What is the sweetest taste on earth?
Biologists investigate why the sweet taste of sugary foods diminishes when they’re cool
Date: April 23, 2020 Source: University of California — Santa Barbara Summary: Have you ever noticed how a bite of warm cherry pie fills your mouth with sweetness, but that same slice right out of the refrigerator isn’t nearly as tempting? Scientists know this phenomenon to be true, but the mechanism behind it has been poorly understood. Share:
Have you ever noticed how a bite of warm cherry pie fills your mouth with sweetness, but that same slice right out of the refrigerator isn’t nearly as tempting? Scientists know this phenomenon to be true, but the mechanism behind it has been poorly understood.
Now, using fruit flies as his subjects, UC Santa Barbara Distinguished Professor Craig Montell has discovered one process responsible for this occurrence. Montell’s team, which includes Qiaoran Li, Nicolas DeBeaubien and Takaaki Sokabe, found that cool temperatures suppress the appeal of sweetness. However, these conditions did not affect the sugar neurons themselves. Rather, they acted via other sensory cells by way of a protein originally discovered to sense light in the eye. Despite this, the perception of coolness in sugary food is not altered by light. The results appear in the journal Current Biology.
«The appeal of food is influenced by more than just chemical composition,» said Montell, the Duggan professor in the Department of Molecular, Cellular, and Developmental Biology. «We already know that cool temperatures reduce the delectability of sweetness in humans.» He and his colleagues wondered whether this was also true in fruit flies, and if so, what were the underlying mechanisms?
The team found a significant difference in fruit flies’ interest in feeding between 23 degrees Celsius (73.4° Fahrenheit) and 19° C (66.2° F). That said, they measured no difference in the activity of the flies’ sweet-sensing taste neurons, despite the change in behavior.
«Since the temperature is not directly affecting the sugar neurons, it must be affecting some other types of cells, which then indirectly affect the propensity to consume sugar,» Montell noted.
Fruit flies detect sugar with one type of taste neuron. Bitter is sensed by another type of neuron, and mechanosensory neurons detect the texture of food, such as hardness. However, temperature sensation is not quite as simple. Both bitter and mechanosensory neurons are also involved in detecting coolness. Only if both are activated does the brain interpret that as a cool signal.
All of these stimuli seem to reduce the animal’s desire to feed, explained Montell. Bitter compounds trigger bitter neurons, which tell the fly to stop feeding. Hard foods trigger the mechanosensory neurons, which also tell the fly to stop feeding. And cool temperatures trigger both, to the same effect.
Critical to this response is a protein called rhodopsin 6. Rhodopsins are most commonly associated with vision, but over the past few years the Montell group has connected rhodopsins to a variety of other senses. Indeed, just a couple weeks prior, Montell’s lab published the first study connecting different members of this class of protein to chemical taste.
«The bitter neurons express this rhodopsin called Rh6, and if you get rid of it, then cool temperatures no longer suppress the appeal of sugar,» he said.
Without Rh6, the bitter-and-cool-detecting neurons are no longer turned on by low temperatures. And since cool-sensation requires activating multiple, different types of neurons, loss of Rh6 prevents the fly from recognizing the lower temperature, thereby eliminating the decreased attraction to sugary food.
«The surprise was finding that it was really the other neurons, not the sugar neurons, whose activity went up,» Montell said, «and that the cool activation of other neurons was indirectly suppressing the sugar neurons.»
The sweet-sensing neurons are still activated by sugars at low temperatures; however, the activation of these other neurons by decreased temperature suppresses the communication between the sweet-detecting neurons and the animal’s brain. This is likely achieved by an inhibitory neurotransmitter released by the bitter/cool-activated neurons.
As for why fruit flies avoid food when it’s chilly, Montell suspects it’s due to their metabolism. Fruit flies’ metabolism, and thus food requirements, are affected by temperature. Lower temperatures mean slower metabolisms, and less need for food. And generally, if the food is cold, so is the fly.
In fact, the fly generation time — the time it takes an egg to turn into an adult fly — doubles from 10 days to 20 when the temperature is lowered from 25 to 18 degrees Celsius. «Everything is just slowed down,» Montell said, «and that’s why feeding is reduced. You don’t want to eat the same amount when your metabolism is slowed down.» This explanation doesn’t hold true for warm-blooded animals like humans, even if we show a similar behavior.
In the future, Montell and first author Qiaoran Li plan to further investigate the mechanosensory side of food appeal by looking at how particle size influences feeding behavior. As an example, he offers the distinct difference between fresh and refrozen ice cream. Despite having the same chemical composition and temperature, most people prefer ice cream that hasn’t melted and refrozen into a block.
Reflecting on the surprising finding, Montell remarked, «It’s great for your expectations to be wrong, as long as you can then figure out what’s right.»
The Sweetest Thing
A famous cola company launched a new product contained in a gleaming green can last year. As a regular cola drinker, I was intrigued by the packaging. After doing some research, I discovered that this variety of cola contains a sweetener called Stevia.
Figure 1. Stevia rebaudiana. Ethel Aardvark, Wikimedia
Stevia is extracted from a plant, Stevia rebaudiana, found in Brazil and Paraguay. The leaves of the Stevia plant have been used for hundreds of years in both countries to sweeten local teas and medicines. The sweet taste is mainly from steviol glycoside compounds, which have up to 150 times the sweetness of sugar, but zero calories .
The story of Stevia gave me, a protein database curator, the idea to search for the sweetest proteins to date. I found one such protein, thaumatin (IPR001938), produced by Thaumatococcus daniellii (also known as Katemfe), a shrub from West Africa. Thaumatin is around 2,000 times sweeter than sugar !
Similar to Stevia, Katemfe plants have been used by the locals for a long time; they use its leaves for wrapping food and its fruits for sweetening breads, palm wine and sour food. Their sweet proteins, thaumatin I and thaumatin II, were first identified in the 1970s in the search for non-toxic, non-calorific ‘natural’ sweeteners to replace synthetic ones .
Figure 2. Katemfe plant ~from Engler et al. Marantaceae, vol. 48: [Heft 11], p. 40, fig. 8 (1902).
Why do plants like Katemfe produce extremely sweet proteins? The answer may lie in the plant defence systems. Under environmental stresses or pathogen attack, plants can produce proteins that help them stay alive. In the case of Katemfe, attack by a viroid (a sub-viral pathogen) induces thaumatin production. Thaumatin has also been shown to have antifungal activities, which suggests it may be part of a defence mechanism that prevents further pathogen attacks .
In fact, thaumatin shares a conserved site (IPR017949) with a group of pathogenesis-related proteins, also known as thaumatin-like proteins (TLPs) , including tobacco salt-induced protein osmotin  and maize antifungal protein zeamatin . Like thaumatin, this group of proteins plays an essential part in plant defence against either environment stress or pathogen attack .
Another question is, why does thaumatin taste sweet to us? This is down to the sweetness receptors in our taste buds on our tongues. The sweet molecules (chemicals or proteins) are perceived by G-protein-coupled receptors, consisting of two subunits, T1R2 and T1R3. Certain amino acid residues in these subunits affect their ability to recognise the sweet molecules . Interestingly, apes and Old World monkeys can perceive thaumatin as a sweet protein, while New World monkeys and rodents cannot . In other words, the sweet taste of thaumatin for us humans could be just an evolutionary coincidence.
Figure 3. Sweet receptor, the peptide region involved in the response for thaumatin is shown in red .
So far, several chemical sweeteners have been commercialised, such as aspartame, sucralose and saccharin, and many more products may yet emerge. We all know that our sugar consumption causes health problems like obesity, diabetes and tooth decay. To avoid such health issues, scientists have searched far and wide to find alternatives. Natural sweeteners, such as Stevia and thaumatin, have provided new options for us. However, with so many different products on the market, as a consumer, I am still sitting on the fence to see which ones provide the best health benefits.
Figure 4. What should be in yours?
I. Katemfe: Katemfe is a 3-4 metre tall shrub from the rain forests of West Africa. It bears light purple flowers and a soft fruit containing shiny black seeds. The fruit is covered in a fleshy red aril, the part that contains thaumatin.
II. E numbers: Thaumatin has been approved by the European as a sweetener, known as E957. It is usually used in processed foods and has a slight licorice aftertaste.
III. Calories: Despite thaumatin containing 4 calories/gram (3.87 calories/gram for sucrose), the amount needed to be used in food or drink is extremely small, due to its high potency.
IV. Other sweet proteins: Besides thaumatin, there are a few other sweet proteins such as monellin (IPR015283), pentadin, mabinlin and brazzein [10,11].
V. Further reading: How did stevia get mainstream? -By Tom Heyden http://www.bbc.co.uk/news/magazine-22758059
- Cardello HM, Da Silva MA, Damasio MH., Measurement of the relative sweetness of stevia extract, aspartame and cyclamate/saccharin blend as compared to sucrose at different concentrations. Plant Foods Hum Nutr. 54(2):119-30., 1999. [PMID:10646559]
- Masuda T, Taguchi W, Sano A, Ohta K, Kitabatake N, Tani F., Five amino acid residues in cysteine-rich domain of human T1R3 were involved in the response for sweet-tasting protein, thaumatin. Biochimie. 95(7):1502-5., 2013. [PMID:23370115]
- van der Wel H, Loeve K., Isolation and characterization of thaumatin I and II, the sweet-tasting proteins from Thaumatococcus daniellii Benth. Eur J Biochem. 31(2):221-5., 1972. [PMID:4647176]
- Rodrigo I, Vera P, Frank R, Conejero V., Identification of the viroid-induced tomato pathogenesis-related (PR) protein P23 as the thaumatin-like tomato protein NP24 associated with osmotic stress. Plant Mol Biol. 16(5):931-4., 1991. [PMID:1859873]
- Liu JJ, Sturrock R, Ekramoddoullah AK., The superfamily of thaumatin-like proteins: its origin, evolution, and expression towards biological function. Plant Cell Rep. 29(5):419-36., 2010. [PMID:20204373]
- Subramanyam K, Arun M, Mariashibu TS, Theboral J, Rajesh M, Singh NK, Manickavasagam M, Ganapathi A., Overexpression of tobacco osmotin (Tbosm) in soybean conferred resistance to salinity stress and fungal infections. Planta. 236(6):1909-25., 2012. [PMID:22936305]
- Schimoler-O’Rourke R, Richardson M, Selitrennikoff CP., Zeamatin inhibits trypsin and alpha-amylase activities. Appl Environ Microbiol. 67(5):2365-6., 2001. [PMID:11319124]
- Monteiro S, Barakat M, Piçarra-Pereira MA, Teixeira AR, Ferreira RB. Osmotin and thaumatin from grape: a putative general defense mechanism against pathogenic fungi. Phytopathology. 93(12):1505-12, 2003. [PMID:18943614]
- Masuda T, Mikami B, Tani F., Atomic structure of recombinant thaumatin II reveals flexible conformations in two residues critical for sweetness and three consecutive glycine residues. Biochimie. 106:33-8, 2014. [PMID:25066915]
- Faus I, Recent developments in the characterization and biotechnological production of sweet-tasting proteins. Appl Microbiol Biotechnol. 53(2):145-51., 2000. [PMID:10709975]
- Masuda T, Kitabatake N. Developments in biotechnological production of sweet proteins. 102(5):375-89. J Biosci Bioeng. 2006. [PMID:17189164]
Do Dogs Have Taste Buds?
Our canine companions use their sense of taste in combination with their other senses to explore the world around them. Sometimes it seems like dogs will eat anything, from garbage and fecal matter to undigestible items like toys and fabric. And other times dogs may be very picky about their food.
So how do they determine what tastes good to them? Do dogs have taste buds like we do? Why do dogs want to eat things that we would never eat?
Do Dogs Have Taste Buds?
Yes, dogs have taste buds that give them the ability to taste things. Taste buds are found on papillae—small, visible bumps on the tongue. Dogs have about 1700 taste buds, while human mouths have approximately 9000.
Puppies develop their ability to taste after a few weeks of life. This is one of the earlier senses that develops, even before hearing and vision. As dogs mature in age, their number of taste buds decreases, along with a decreased sense of smell, which may play a role in picky eating or decreased appetite.
Each taste bud has an ability to sense all tastes if the flavor is strong enough. Taste buds in different areas on the tongue are slightly more sensitive to certain flavors in comparison to others. Bitter and sour taste buds are located toward the back of the tongue. Salty and sweet taste buds are found toward the front of the tongue.
Dogs have specific taste receptors that are fine-tuned to meats, fats, and meat-related chemicals due to their ancestral diet being primarily comprised of meat. The reduced number of taste buds in dogs as compared to humans may explain their decreased ability to distinguish between subtle flavors, like the differences between types of meat (chicken, pork, or beef) or different berries (strawberries, blackberries, raspberries, and blueberries).
Dogs also have taste buds that are fine-tuned to water. This ability is also seen in cats and other carnivores, but not in humans. Special taste buds on the tip of a dog’s tongue react to water as they drink and become more sensitive when thirsty or after eating a meal, which encourages them to drink more water.
Dogs’ Taste Buds vs. Their Sense of Smell
Taste is directly linked to smell, and an item’s scent can enhance its taste. The smell of a food item plays a much larger role in how dogs experience the flavor of their food.
Dogs also have a special scent organ along their palate that helps them “taste” through smell. When a dog smells something, they capture molecules that tell them how a food will taste. Dogs can taste without smelling, but not as well as people, due to fewer taste buds. However, their sense of smell is much more defined. They intuitively know when food isn’t safe for consumption by combining their senses of smell and taste.
Can Dogs Taste Spicy, Sweet, Sour, and Salty Food?
Dogs have receptors for the same taste types as humans, including spicy, sweet, sour, bitter, and salty foods. However, dogs never developed the highly tuned salt receptors that humans have. This is a result of their heavily meat-based ancestral diet being naturally high in salt. This meant they did not need to seek additional salt sources in their diet and have less of an affinity for salty foods.
Sweet flavors are especially preferred by dogs, which likely stems from their ancestral diet including wild fruits and vegetables. However, this does not mean that they should overindulge in pet-safe fruits and veggies. Too much sugar is detrimental for dogs, so sweet produce should be offered in moderation. Dogs should not have other sugary human foods.
What Tastes Bad to Dogs?
Dogs generally avoid salty, spicy, sour, or bitter tastes. Many of these may be unsafe to eat. The presence of toxins or spoilage from bacterial contamination will cause food to taste bad to dogs.
This is why many chew-deterrent sprays for dogs include bitter ingredients. Dogs may also reject many medications due to their bitter tastes.
The burning heat from spicy foods is caused by a compound called capsaicin and can cause physical reactions in dogs despite an inability to detect much of the flavor.
Featured Image: iStock.com/ti-ja
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