Jo Robinson of Eating on the Wild Side explains nutrients in fruits and vegetables, while scientists use a crop modeling system to help guide future food production in response to growing populations and changing climates.
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Mustard Greens (photo credit: Melissa McClellan/Flickr)
Through exploration of the ancestral context of taste, scientists can better understand how modern humans use the sense of taste to make decisions and survive. Evolution has shaped our sense of taste to guide us to seek the food we need to survive, while steering clear of foods harmful to us. It is understandable that early humans who avoided spoiled meat and poisonous berries were able to pass down their genes, giving modern humans the ability to avoid them too. But what explains the countless humans who voluntarily consume, and even enjoy, some bitter foods? Why do we eat bitter greens? Brussels sprouts? Hoppy beers? Why do we tolerate some bitter flavors and not others?
Tastes can be positively or negatively palatable depending upon their context among other food flavors. Sour fruit flavors like grapefruit or cranberry can be refreshing and delicious to eat, but sour milk clearly signals that the food has expired. These matches between tastes and flavors are called flavor congruencies.
Most taste-odor flavor pairings are learned associatively through eating. Flavors associated with calories and nutrients become more pleasurable with time, whereas poisoning and illness teach us to associate foods with an unpleasant taste or disgust. For omnivores like us, learning the consequences of eating different foods is an indispensable survival tool. Because our range of food option is so vast, it is essential to sample many foods and connect their post-ingestive consequences with their perceived tastes. Bitter-tasting substances are innately disliked by infants and children presumably because most bitter compounds are toxic. Most children are drawn to all things sickeningly sweet, but as adults enjoy eating eat bitter Brussels sprouts. We learn to enjoy the taste of mildly bitter foods, especially when paired with positive metabolic and pharmacological outcomes. The more your body benefits from an ingested food, the more palatable it becomes [1].
Our bodies require phytonutrients such as flavonoids that cannot be physically separated from their vegetable carriers. Humans learn to tolerate low levels of bitterness in foods as they co-occur with nutrients in plants through a post-digestive reward/punishment system. For example, rhubarb contains 0.5% oxalic acid by weight, a substance that in large doses can cause joint pain and fatal kidney stones. The first time a child eats rhubarb, the initial taste response tells the brain that the food is bitter, toxic, and should be avoided. However, as the body begins to benefit from the essential nutrients in rhubarb without suffering any damage, the rhubarb becomes more and more palatable. Experiments show that rats can very quickly learn associations between tastes and metabolic and physiological consequences, perhaps in a matter of days. These associations occur after only a single trial and are strong enough to resist fading away even after multiple presentations of the food with no physiological consequences [2].
In humans, a large sugar molecule called maltooligosaccharide (MOS) presents a sweeter case of taste association. Human saliva transforms starch into MOS. Although MOS is tasteless, it activates brain reward centers in a manner similar to sugar, while non-nutritive sweeteners do not. Thus, a tasteless molecule that has positive metabolic outcomes can activate brain reward areas more effectively than a sweet-tasting substance that has little nutritional value [3].
The next time you eat mustard greens, stop to appreciate the complex process that allows you to taste and enjoy your leafy meal. Consider how your perception of taste has evolved, which has protected your ancestors from poisoning themselves. Reflect upon the incredible and complex mechanisms humans have developed to keep you well nourished. And if you still haven’t warmed up to greens, consider introducing them gradually into your diet. By exploiting the body’s associative adaptation to taste, you could learn to love them.
References Cited
About the author: Elsbeth Sites is pursuing her B.S. in Biology at UCLA. Her addiction to the Food Network has developed into a love of learning about the science behind food.
Beauty rest isn’t just for people—cabbages also benefit from a good night’s sleep. (photobear/Flickr)
In 400 BCE, the Greek admiral Androsthenes wrote* of a tree that
“opens together with the rising sun . . . and closes for the night. And the country-dwellers say that it goes to sleep.”
Over the next 2000 years, researchers discovered that the daily cycles first observed by Androsthenes fall into 24-hour periods similar to our own cycles of waking and sleeping [1]. In plants, these circadian rhythms help control everything from the time a plant flowers to its ability to adapt to cold weather [2]. Plants can even use their internal clocks to do arithmetic calculations to budget their energy supplies through the night [3].
But what happens when part of a plant is harvested for food? In a recent study, researchers at Rice University and UC Davis showed that cabbages can exhibit circadian rhythms as long as a week after harvest.
As with any plant, cabbages experience circadian rhythms while growing out in the field; however, cabbages stuck in the constant dark of a delivery truck or light of a 24-hour grocery store will inevitably lose their sense of time. Like travelers adjusting to a new time zone, cabbages deprived of cyclic light conditions suffer a severe bout of veggie jet lag. And just as travelers overcome jet lag by readjusting their sleep cycles, cabbages can “re-entrain” their circadian rhythms by being exposed to cyclic light conditions. This also works with spinach, zucchini, sweet potato, carrots, and blueberries, suggesting that post-harvest circadian rhythms are a general characteristic of many, if not all, fruits and vegetables.
The ability to re-entrain circadian rhythms in produce presents an intriguing new way to improve the palatability and even nutrition of our fruits and vegetables. In the wild, circadian rhythms can help plants defend themselves against hungry herbivores. The researchers showed that cabbages with re-entrained circadian rhythms use a similar mechanism to avoid becoming an afternoon snack for plant-eating larvae—with less damage from hungry larvae, re-entrained cabbages appear fresher and tastier than cabbages kept under constant light or dark conditions.
 |
| Circadian rhythms help protect produce from herbivores. Samples from cabbages kept in (A) cyclic “in phase” light, (B) constant light, or (C) constant dark conditions were fed to larvae. Cabbages kept in constant light or constant dark sustained the most damage. |
Cabbages fight off larvae and other pests thanks to molecules called glucosinolates. Any cabbage can produce these molecules, but re-entrained cabbages produce glucosinolates in sync with their circadian rhythms. Because larvae also experience circadian rhythms, re-entrained cabbages get an extra boost of molecular larvae-fighting power just when they need it the most.
While glucosinolates are bad news for larvae, they have valuable anti-cancer properties when consumed by humans. In fact, the very molecules that plants create to defend themselves against their environment are often beneficial for our own health. Future research will show whether such phytonutrients in other types of produce can also be reconditioned to accumulate in predictable 24-hour cycles. Taking advantage of circadian rhythms in fresh produce could then give us more control over the way phytonutrients accumulate over time, helping us maximize the nutritional benefits of our fruits and vegetables. Improving the nutrition of our food could be as simple as giving our produce a good night’s sleep.
*The original Greek passage comes from Botanische forschungen des Alexanderzuges [4] with a very special thank you to Tovah Keynton for the English translation. The drawings (also from Botanische) depict the tree leaves transitioning into and then assuming their “sleeping position.”
References Cited
About the author: Liz Roth-Johnson is a Ph.D. candidate in Molecular Biology at UCLA. If she’s not in the lab, you can usually find her experimenting in the kitchen.
Author Jo Robinson explores the agricultural history of phytonutrients, while Harvard researchers move us a step closer toward understanding how the resveratrol in red wine and chocolate could be hindering the aging process. Read more
Dena Herman, RD, PhD, MPH, is an Adjunct Assistant Professor in the Department of Community Health Sciences at the UCLA Fielding School of Public Health. Her research has focused on improving dietary quality among low-income populations, as well as the development of interventions to reduce childhood obesity.
