When I was very young I used to complain that certain vegetables like brinjals (egg plant, aubergine), cucumbers, drumsticks and some leafy vegetables tasted bitter in summer and refused to eat curries  prepared by my mother with them. My parents would think that as I  was a very picky and poor eater,  my grievances were excuses for refusing to eat things I didn't like! My mother used to do her best to decrease the bitterness while cooking but somehow my hyper sensitive taste buds always picked it!

I now want to tell my mom and dad, who sadly no longer alive, that I am right and  my protests were not just excuses and there is a scientific explanation behind these vegetables tasting bitter in summer.

And that explanation is " Heat Stress" or "Environmental Stress" experienced by these plants that produce fruits, vegetables and grains. Some plants produce chemicals that are bitter and toxic because of attack by pests.

Take, for example, cucumbers and musk melons. They are part of the Cucurbit family, along with squash.

 These plants naturally produce chemicals called cucurbitacins, which are very bitter, and in large quantities can make a person ill. Most of the time, these chemicals are confined to the leaves and stem of the plant, but can work their way into the fruit of the plant in certain conditions causing bitter cucumbers.

The most important reason why a cucumber is bitter in the first place is simple genetics; there is a recessive trait that can cause a plant to produce bitter fruit from the start. You may plant seeds from the same packet and treat them all the same, only to discover one of the plants produces bitter cucumbers.

But one of the most common reasons why a cucumber is bitter is due to heat stress. If a plant is stressed due to heat, it may start producing bitter cucumbers. Another possibility for what causes bitter cucumber is if a cucumber goes through alternating periods of drought and over-watering. The stress can cause the plant to produce bitter fruit. If the temperature fluctuates dramatically from one extreme to another over an extended period of time, the plant may start producing bitter cucumbers. 

Egg plants or brinjals  contain  the toxin solanine which - when consumed in high doses ( the toxic dose is 20 to 25 mg. according to FDA, US ) (ref 1) - can cause gastro problems among other things. Solanine is a glycoalkaloid poison found in species of the nightshade family (Solanaceae), such as the potato (Solanum tuberosum), the tomato (Solanum lycopersicum), paprika, peppers and the eggplant. It can occur naturally in any part of the plant, including the leaves, fruit, and tubers. Solanine has pesticidal properties, and it is one of the plant's natural defenses.  

Solanine poisoning is primarily displayed by gastrointestinal and neurological disorders. Symptoms include nausea, diarrhea, vomiting, stomach cramps, burning of the throat, cardiac dysrhythmia, nightmare, headache and dizziness. In more severe cases, hallucinations, loss of sensation, paralysis, fever, jaundice, dilated pupils, hypothermia. 

(The conditions caused by Solanine are collectively called  SOLANINE TOXICITY SYNDROME [STS] by people who practice alternate medicine.   According to them different people have different degrees of sensitivity to them, and different efficiencies in being able to excrete them. How or in what way they will affect you will be a matter of genetics, as well as lifestyle and nutritional status.  It can do the following in sensitive patients; 1) act as an endocrine disruptor especially to the thyroid, 2) cause chronic joint pain, arthritis (all forms), joint inflammation- this is due to solanines ability to remove calcium from the bones and deposit it in any weak or genetically predisposed area of the body, 3) for the same reason it can be a major contributor to osteoporosis (since it removes calcium from the bones) and arteriosclerosis (it can deposit the calcium in the blood vessels), 4) “leaky gut” as well as IBS, 5) appendicitis, 6) birth defects including spina bifida, 7) depression, 8) migraines, 9) can greatly interfere with calcium and vitamin D absorption, despite supplementation.)

But you don't find clear scientific evidence for STS!

Therefore, there is a need for vigorous testing and research for STS authenticity which is endorsed by even Ayurveda. 

A bitter taste in a potato is another, potentially more reliable indicator of toxicity.  Potatoes that are diseased with blight or that have sprouted have a larger than usual amount of solanine. They will have a bitter taste and often a green discoloration; such potatoes should be avoided.

When exposed to light, potatoes manufacture increasing amounts of chlorophyll as well as two bitter-tasting alkaloid compounds, solanine and chaconine. While chlorophyll is a harmless chemical, its presence in potatoes indicates that the tubers have also increased their production of a glycoalkaloid known as solanine. Solanine protects potatoes and other plants in the family Solanaceae from herbivory and serves to preserve the sprouting spud from hungry animal mouths. Solanine is considered a neurotoxin, and ingestion by humans can cause nausea and headaches and can lead to serious neurological problems and even death if enough is consumed. In high concentrations, these can not only make potatoes taste bitter but also can cause headaches and stomachaches. Cooking does not destroy the solanine toxin, so the green parts of potatoes should be removed entirely. Green potatoes should especially not be served to children, whose smaller bodies make them more susceptible to poisoning. To reduce the risks, potatoes should be stored in cool dark areas, and when in doubt, throw them out .

Sprouted potatoes contain higher levels of glycoalkaloids, which can be toxic to humans when eaten in excess. Health problems linked to eating sprouted potatoes range from stomach upset to heart and nervous system problems

Even sweet potatoes become bitter sometimes. Uneven rainfall without compensatory watering causes the excess bitterness. Black rot, even if barely detectable on the sweet potato, can cause bitterness. This is a common disease of sweet potatoes.

Then insect attack make these tubers produce toxic chemicals. So if you find dark spots, or insect holes in sweet potato tubers, just throw them away and don't consume them.

Extreme climate also increases toxins in other crops. Food crops can  generate more chemical compounds in response to extreme weather - and this could in turn damage our health too!

Crops such as wheat and maize are generating more potential toxins as a reaction to protect themselves from extreme weather.

But these chemical compounds are harmful to people and animals if consumed for a prolonged period of time, according to the document released recently by United Nations Environment Assembly meeting in Nairobi (2).

Crops are responding to drought conditions and increases in temperature just like humans do when faced with a stressful situation.

Under normal conditions, for instance, plants convert nitrates they absorb into nutritious amino acids and proteins. But prolonged drought slows or prevents this conversion, leading to more potentially problematic nitrate accumulating in the plant. If people eat too much nitrate in their diets, it can interfere with the ability of red blood cells to transport oxygen in the body.

Crops susceptible to accumulating too much nitrate in times of stress include maize, wheat, barley, soybeans, millet and sorghum. 

Some drought-stressed crops, when then exposed to sudden large amounts of rain that lead to rapid growth, in turn accumulate hydrogen cyanide, more commonly known as prussic acid. Prussic acid - one of the ingredients used in some types of chemical warfare - interferes with oxygen flow in humans. Even short-term exposure can be debilitating for people. Plants such as cassava, flax, maize and sorghum are most vulnerable to dangerous prussic acid accumulation.

Mycotoxins, produced by molds that can affect plant crops and raise the risk of liver damage, cancer and blindness, as well as stunting foetuses and infants, also are spreading to more areas as a result of shifting weather patterns as a result of climate change. They are responsible for conditions which are collectively called mycotoxicoses.

Toxic crops can lead to neurological diseases among humans but the greatest challenge is the incidence of cancer.

Scientists suggest that developing crop varieties designed to cope with extreme weather could help reduce the levels of toxic chemicals in food.

Some modern varieties of fruits and vegetables have the bitterness bred out of them  already by scientists! People of science are trying their best to help. 

Meanwhile, if you find your food bitter, just throw it away! Listen to your taste buds that say the food is not good for you!

References: 

1. http://www.accessdata.fda.gov/scripts/plantox/detail.cfm?id=1364

2. http://web.unep.org/frontiers/sites/unep.org.frontiers/files/docume...