, A Treatise of the Scurvy in Three Parts.

Unfortunately, why citrus (and vitamin C in it) would prevent or cure scurvy was not understood at the time, and it took a long time for this result to be accepted. Thankfully, today with a balanced diet, scurvy is a specter from the past.

In addition to being chock-full of vitamin C, citrus is also full of citric acid, so named because it was initially isolated from citrus. Until modern times, citric acid was mainly isolated from lemons.

Image via Agricolae [CC BY-SA 4.0], from Wikipedia Commons

And now, a little citrus chemistry...

Although sweet oranges have been bred for their saccharine taste, you may have noticed that juice that has been left to sit out had a distinct tang or bitterness. This is due to a chemical component called limonin and a process called "delayed bitterness". Almost all citrus have limonin, which is why this occurs in almost every citrus drink (think: buying way too bitter lemonade at your neighborhood lemonade stand). Limonin gets produced when the fruit is squeezed and enzymes break down the limonin precursor: limonin glucoside. This enzymatic action takes time, hence "delayed bitterness". In order to have long lasting sweet fruit juices, the limonin, and limonin glucoside have to be extracted.
 

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Under threat

Citrus, for all its marvelous mutability and variation, faces a number of threats, of which probably the most well known is “citrus greening disease” (also called huánglóngbìng or HLB). It is a bacterial disease transmitted tree to tree by sap-sucking insects (phyllids) that originated in Asia and spread rapidly around the world, reaching the United States in 2005. Its causes shrunken, misshaped, and bitter fruits to develop, before eventually killing the tree. To prevent the spread of greening disease and other citrus diseases (and to avoid a hefty fine), it is incredibly important that plant material (whole plants, twigs, leaves, and yes, even fruits) not be transported, even neighborhood to neighborhood within a city, and to purchase citrus plants from a reputable nursery. For more information, see SaveOurCitrus Resources.

Biodiversity and mini citrus

As mentioned previously, citrus is known for its high-levels of successful cross breeding. The majority of commercial citrus are results of crosses, and back crosses, between pomelos, mandarins, and citron. In fact, most of the citrus that you know and love are results of hybrids! Take, for example, the common sweet orange. Every sweet orange you pick up from the grocery store contains a blend of pomelo genes and mandarin genes.

This cross breeding has brought into the world many wonderful types of mini citrus. Good Land Organics, a farm located in Goleta, CA, grows caviar limes, named for the little “balls” inside the outer peel. They’re a tart treat that is perfect for flavoring water or serving on top of fish.

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Whether you’re looking for something sweet, something sour, or something unique, the citrus Genus has something for you.

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Topics: Food Science