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Many fruits (like apples, berries, citrus fruits etc.) contain high levels of organic acids, especially malic acid and citric acid. Are there any evolutionary functions of those acids in ripe fruits? In unripe fruits the sour taste may protect fruits from being eaten prematurely by herbivores, but why are those acids still present in ripe fruits? Are they there for increasing osmotic pressure so plants can make bigger fruits in a cheap way by just inserting more water in them? Or do organic acids perhaps play a role in pathogen protection? Or are they just a by-product of some other biochemical mechanism in fruit development?
Great question! I have done some research on the topic and found so much interesting information. It still seems like a topic that i being researched on, but so far, the research shows that malate is created by a CAM and C4 like pathway while citrate is created from TCA cycle. Their functions are that they are important biomolecules in metabolism, in particular respiration.
Malic acid and citric acid are the two main organic acids that are accumulated in the fruits and cause the sour taste. Citrus fruits contain more citric acid and other fruits like apples and pears contain malic acid. I will provide a detailed analysis for both cases.
In an article by Deshpande & Ramakrishnan they mention two hypotheses about the accumulation of the citric acid in fruits. One was that the organic acids are not formed in the fruit, but transported to the fruit, in which the fruit acts like a storage organ. The other hypothesis is that the carbohydrates in the fruit is converted to the organic acid. In their research. they tried to isolate some enzymes from the Garcinia fruit to support the second hypothesis. They say:
This [reciprocal relation between acid and sugar content] would indicate the transformation of sugar to citric acid in the fruit tissue itself due to the presence of the requisite enzyme systems. They suggest that the TCA cycle is how the citric acid is being accumulated and stored.
The article in the comments by Lobit et al. also supports a similar hypothesis of the accumulation of malic acid. The authors tell us that the malic acid is most likely produced in the fruit itself because the pH of the xylem and phloem (the transporting organs of plants) have a pH of close to 7 or higher. It seems that the malic acid is stored in the vacuoles of the mesocarp cells and that the amount of malic acid in the fruit depends on the transport of the malic acid to the vacuoles in the mesocarp cells.
The actual pathways for the creation of citric acid and malic acid are very complex. Here is a picture of the different pathways in the cytosol of the mesocarp cells thst create these organic acids (picture taken from Etienne et al.)
The first pathway described is the PEP pathway, which is really a modified CAM or C4 pathway (which are teo ways to deal with photorespiratin, and are adaptations to hot, dry environments). The PEP is converted to oxaloacetate by PEPC, which also takes in CO2, then that oxaloacetate is converted to malate by NAD-MDH. PEPC is regulated by malate concentration and cystolic pH, showing that the activity of PEPC is to indirectly increase the malate concentrations, not just to create more oxaloacetate.Let me note that I still don't fully understand why C4 and CAM-like pathways are present in Fruits from C3 plants.
The major pathway for citrate synthesis is the TCA cycle. Both are also involved in metabolism. TCA cycle (the citric acid cycle) is an important cycle and the second pathway in aerobic respiration. This pathway occurs in the mitochondria. Malate reacts with acetyl-CoA to make citrate and the CoA is released. The citrate then goes on to be repeatedly oxidized and ends up back to oxaloacetate. Now, for metabolic reasons, for example, if the amount of ATP created is top much at the end of aerobic respiration, the citrate could be transported out of the mitochondria, leading to citrate accumulation.
Control of acid accumulation
The control of the organic acid accumulation is different for both malic acid and citric acid. Malic acid accumulation in the vacuoles is controlled by vacuolar storage and vacuolar transport (Etienne et al). This hypothesis was really the subject of the article in the comments. Their article was about modeling the malic acid based on this hypothesis and compare it to actual biochemical analysis of fruits. Their results supported the hypothesis.
The accumulation of citric acid is more dependent on metabolism, therefore respiration. This is since the vacuolar transport is slower for citrate than malate, and so whatever limits thr speed of malate vauolar transport affects citrate. But citrate concentration in thr cytosol would be the only other main factor affecting the vacuolar transport. A model by Wu et al. was created based on this hypothesis and compared to the actual citrate concentrations, and once again, this hypothesis also supported.
Functions of these organic acids
Suprisingly, the only functions of malate and citric acid cycle is mainly for respiration(mainly TCA cycle), and other metabolic processes. Malate does affect the texture, ripening, and starch metabolism and more of fruits, as described in this article. But, malate and citrate are important for all respiring organisms, including us.
Soursop (also graviola, guyabano, and in Hispanic America, guanábana) is the fruit of Annona muricata, a broadleaf, flowering, evergreen tree.   The exact origin is unknown it is native to the tropical regions of the Americas and the Caribbean and is widely propagated.  It is in the same genus, Annona, as cherimoya and is in the Annonaceae family. Soursop is known as sirsak in Indonesia, where it is widely available and believed to have medicinal benefits.
Annona macrocarpa Wercklé
Annona crassiflora Mart. 
Guanabanus muricatus M.Gómez
Guanabanus muricatus (L.) M.Gómez 
Annona bonplandiana Kunth
Annona cearensis Barb. Rodr.
Annona muricata Vell. 
The soursop is adapted to areas of high humidity and relatively warm winters temperatures below 5 °C (41 °F) will cause damage to leaves and small branches, and temperatures below 3 °C (37 °F) can be fatal. The fruit becomes dry and is no longer good for concentrate.
With an aroma similar to pineapple,  the flavor of the fruit has been described as a combination of strawberries and apple with sour citrus flavor notes, contrasting with an underlying thick creamy texture reminiscent of banana.
Soursop is widely promoted (sometimes as "graviola") as an alternative cancer treatment, but there is no medical evidence it is effective for treating cancer or any disease. 
The name derives from Arabic: تمر هندي , romanized tamar hindi, "Indian date". Several early medieval herbalists and physicians wrote tamar indi, medieval Latin use was tamarindus, and Marco Polo wrote of tamarandi. 
In Colombia, Ecuador, Cuba, Dominica, Guatemala, El Salvador, Honduras, Mexico, Peru, Puerto Rico, Venezuela, Italy, Spain, and throughout the Lusosphere, it is called tamarindo. In those countries it is often used to make the beverage of the same name (or agua de tamarindo). In the Caribbean, tamarind is sometimes called tamón. In Indonesia and throughout another Malay world countries, it is called asam jawa (Javanese tamarind) or simply asam, and sukaer in Timor.  While in the Philippines, it is called sampalok or sampaloc in Filipino, and sambag in Cebuano.  Tamarind (Tamarindus indica) is sometimes confused with "Manila tamarind" (Pithecellobium dulce). While in the same taxonomic family Fabaceae, Manila tamarind is a different plant native to Mexico and known locally as guamúchili.
Tamarindus indica is probably indigenous to tropical Africa,  but has been cultivated for so long on the Indian subcontinent that it is sometimes reported to be indigenous there.  It grows wild in Africa in locales as diverse as Sudan,  Cameroon, Nigeria, Kenya, Zambia, Somalia, Tanzania and Malawi. In Arabia, it is found growing wild in Oman, especially Dhofar, where it grows on the sea-facing slopes of mountains. [ citation needed ] It reached South Asia likely through human transportation and cultivation several thousand years ago.   [ failed verification ] It is widely distributed throughout the tropics,  from Africa to South Asia and throughout Oceania, Southeast Asia, Taiwan and China. [ citation needed ]
In the 16th century, it was introduced to Mexico and Central America, and to a lesser degree to South America, by Spanish and Portuguese colonists, to the degree that it became a staple ingredient in the region's cuisine. 
Today, India is the largest producer of tamarind.  The consumption of tamarind is widespread due to its central role in the cuisines of the Indian subcontinent, Southeast Asia, and the Americas, especially Mexico.
The tamarind is a long-lived, medium-growth tree, which attains a maximum crown height of 12 to 18 metres (40 to 60 feet). The crown has an irregular, vase-shaped outline of dense foliage. The tree grows well in full sun. It prefers clay, loam, sandy, and acidic soil types, with a high resistance to drought and aerosol salt (wind-borne salt as found in coastal areas). 
The evergreen leaves are alternately arranged and pinnately lobed. The leaflets are bright green, elliptic-ovular, pinnately veined, and less than 5 cm (2 in) in length. The branches droop from a single, central trunk as the tree matures, and are often pruned in agriculture to optimize tree density and ease of fruit harvest. At night, the leaflets close up. 
As a tropical species, it is frost-sensitive. The pinnate leaves with opposite leaflets give a billowing effect in the wind. Tamarind timber consists of hard, dark red heartwood and softer, yellowish sapwood. 
The tamarind flowers (although inconspicuously), with red and yellow elongated flowers. Flowers are 2.5 cm wide (one inch), five-petalled, borne in small racemes, and yellow with orange or red streaks. Buds are pink as the four sepals are pink and are lost when the flower blooms. 
The fruit is an indehiscent legume, sometimes called a pod, 12 to 15 cm ( 4 + 1 ⁄ 2 to 6 in) in length, with a hard, brown shell.   
The fruit has a fleshy, juicy, acidic pulp. It is mature when the flesh is coloured brown or reddish brown. The tamarinds of Asia have longer pods (containing six to 12 seeds), whereas African and West Indian varieties have shorter pods (containing one to six seeds). The seeds are somewhat flattened, and a glossy brown. The fruit is best described as sweet and sour in taste, and is high in tartaric acid, sugar, B vitamins, and, unusually for a fruit, calcium. 
The fruit is harvested by pulling the pod from its stalk. A mature tree may be capable of producing up to 175 kg (386 lb) of fruit per year. Veneer grafting, shield (T or inverted T) budding, and air layering may be used to propagate desirable cultivars. Such trees will usually fruit within three to four years if provided optimum growing conditions. 
Culinary use Edit
The fruit pulp is edible. The hard green pulp of a young fruit is considered by many to be too sour, but is often used as a component of savory dishes, as a pickling agent or as a means of making certain poisonous yams in Ghana safe for human consumption.  As the fruit matures it becomes sweeter and less sour (acidic) and the ripened fruit is considered more palatable. The sourness varies between cultivars and some sweet tamarind ones have almost no acidity when ripe. In Western cuisine, tamarind pulp is found in Worcestershire Sauce  and HP Sauce.
Tamarind paste has many culinary uses including a flavoring for chutneys, curries, and the traditional sharbat syrup drink.  Tamarind sweet chutney is popular in India and Pakistan  as a dressing for many snacks and often served with samosa. Tamarind pulp is a key ingredient in flavoring curries and rice in south Indian cuisine, in the Chigali lollipop, in rasam, and in certain varieties of masala chai tea. Across the Middle East, from the Levant to Iran, tamarind is used in savory dishes, notably meat-based stews, and often combined with dried fruits to achieve a sweet-sour tang.   In the Philippines, the whole fruit is used as an ingredient in the traditional dish called sinigang to add a unique sour taste, unlike that of dishes that use vinegar instead. Indonesia also has a similarly sour, tamarind-based soup dish called sayur asem.
In Mexico and the Caribbean, the pulp is diluted with water and sugared to make an agua fresca drink.
Tamarind seed oil Edit
Tamarind seed oil is the oil made from the kernel of tamarind seeds.  Isolation of the kernel without the thin but tough shell (or testa) is difficult. Tamarind kernel powder is used as sizing material for textile and jute processing, and in the manufacture of industrial gums and adhesives. It is de-oiled to stabilize its colour and odor on storage.
|Acid insoluble ash||0.4%||0.3%|
|The fatty acid composition of the oil is linoleic 46.5%, oleic 27.2%, |
and saturated fatty acids 26.4%. The oil is usually bleached after refining.
|Fatty acid||(%) Range reported|
|Lauric acid (C12:0)||tr-0.3|
|Myristic acid (C14:0)||tr-0.4|
|Palmitic acid (C16:0)||8.7–14.8|
|Stearic acid (C18:0)||4.4–6.6|
|Arachidic acid (C20:0)||3.7–12.2|
|Lignoceric acid (C24:0)||4.0–22.3|
|Oleic acid (C18:1)||19.6–27.0|
|Linoleic acid (18:2)||7.5–55.4|
|Linolenic acid (C18:3)||2.8–5.6|
Seeds can be scarified or briefly boiled to enhance germination. They retain their germination capability for several months if kept dry. [ citation needed ]
The tamarind has long been naturalized in Indonesia, Malaysia, Sri Lanka, the Philippines, the Caribbean, and Pacific Islands. Thailand has the largest plantations of the ASEAN nations, followed by Indonesia, Myanmar, and the Philippines. In parts of Southeast Asia, tamarind is called asam.  It is cultivated all over India, especially in Maharashtra, Chhattisgarh, Karnataka, Telangana, Andhra Pradesh, and Tamil Nadu. Extensive tamarind orchards in India produce 275,500 tons (250,000 MT) annually. 
In the United States, it is a large-scale crop introduced for commercial use (second in net production quantity only to India), mainly in southern states, notably south Florida, and as a shade tree, along roadsides, in dooryards and in parks. 
A traditional food plant in Africa, tamarind has the potential to improve nutrition, boost food security, foster rural development and support sustainable landcare.  In Madagascar, its fruit and leaves are a well-known favorite of the ring-tailed lemur, providing as much as 50 percent of their food resources during the year if available. 
Folk medicine Edit
Throughout Southeast Asia, the fruit of the tamarind is used as a poultice applied to foreheads of fever sufferers.  The fruit exhibits laxative effects due to its high quantities of malic acid, tartaric acid, and potassium bitartrate. Its use for the relief of constipation has been documented throughout the world.  
Tamarind lumber is used to make furniture, carvings, turned objects such as mortars and pestles, chopping blocks, and other small specialty wood items. Tamarind heartwood is reddish brown, sometimes with a purplish hue. The heartwood in tamarind tends to be narrow and is usually only present in older and larger trees. The pale yellow sapwood is sharply demarcated from the heartwood. Heartwood is said to be durable to very durable in decay resistance, and is also resistant to insects. Its sapwood is not durable and is prone to attack by insects and fungi as well as spalting. Due to its density and interlocked grain, tamarind is considered difficult to work. Heartwood has a pronounced blunting effect on cutting edges. Tamarind turns, glues, and finishes well. The heartwood is able to take a high natural polish. 
Metal polish Edit
In homes and temples, especially in Buddhist Asian countries, the fruit pulp is used to polish brass shrine statues and lamps, and copper, brass, and bronze utensils. The copper alone or in brass reacts with moist carbon dioxide to gain a green coat of copper carbonate. Tamarind contains tartaric acid, a weak acid that can remove the coat of copper carbonate. Hence, tarnished copper utensils are cleaned with tamarind or lime, another acidic fruit. 
Throughout South Asia and the tropical world, tamarind trees are used as ornamental, garden, and cash crop plantings. Commonly used as a bonsai species in many Asian countries, it is also grown as an indoor bonsai in temperate parts of the world. 
In hens, tamarind has been found to lower cholesterol in their serum, and in the yolks of the eggs they laid.   Due to a lack of available human clinical trials, there is insufficient evidence to recommend tamarind for the treatment of hypercholesterolemia or diabetes.  Different parts of tamarind (T. indica) are recognized for their various medicinal properties. A previous study reported that the seed, leaf, leaf veins, fruit pulp and skin extracts of tamarind possessed high phenolic content and antioxidant activities.  The presence of lupanone and lupeol,  catechin, epicatechin, quercetin and isorhamnetin  in the leaf extract could have contributed towards the diverse range of the medicinal activities. On the other hand, ultra-high performance liquid chromatography (UHPLC) analyses revealed that tamarind seeds contained catechin, procyanidin B2, caffeic acid, ferulic acid, chloramphenicol, myricetin, morin, quercetin, apigenin and kaempferol.  The treatment of tamarind leaves on liver HepG2 cells significantly regulated the expression of genes and proteins involved with consequential impact on the coagulation system, cholesterol biosynthesis, xenobiotic metabolism signaling and antimicrobial response. 
ELI5: why is the sour taste at least to some degree positive
I understand there have been some questions about sour foods, but could not find this being adressed. the sour taste is a negative response to prevent the consumption of bad foods and acids right? so why do people eat sour candy
I love sour candy, tart foods, and cooking with vinegar. I know some people really don't. For me there's a difference between good sour (lime juice) and bad sour (rotting milk).
TLDR Humans have to get vitamin C , an essential nutrient, by diet alone. Citrus fruit is a great source of vitamin C and has a sour/tart taste to it so it could be an evolutionary advantage to taste it and even enjoy it. Could also just be coincidence/cultural.
Most acids commonly found in food are not bad for us, they are actually a source of energy. Truly dangerous acids are not commonly found in nature, and they typicaly also smell pretty bad so we don't eat them.
Also, many fruit are sour, and our distant ancestors evolved around a diet based on fruit, so fruit taste good to us. That's also why we now have color vision, whereas most other mammals don't: Otherwise, it would be hard to distinguish ripe fruit from a distance.
Seedless fruit is not something new
Seedlessness in many fruits is a highly desirable trait and is due to natural causes, not genetic engineering techniques.
Wild banana with many seeds.
Let&rsquos admit it&mdashwe really don&rsquot like seeds. That is not universally true, of course. After all, many food items are actually seeds (beans, peas, rice, corn, coffee, cacao) or come from seeds (flour, oil), and we need seeds to propagate many plants. However, when it comes to grapes, watermelon, banana, citrus and some other fruit and vegetables, seeds can be a nuisance. Seeds in many fruits are intermingled with the part we eat, and not confined to the inedible portion like apples, or small like blueberries and strawberries. The crunch of a large seed is not enjoyable and unless it is a contest, it is often socially awkward to spit them out. Therefore, we jump at the chance to get rid of seeds, or at least reduce them to a manageable number.
Seedless plants are not common, but they do exist naturally or can be manipulated by plant breeders without using genetic engineering techniques. No current seedless plants are genetically modified organisms (GMOs). As with many plant systems, several steps must work correctly in the &ldquopathway&rdquo for production of the final product (seeds in this case). Compromise in any one step leads to failure. Seedlessness to the plant is useless since it fails to produce offspring, that is why most seedless plants are propagated through grafting or cuttings (cucumber and watermelon being exceptions). However, it is a heritable trait carried on through pollen and maintained in the gene pool until the right parental combination again occurs to produce a plant with seedless fruit. Since these occur naturally, and humans being observant, curious and resourceful creatures, once we find something we like, we take full advantage of it. I am sure the first person to discover seedless grapes had a corner on the raisin market. So, why are some fruit seedless?
All seedless fruit fall under a general category called parthenocarpy. Parthenocarpy is a Greek word meaning &ldquovirgin fruit.&rdquo This is a situation where fruit develops without fertilization of the ovule (the part of the flower that when fertilized develops into a seed). In these plants, pollination may or may not be necessary to trigger hormone production to stimulate the ovary wall to swell and form fruit. However, fertilization and seed development does not occur and there are no &ldquoseed traces&rdquo or seed remnants. In some cases, fruit development can be stimulated in the absence of pollen through external hormone applications. This seedlessness is present in some varieties of cucumbers, persimmons, grapes, citrus, pineapples and others. This type of seedlessness often produces smaller fruit than their seeded counterparts.
Some plants capable of producing seed may have sterile pollen or other reasons that render them incapable of forming seed, and to produce seed they require pollination by another, genetically different member of that species. When planted in large orchards, they are surrounded by genetically identical copies of themselves, causing them produce parthenocarpic fruit. Many citrus operate this way.
Stenospermocarpy is a type of parthenocarpy where fertilization occurs and the seed begins to develop but eventually aborts, leaving behind a noticeable &ldquoseed trace.&rdquo Seed traces vary in size depending on how far seed development progressed before abortion and are generally soft enough that they do not have the crunch of fully developed seed. This occurs in most seedless grapes, watermelon and other fruits. Breeders of seedless grapes capitalize off this partial development process by removing developing seeds prior to abortion and growing them into plants using tissue culture techniques. This way, both parents possess the seedless trait thereby producing a higher number of seedless offspring.
Disruption of the seed development process occurs for a number of reasons. Watermelon and banana are seedless because they have three sets of chromosomes, giving them an odd number to work with when they produce pollen and egg cells. Most organisms have an even number of chromosomes, so the resulting egg and pollen cells receive an even number of chromosomes that contain the genetic material, e.g., DNA, to combine to make offspring. When triploids form eggs and pollen, the process produces an odd number, resulting in egg and pollen not receiving an equal chromosome compliment, therefore they lack information needed to be viable. Pollen from triploids often appears shriveled and poorly formed.
Triploid organisms occur naturally or they can be developed by crossing a diploid (two sets of chromosomes) with a tetraploid (four sets of chromosomes) to produce a triploid. In the case of watermelon, pollination needs to occur for fruit to develop and since triploid pollen does not germinate, diploid varieties are interplanted to provide viable pollen to induce fruit without complete seed development. The white seed traces are readily visible in watermelon
Stenospermocarpic seedlessness in all grapes studied so far are all due to a naturally occurring harmful &ldquopoint mutation&rdquo in the section on the grape chromosome responsible for seed development. Many use the word mutation or mutant in a negative context, but most changes we find desirable occurred naturally.
An effort was made to develop seedless cherries. However, there is a difference between a &ldquopit&rdquo and a seed. A pit is the hard, stony tissue surrounding the seed in olives, cherries, peaches, plums and apricots and is not part of the seed. Researchers were able to develop seedless but not pitless cherries.
Seedlessness may or may not change the character of the fruit. Seed in a fruit can help draw energy and nutrients into the fruit changing characteristics such as nutrient and sugar levels, fruit size, fruit number, time of maturity and others. Breeders and horticulturalists have done a good job using standard breeding and production techniques to overcome these limitations.
For you raspberry and blackberry lovers, as far as I know there is still no seedless members, but we will keep looking.
Why Do Sour Things Make Me Pucker?
Have you ever sucked on a lemon and felt your face scrunch up? Foods that are very sour contain a lot of acid and can make you pucker—wrinkle your face, squint your eyes, and press your lips together. When things like lemons, vinegar, and unripened fruit touch your tongue, your brain gets a signal that you’re eating something sour. It could be your body's way of saying "watch out!"
Your tongue has thousands of little bumps with tiny sensors called taste buds. Taste buds let you know when something is sweet, salty, sour, bitter, or savory. (Savory is also called umami. Say: ooo-MOM-eee.) Each taste bud has dozens of taste cells that have little sprouts on them that look like hair that can only be seen with a microscope. When foods dissolved in your saliva touch them, they tell the brain about the flavor of what you are eating. When they come in contact with very sour foods, your face might pucker up because the taste is strong and acidic.
Puckering when you taste something sour is often involuntary (in-VAWL-uhn-ter-ee). That means you do it without trying. It may happen because we have an instinct not to eat things that are dangerous. Of course, not all sour foods are bad for us. But some sour foods can make us sick—spoiled milk or fruit that is not ripe, for example. Reacting with a wrinkled-up face may be our body’s way of trying to warn ourselves and others to stay away from foods that might hurt us.
For further reading, check out “Why Are Lemons Sour?” over at Wonderopolis.
Scientists Get to the Genetic Roots of Why Citrus Fruits Taste Sour
(Credit: Abigail Malate, Staff Illustrator) (Inside Science) -- Lemons are known for their face-puckering sour taste. Now scientists have uncovered the mysterious genes behind this acidity, new findings that could help farmers breed sweeter oranges, lemons, limes, grapefruit and other citrus fruit. The oldest known reference to citruses dates back to roughly 2200 B.C., when tributes of mandarins and pomelos wrapped in ornamental silks were presented to the imperial court of Yu the Great in China. More citruses are now grown than any other kind of fruit worldwide for example, in 2014, people in the United States consumed roughly 35.6 kilograms of citrus fruit per person, according to the Agricultural Marketing Resource Center.
Citruses are known for their acidity. The sour taste of a fruit depends on compartments within plant cells known as vacuoles, which are acidic because positively charged hydrogen ions (essentially, protons) get pumped into them. In most plant species, these vacuoles are only mildly acidic compared to the rest of the cell's innards. It was long a mystery how citrus vacuoles became extremely acidic. The new discovery regarding citruses began with distant relatives of citrus plants, the petunias. Husband-and-wife team Ronald Koes and Francesca Quattrocchio, molecular geneticists at the University of Amsterdam, and their colleagues found mutant versions of genes known as PH1 and PH5 could alter the color of the flowers by hyperacidifying their petals. "Petals with more acidic vacuoles are reddish petals with less acidic vacuoles are bluish," Quattrocchio said. These genes produced molecules known as P-ATPases on the membranes of the vacuoles, increasing the number of protons that are pumped into the compartments. Versions of these genes are found not only widely across flowering plants, including species without colorful petals, but also in plants without flowers at all, such as conifers. The widespread nature of these acidity genes suggested they might play roles beyond flower color. This spurred the scientists to explore whether they might be responsible for the acidic taste of citruses. "We looked at the most acid plant we could think of, lemons," Koes said. The researchers investigated CitPH1 and CitPH5, the citrus versions of these petunia genes. They found these genes were highly active in sour lemons, oranges, pomelos and rangpur limes, but much less active in sweet-tasting "acidless" varieties of citrus, such as Lima oranges and Millsweet limettas, due to a variety of hindering mutations. "People will see this work as a solution to a puzzle that was out there for quite a long time," Quattrocchio said. Previous attempts to isolate the proteins behind citrus acidity likely faced problems because these molecules are embedded inside cell membranes and therefore difficult to purify and analyze, Koes said. In addition, the complete pump is made of dozens of proteins, and it tends to fall apart during purification, he added. Moreover, the acid within citrus vacuoles would itself wreck many attempts to examine their membranes, said plant physiologist Lincoln Taiz at the University of California, Santa Cruz. "This is an exciting discovery -- it explains why the lemon fruit is able to hyperacidify the vacuole," said Taiz, who did not take part in this research.
Bearing Better Fruit
These findings could help speed up the breeding of new varieties of fruit, Koes said. By analyzing the DNA of young saplings, breeders may one day predict the sweetness or sourness of their fruit "many years before the trees set fruit that one could examine for acidity or taste in the conventional way," Koes explained. Such improved breeding is potentially not limited to citruses. "For example, the acidity of wine grapes could be varied to create different wine flavors," Taiz said. "Another application might be to vary the colors of flowers." In addition, there are hints these genes are linked to key parts of plant development. "We see them active in stem cells, and we have no clue why yet," said study lead author Pamela Strazzer, a molecular geneticist at the University of Amsterdam. The scientists detailed their findings online Feb. 26 in the journal Nature Communications . [This article originally appeared on Inside Science News.]
Why certain fruits don't mix
Acid, sub-acid, sweet. If these aren’t terms you generally think of when picking out your pomegranates, papayas, and persimmons then you may need a lesson in how you pick and pair the fruits you eat.
Fruit typing is part of the overall concept of food combining, which follows the thinking that foods digest at different rates and ingesting an improper mix of foods can cause fermentation within the digestive system, slowing transit time and leading to bloating and possible bacterial imbalances.
The basic rules: Fruits should be consumed alone on an empty stomach starches with cooked non-starchy vegetables flesh proteins and dairy with cooked non-starchy vegetables and nuts and seeds with raw vegetables.
“Proper food combining puts less effort and strain on our digestive system and faster, easier digestion increases the bioavailability of food,” says Mark Hendricks, group fitness manager at Toronto's Bay Street Club, who follows the protocol himself and advises clients to do the same. “We want our bodies to utilize the nutrients we ingest and this assists in the process and leads to healthy elimination as well.”
As a protocol, it can be particularly beneficial in helping to heal a digestive tract already compromised by an imbalance of flora, notes nutritionist Haylie Pomroy, author of the Fast Metabolism Diet and Fast Metabolism Diet Cookbook. But it’s a restrictive way to eat, nixing snack ideas like an apple with almond butter (a nutritionist favorite), all juice-smoothies that blend veggies with fruit, and arguably every unmodified restaurant meal.
Fruit typing, however, is an easy way to dip your toe into the idea and reap some of the benefits, Hendricks says. Here, his tips:
Separate acid from sweet.
Not all fruits play well together. Acid fruits like grapefruit and other citrus, pineapples, pomegranates, sour apples and plums, strawberries and tomatoes don’t pair well with sweet fruits like grapes, bananas, persimmon, figs, prunes, and dates.
. But buffer your acid.
Acidic fruits like grapefruit can be hard on the stomach. In that case, Hendricks tells clients to mix in a sub-acid fruit like blueberries. Subacid fruits—sweet apples, apricots, cherries, mangoes, nectarines, pears, papayas and berries—can mix with either acid or sweet fruits.
Enjoy melons on their own.
Melons like honeydew, cantaloupe, Crenshaw, and watermelon have a very high water content and digest even faster than the other fruits.
Skip the standard fruit salad.
It’s usually a digestion-slowing mix of melon, apple, pineapple, banana, and strawberry, says Hendricks. Instead, make a berry bowl using blueberries and raspberries.
Pick fiber-rich fruits pre-workout.
Melons are great for hydration but digest too quickly to sustain you, while the sugar in sweet fruits offers only short-lived energy. Instead choose a sub-acid fruit like a red apple or a nectarine with more density and fiber for staying power.
Incompatible Food Combining
It is no surprise to see on the market today so many digestive and dietary aids for the stomach, along with pills for gas and indigestion. Most of these conditions likely begin with poor food combining. This is a subject of much debate amid the growing concern about diet and the many theories on the topic.
Ayurveda, an ancient holistic science of healing, offers a logical approach for determining correct diet based upon the elements comprising an individual’s constitution: vata, pitta and kapha. This approach is quite different from the contemporary view of a balanced diet, based on eating from various food groups. Ayurveda believes that understanding the individual is the key to finding a truly balanced diet. It teaches that the gastric fire or agni in the stomach and digestive tract is the main gate through which nutrients enter the tissues and then pass along to individual cells, to maintain the life functions.
According to Ayurveda, every food has its own taste (rasa), a heating or cooling energy (virya) and a post-digestive effect (vipaka). Some also possess prabhava, an unexplained effect. So while it is true that an individual’s agni largely determines how well or poorly food is digested, food combinations are also of great importance. When two or more foods having different taste, energy and post-digestive effect are combined, agni can become overloaded, inhibiting the enzyme system and resulting in the production of toxins. Yet these same foods, if eaten separately, might well stimulate agni, be digested more quickly and even help to burn ama.
Poor combining can produce indigestion, fermentation, putrefaction and gas formation and, if prolonged, can lead to toxemia and disease. For example, eating bananas with milk can diminish agni, change the intestinal flora, produce toxins and may cause sinus congestion, cold, cough and allergies. Although both of these foods have a sweet taste and a cooling energy, their post-digestive effect is very different - bananas are sour while milk is sweet. This causes confusion to our digestive system and may result in toxins, allergies and other imbalances.
Similarly, milk and melons should not be eaten together. Both are cooling, but milk is laxative and melon diuretic. Milk requires more time for digestion. Moreover the stomach acid required to digest the melon causes the milk to curdle, so Ayurveda advises against taking milk with sour foods. These incompatible food combinations not only disturb the digestion but also cause confusion in the intelligence of our cells, which can lead to many different diseases.
Before you say “This is MUCH too complicated, how will I ever figure it out?”, there are some useful guidelines to introduce you to these concepts. And remember that Ayurveda is a strong proponent of the “go slowly” school of thought.
You might want to introduce yourself to food combining by eating fruit by itself, as many fruits create a sour and indigestible “wine” in the stomach when mixed with other food. Once you have adopted this change into your eating habits, try other suggestions from the list below. As a general principal, avoid eating lots of raw and cooked foods together or fresh foods with leftovers.
Various Factors that Can Lessen the Effects of Bad Food Combinations
- A strong digestive fire (if we are so blessed) can be the most powerful tool of all to deal with “bad” food combinations.
- Different quantities of each food involved in a combination can sometimes help significantly. For instance equal quantities by weight of ghee and honey are a bad combination&mdashghee is cooling, but honey is heating&mdashwhereas mixing a 2:1 ratio is not toxic. The reason? Prahbav, the unexplainable.
- Very often spices and herbs are added in Ayurvedic cooking to help make foods compatible or to ease a powerful effect, e.g., cooling cilantro in very spicy food.
- If our bodies have become accustomed to a certain food combination through many years of use, such as eating cheese with apples, then it is likely that our body has made some adaptation or become accustomed to this. Which is not to say that we should continue this practice, but to explain why the newcomer to apples and cheese may experience a strong case of indigestion whilst the “old-timer” digests it adequately.
- Antidotes, like cardamom in coffee, or ghee and black pepper with potatoes, often can help alleviate some of the negative effects. (Coffee is stimulating and ultimately depressing to the system, and potatoes cause gas).
- If foods with different and possibly aggravating qualities, such as a mixture of vegetables, are cooked together in the same pot, the foods tend to learn how to get along. Using appropriate spices and herbs helps with this too.
- Eating a ‘bad’ combination occasionally usually does not upset the digestion too much.
- Eat ½ teaspoon fresh grated ginger with a pinch of rock salt before each meal to stimulate agni.
- Salt also aids digestion, and helps to retain water.
- Alkalis help digestion and regulate gastric fire.
- Ghee stimulates agni and improves digestion.
- Small sips of warm water during a meal will aid digestion and absorption of food. Do not drink iced water as it slows agni and digestion. Indeed ice water should not be taken under most circumstances, as it is too shocking to the system.
- Proper chewing is essential to good digestion, ensuring food gets thoroughly mixed with saliva.
- A cup of lassi at the end of a meal also aids the digestive process. Make by blending ¼ cup yogurt with 2 pinches of ginger and cumin powder in 1 cup water.
- Ideally, one should fill the stomach with one-third food, one-third liquid and one-third should be empty.
The following table lists some* of the incompatible food combinations worth avoiding.
|Beans||fruit cheese, eggs, fish, milk, meat, yogurt|
|Eggs||fruit, especially melons beans, cheese, fish, kitchari, MILK, meat, yogurt|
|Fruit||As a rule, with any other food. (There are exceptions, such as certain cooked combinations, as well as dates and milk, which have the same rasa, virya and vipaka.)|
|Honey**||With equal GHEE by weight (e.g. 1 tsp. honey with 3 tsp. ghee) boiled or cooked honey.|
|Hot Drinks||mangos cheese, fish, meat, starch, yogurt|
|Lemon||cucumbers, milk, tomatoes, yogurt|
|Melons||EVERYTHING – especially dairy, eggs, fried food, grains, starches. Melons more than most fruit should be eaten alone or left alone.|
|Milk||BANANAS, cherries, melons, sour fruits bread containing yeast, fish, kitchari, meat, yogurt|
|Nightshades, e.g., potato, tomato||melon cucumber, dairy products|
|Radishes||bananas, raisins milk|
|Tapioca||fruit, especially banana and mango beans, raisins, jaggary|
|Yogurt||fruit cheese, eggs, fish, hot drinks, meat, MILK, nightshades|
*Foods in CAPITALS are the most difficult combinations.
**According to ancient Ayurvedic literature, honey should never be cooked. If cooked, the molecules become a non-homogenized glue that adheres to mucous membranes and clogs subtle channels, producing toxins. Uncooked honey is nectar. Cooked honey is considered poison.
© 1997, 2016. Amended extracts reprinted with permission from: Ayurvedic Cooking for Self-Healing by Usha and Dr. Vasant Lad, 1997.
The Strange Story Of The Man Behind 'Strange Fruit'
Abel Meeropol watches as his sons, Robert and Michael, play with a train set.
Courtesy of Robert and Michael Meeropol
One of Billie Holiday's most iconic songs is "Strange Fruit," a haunting protest against the inhumanity of racism. Many people know that the man who wrote the song was inspired by a photograph of a lynching. But they might not realize that he's also tied to another watershed moment in America's history.
The man behind "Strange Fruit" is New York City's Abel Meeropol, and he really has two stories. They both begin at Dewitt Clinton High School, a public high school in the Bronx that has an astonishing number of famous people in its alumni. James Baldwin went there. So did Countee Cullen, Richard Rodgers, Burt Lancaster, Stan Lee, Neil Simon, Richard Avedon and Ralph Lauren.
Meeropol graduated from Dewitt Clinton in 1921 he went on to teach English there for 17 years. He was also a poet and a social activist, says Gerard Pelisson, who wrote a book about the school.
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Evolution Of A Song: 'Strange Fruit'
In the late 1930s, Pellison says, Meeropol "was very disturbed at the continuation of racism in America, and seeing a photograph of a lynching sort of put him over the edge."
Meeropol once said the photograph "haunted" him "for days." So he wrote a poem about it, which was then printed in a teachers union publication. An amateur composer, Meeropol also set his words to music. He played it for a New York club owner — who ultimately gave it to Billie Holiday.
When Holiday decided to sing "Strange Fruit," the song reached millions of people. While the lyrics never mention lynching, the metaphor is painfully clear:
Southern trees bear a strange fruit,
Blood on the leaves and blood at the root,
Black body swinging in the Southern breeze,
Strange fruit hanging from the poplar trees.
Pastoral scene of the gallant South,
The bulging eyes and the twisted mouth,
Scent of magnolia sweet and fresh,
And the sudden smell of burning flesh!
Here is a fruit for the crows to pluck,
For the rain to gather, for the wind to suck,
For the sun to rot, for a tree to drop,
Here is a strange and bitter crop.
In 1999, Time magazine named "Strange Fruit" the "song of the century." The Library of Congress put it in the National Recording Registry. It's been recorded dozens of times. Herbie Hancock and Marcus Miller did an instrumental version, with Miller evoking the poem on his mournful bass clarinet.
Miller says he was surprised to learn the song was written by a white Jewish guy from the Bronx. "Strange Fruit," he says, took extraordinary courage both for Meeropol to write and for Holiday to sing.
"The '60s hadn't happened yet," he says. "Things like that weren't talked about. They certainly weren't sung about."
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New York lawmakers didn't like "Strange Fruit." In 1940, Meeropol was called to testify before a committee investigating communism in public schools. They wanted to know whether the American Communist Party had paid him to write the song. They had not — but, like many New York teachers in his day, Meeropol was a Communist.
Journalist David Margolick, who wrote Strange Fruit: The Biography of a Song, says, "There are a million reasons to disparage communism now. But American Communism, one point it had in its favor was that it was concerned about civil rights very early."
Meeropol left his teaching job at Dewitt Clinton in 1945. He eventually quit the Communist Party.
And that's where the second part of Meeropol's story begins. The link is the pseudonym he used when writing poetry and music: Lewis Allan.
"Abel Meeropol's pen name 'Lewis Allan' were the names of their children who were stillborn, who never lived," says his son, Robert Meeropol. He and his older brother, Michael, were raised by Abel and his wife, Anne Meeropol, after the boys' parents — Ethel and Julius Rosenberg — were executed for espionage in 1953.
Julius and Ethel Rosenberg were sentenced to death for conspiring to give atomic secrets to the Soviet Union. The Rosenbergs had also been Communists.
Julius and Ethel Rosenberg are taken to prison after being found guilty of nuclear espionage. They were subsequently executed. Keystone/Getty Images hide caption
Julius and Ethel Rosenberg are taken to prison after being found guilty of nuclear espionage. They were subsequently executed.
The couple's trial and execution made national headlines, and there was also something of a salacious element, given that the Rosenbergs were a married couple. News accounts described it as "the first husband and wife to die in the electric chair."
At the time, the Rosenberg sons, Robert and Michael, were 6 and 10, respectively. News photographs of the boys show them dressed in suits visiting their parents in prison.
"They're these little boys and they're wearing these caps, and they look so young and so vulnerable. It's really a very poignant image," says Margolick.
Robert Meeropol says that in the months following his parents' execution, it was unclear who would take care of him and his brother. It was the height of McCarthyism. Even family members were fearful of being in any way associated with the Rosenbergs or Communism.
Then, at a Christmas party at the home of W.E.B. Du Bois, the boys were introduced to Abel and Anne Meeropol. A few weeks later, they were living with them.
"One of the most remarkable things was how quickly we adapted," Robert says. "First of all, Abel, what I remember about him as a 6-year-old was that he was a real jokester. He liked to tell silly jokes and play word games, and he would put on these comedy shows that would leave me rolling."
There is something else about Abel Meeropol that seems to connect the man who wrote "Strange Fruit" to the man who created a loving family out of a national scandal. "He was incredibly softhearted," Robert says.
Anne Meeropol plays a song on guitar for her sons, Robert and Michael. Courtesy of Robert and Michael Meeropol hide caption
Anne Meeropol plays a song on guitar for her sons, Robert and Michael.
Courtesy of Robert and Michael Meeropol
For example, there was an old Japanese maple tree in their backyard, which sent out many new seedlings every year.
"I was the official lawnmower," Robert says, "and I was going to mow over them, and he said, 'Oh, no, you can't kill the seedlings!' I said, 'What are you going to do with them, Dad? There are dozens of them.'
"Well, he dug them up and put them in coffee cans and lined them up along the side of the house. And there were hundreds of them. But he couldn't bring himself to just kill them. It was just something he couldn't do."
Abel Meeropol died in 1986. His sons, Robert and Michael, both became college professors. They're also both involved in social issues. Robert founded the Rosenberg Fund for Children. And he says that even after all these years, he still finds himself unable to kill things in his own garden.