At our 2014 Science of Sushi event, Dr. Ole Mouritsen and Chef Morihiro Onodera illuminated the science underlying some of our favorite components of sushi. In case you still haven’t had your fill, here are 10 scientific facts related to sushi: Read more
(Steve Evans/Flickr)
Nothing says “summer” quite like a big, juicy slice of watermelon. Even if you prefer it charred on the grill or blended into an icy agua fresca, watermelon is one of the best ways to beat the late-summer heat.
So what gives watermelon its refreshingly delicate flavor?
Turns out the answer is pretty complicated. Over the last few decades, scientists have identified dozens of flavor and aroma molecules that contribute to watermelon’s unique taste [1].
And here’s an interesting twist: a watermelon’s flavor has a lot to do with its color. Chow down on a yellow ‘Early Moonbeam,’ a pale ‘Cream of Saskatchewan,’ or a deep red ‘Crimson Sweet’ and you’ll likely notice different flavor profiles for each melon.
These watermelons don’t just look different, they taste different, too! (David MacTavish/Hutchinson Farm)
Several of watermelon’s flavor molecules form when colorful chemicals called carotenoids break down into smaller chemical compounds [2,3].
For example, the classic color of red watermelons comes from lycopene, the same molecule responsible for the color of red tomatoes. When lycopene breaks down, it forms key flavor compounds such as lemon-scented citral.
Orange melons don’t have much lycopene, but they make up for it with extra beta-carotene. This chemical – the same one that makes carrots orange – leads to a completely different set of flavor molecules, including floral beta-ionone.
 |
| Colorful molecules called carotenoids break down into different flavor compounds. Figure adapted from [2]. |
The chemistry of watermelon flavor is clearly complex, but scientists are still searching for individual molecules that mimic watermelon’s characteristic taste.
Most recently, a study identified a single molecule – dubbed “watermelon aldehyde” – that has a very distinct watermelon aroma [4]. Unfortunately (or fortunately, depending on your perspective), the molecule is too unstable to be used as a food additive. So for now, artificially flavored “watermelon” products will just have to keep on tasting nothing like watermelon.
Good thing there’s plenty of real, chemically complex watermelon to go around.
References
About the author: Liz Roth-Johnson received her Ph.D. in Molecular Biology at UCLA. If she’s not in the lab, you can usually find her experimenting in the kitchen.
photo credits (sake puppets/flickr)
Whether or not you like eating sashimi, such a fine specimen of fish is undeniably an incredibly beautiful food. The subtle flavors, delicate texture and vivid colors make sushi and sashimi such a unique eating experience. To whet your appetite for The Science of Sushi at UCLA, here are some bits of sashimi science we learned from Ole G. Mouritsen’s book, Sushi: Food for the Eye, the Body, and the Soul.
Sashimi 刺身
Salmon and Tuna Sashimi – Photo Credits: (avlxyz/flickr)
Why are fish muscles soft?
If you used your finger to poke a raw filet of a bony fish like salmon or tuna, then tried this on meat from a terrestrial animal like beef or pork, you would notice that fish muscle is significantly softer than terrestrial meat. On a very fresh piece of fish, you could poke your finger through the muscle. From a basic understanding of meat texture, it seems strange that the meat of a fast-swimming predator is soft while the flesh of a slow-moving grazer is firm; typically the more an animal uses its muscles, the tougher its muscles.
Yet fish tend to have the same density as the water in which they live, so they do not use their muscles to bear their own weight; fish need only to exert their muscles when they want to move. By contrast, terrestrial animals frequently use their muscles to counter gravity and remain upright. Fish simply have less work to do, and so their muscles do not develop the same chewy texture that land animals do. But not all fish have smooth and tender muscles; some species like shark have tougher meat. Why? Sharks’ bodies happen to have a specific gravity greater than the water they inhabit, so they must exert their muscles at all times to keep afloat, and thus their muscles more closely resemble a ruminant’s in firmness.
Fresh is best
About six hours after the fish is killed a phenomenon common to all animals, rigor mortis, sets in. During rigor mortis calcium ions of the proteins embedded in the muscle fibers are released, causing the muscle fibers to contract and become stiff.
To delay rigor mortis for up to a few days, fish can be deep-frozen immediately after they are caught. Once the process of rigor mortis has run its course, the fish begins to decompose, the muscle fibers separate, and the connective tissue loosens. At this point it is ideal to consume the fish, as it is at its peak of softness and freshness. This type of sushi is called nojime, the type made from fish that are not kept alive after being caught. The opposite is ikijime sushi, prepared from fish with firmer muscles as they are kept alive until the last moment and prepared before rigor mortis can set in.
A rainbow of fish
Orange, pink, red, white; there is remarkable variation in fish meat color. Photo Credit: Kake Pugh (kake/flickr)
Orange and pink
The muscles of wild salmon and sea trout are typically orange-pink in color. The origins of this distinct shade of salmon begins at the bottom of food chain, with plankton. These little organisms contain a pigment astaxanthin. It belongs to the family of pigments called carotenoids, which includes the pigment that makes carrots orange. Tiny crustaceans eat plankton, and thus ingest astaxanthin, whereupon it is bound to proteins called crustacyanins in the animals’ tough shell. While bound to these proteins, the pigment is blue-green or a dark red-brown. This will seem familiar if you have ever seen live crab or lobster. When a fish comes along and eats the crustacean, the crustacyanins are denatured and they release the pigment, allowing its own red-orange color to become visible. The color change that occurs upon cooking crustacean shells is caused by the same protein-denaturation and pigment-release process that occurs in fishes’ digestion systems.
Red Fish
Although the proteins that form the muscles themselves are colorless, a lot of fish meat is deep red, like tuna. These colored muscles are classified as slow muscles, as they take care of work that has to be carried out on an on-going basis, namely, continuous swimming. Since they require a continuous oxygen supply to produce energy, they contain myoglobin. Myoglobin is responsible for the transport of oxygen within muscle tissues. Each myoglobin molecule can bind one oxygen molecule to form oxy-myoglobin, which is bright red.
White fish
In contrast to slow muscles, fast muscles undertake smaller and more rapid movements like the slapping of fins and tail. These muscles do not contain myoglobin; instead they use the colorless starch glycogen to supply energy. No myoglobin means that these muscles stay colorless or white.
Interested in learning more sushi science from the experts? UCLA Science & Food’s public lecture, The Science of Sushi, is on April 23rd. In this lecture, Dr. Ole Mouritsen will illuminate the science underlying sashimi, nori, sushi rice, umami, and more. He will be joined by Chef Morihiro Onodera who will share his approach to sushi as well as an inside look into his partnership with a rice farm in Uruguay.
References:
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.
Photo credit: Jun OHWADA (しそ山葵) (june29/Flickr)
Wasabi packs quite a punch, but where exactly does that wallop of heat come from? That green dollop that accompanies sushi comes from the wasabi plant, also known as Japanese horseradish, which is not to be confused with its distant cousin, the more common and well-known European horseradish (Armoracia rusticana). As a member of the Cruciferae family, wasabi is actually more closely related to cabbage, cauliflower, broccoli, and mustard [1][2].
Grown primarily in Japan, the wild-type species (Wasabia tenuis) are only found mountainside in streambeds and river sand bars [2]. Cultivated wasabi plants (W. japonica), similar to the wild-type variety, comprise a cluster of long-stemmed heart-shaped leaves and delicate, spring-blooming, white flowers branching from a gnarled, thick, root-like stem known as a rhizome [3]. Wasabi grown under semi-aquatic conditions are called sawa, while those grown in fields are called oka [1][3]. Sawa is considered higher quality, as they produce larger rhizomes, thereby often cultivated for culinary purposes. Oka is largely cultivated for nutraceutical purposes, such as herbal supplements [3].
Wasabi leaves and rhizomes. Photo credit: Jaden (Steamy Kitchen)
The plants are notoriously difficult to cultivate, as they thrive best in running water [4]. Even under ideal conditions, wasabi is difficult to farm, especially on large-scale operations for commercial purposes. As such, real wasabi is expensive and rare outside of Japan. Due to the taste similarities between wasabi and horseradish, common wasabi substitutes are usually a mixture of horseradish, mustard, starch, and green food coloring. So how does one differentiate between real and imitation wasabi? Simply taste it.
Real wasabi is made by grating the wasabi rhizome into a fine powder. Due to the high volatility of the flavor compounds, after grating the rhizome, the heat will only last for, at most, fifteen minutes, whereas horseradish-based wasabi can be left overnight and still retain its heat [1]. Additionally, though the chemical makeup of horseradish and wasabi may be similar, it is different enough that each has a unique flavor profile. Both horseradish and wasabi rhizomes contain thioglucosides, a sugar glucose with sulfur-containing organic compounds. Maceration of the rhizome, such as by grating, breaks the cell walls and releases these thioglucosides, as well as an enzyme known as myrosinase [1]. Myrosinase is responsible for breaking the thioglucosides into glucose and a complex mixture of a class of compounds called isothiocyanates. Horseradish and wasabi contain varying isothiocyanate amounts and compositions. There are 1.9g total isothiocyanates per kilogram of horseradish, as opposed to 2.1g/kg in wasabi. The most abundant and stable of these compounds, allyl isothiocyanate, gives real and imitation wasabi its infamous pungency [1][4]. The next most abundant isothiocyanate compound is 2-phenylethyl isothiocyanate, which is only found in horseradish [1]. All other types of isothiocyanates exist in higher concentrations in wasabi than horseradish.
Allyl isothiocyanate produces a hotness in wasabi that is distinct from the spiciness of hot peppers. Hot peppers contain capsaicin, an oil-based molecule which stimulates the tongue. This spiciness can only be washed away with foods containing oils or fats, such as dairy products. Unlike capsaicin, allyl isothiocyanate vapors stimulate the nasal passages. Fortunately for heat-seekers, the amount of pain is directly related to the amount of wasabi consumed, and a little will go a long way. Fortunately for mild-lovers, because allyl isothiocyanate is not oil-based, the burning can easily be cleansed by consuming more of any food or liquid. Although real wasabi is expensive and only found at specialty stores or prepared to order at high-end restaurants, that sinus-opening sharpness is worth experiencing, even if only once.
References Cited
About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.
“There’s so much great food yet to discover that we can grow, so I just love discovering new varieties, crops, things that our customers and myself have never tried before.”
– Alex Weiser, 2013 Science & Food course Read more
Photo credit: Jesús Rodriguez (hezoos/Flickr)
Chocolate-covered strawberries have an innate beauty in their simplicity, making this snack both sweet and decadent. But this gourmet treat does not have to be expensive nor only savored at special events. Although it’s not quite as simple as dipping strawberries into soupy chocolate sauce, you can easily make chocolate-covered strawberries in your very own kitchen with a basket of strawberries, a bag of chocolate, and a little patience.
To perfect the crafting of chocolate-covered strawberries, it helps to first consider the composition of chocolate. Chocolate contains only a few ingredients: fat, sugars, proteins, and soy lecithin as emulsifier that holds everything together [1,2]. Cocoa butter, a fat that is derived from cocoa beans, makes up the majority of chocolate. Like many vegetable fats, cocoa butter is a mixture of fatty molecules called triacylglycerols. Different types of triacylglycerols—saturated, monounsaturated, polyunsaturated—have their own thermal and structural properties. Roughly 80% of cocoa butter are monounsaturated triacylglycerols [3]. The secret to chocolate perfection lies in the microscopic arrangement of these molecules. The texture (smooth vs. lumpy), appearance (glossy vs. dull), and melting temperature of chocolate (in your mouth at 98°F vs. in your hand at 82°F) all depend on how triacylglycerols pack together in the finished chocolate product.
Triacylglycerols are elongated, spindly molecules that can be packed together in different ways, sort of like long, skinny Legos. The three main ways that triacylglycerols can pack together are named α, β’, and β [3]. A pure mixture of triacylglycerols will form the most stable structure, β [4], and quality chocolate that is hard, smooth, and shiny will predominantly contain this β structure. Unfortunately, cocoa butter isn’t purely one type of triacylglycerol: while the 80% monounsaturated triacylglycerols will tend to pack together nicely into perfect β structures, the other 20% of cocoa butter fat molecules can interfere and lead to less stable α or β′ structures. As shown in Table 1, chocolate can take on different combinations of α, β′, and β structures, categorized in order of increasing stability as crystals I-VI [2,3]. Crystal V possesses only the β structure, and so it boasts the most desirable chocolate characteristics, such as good sheen, satisfying snap, and melt-in-your-mouth smoothness.
Table 1. Properties of chocolate crystals (adapted from [2]).
| Crystal | Structure | Melting Temp (°F) | Chocolate Characteristics |
| I | β′sub(α) | 63 | Dull, soft, crumbly, melts too easily |
| II | α | 70 | Dull, soft, crumbly, melts too easily |
| III | β′2 | 79 | Dull, firm, poor snap, melts too easily |
| IV | β′1 | 82 | Dull, firm, poor snap, melts too easily |
| V | β2 | 93 | Glossy, firm, best snap, melts near body temp |
| VI | β1 | 97 | Hard, takes weeks to form |
Unfortunately, getting chocolate to form the desired crystal type is easier said than done. When chocolate is melted and then left alone to re-harden on its own terms, uncontrolled crystallization occurs: any and all of the six crystal types will form at random. Chocolate that has been allowed to set this way ends up clumpy and chalky. To control crystallization and select for crystal V, the chocolate must be tempered. Through the tempering process, chocolate is first heated to 110-130°F to melt all the different crystal types. Most importantly, the temperature has to be higher than 82°F to melt the inferior crystals I-IV. Melted chocolate is then cooled down by adding “seeds” of chocolate that already contain only crystal V. These seeds are usually just pieces of chocolate that has already been tempered. Any piece of chocolate—chips, buttons, or chopped— can be used, as the majority of chocolate on the market has already been tempered. These seeds slowly cool the melted chocolate and act as a molecular template from which additional crystal V structures can grow [3]. As the chocolate cools, the stable crystal V will come together into a dense, even network, creating that lustrous, firm chocolate coating.
But beware: a drop of water can ruin all that hard work and perfectly tempered chocolate by causing it to seize. During the manufacturing process, water is removed from the chocolate, leaving behind a blend of fats and sugars. Introducing water to melted chocolate causes the sugar molecules to clump together in a process known as seizing [1]. These wet, sticky sugar clusters result in a grainy, thick batch of chocolate.
Seizing can happen when chocolate is melted in a double boiler, as water from the steam can get into the chocolate. It can also happen when pockets of chocolate are accidentally burnt. Burning is a chemical reaction that oxidizes the fats and sugars to produce carbon dioxide and water. Water that forms in the burnt pockets of chocolate will cause the rest of the batch to seize. But have no fear! Seized chocolate is not completely ruined: it can be saved by adding even more water or other liquids such as cream. Though it may seem counterintuitive, adding more water actually dissolves the sugar clumps, breaking them apart so that the chocolate can become smooth and creamy again [1]. Unfortunately, because there is now moisture in the chocolate, it will not dry and harden into a chocolate shell anymore. Chocolate rescued in this way can be used for hot chocolate, icings, fillings, or ganaches, which means you can still make an impressive chocolate treat even if the chocolate-covered strawberries don’t work out.
Chocolate-Covered Strawberries
1 lb. strawberries
16oz milk chocolate chips
Thermometer (optional, but would be helpful)
1. Melt half to two-thirds of the chocolate chips…
…In a double boiler: Stir constantly. Be sure steam doesn’t escape and sink into the chocolate. Do not cover.
…In the microwave: Heat on high 1 minute. Do not cover. Remove from the microwave and stir. If all the chocolate has not melted, heat again for 5-10 seconds. Repeat until completely melted
Note: If possible, avoid using a heat-retaining container like glass, which may burn the chocolate. Plastic is preferred.
2. Once completely melted, carefully continue heating until the temperature is 90-95°F.
3. Remove from heat, then add chocolate chips. Stir until the chips have melted and the chocolate is 82-88°F.
4. To test if the chocolate is ready, spread a thin layer on the back of a spoon or a piece of paper. It should harden in less than 3 minutes. If it doesn’t, stir in more chocolate chips.
5. When the chocolate is ready, carefully dip in strawberries. Make sure the strawberries are dry, before dipping. Allow dipped strawberries to dry on a sheet of parchment paper.
References Cited
About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.
Photo credit: Eli Duke (eliduke/Flickr)
There are few things sweeter in life than chocolate, which is probably why it’s one of the most popular flavors in the world. We can thank the cacao trees (Theobroma cacao) for this gift, which are only grown within a region known as the Cocoa Belt, 10° to 20° north and south of the equator [1]. Chocolate is produced from the seeds of the pods that grow from the cacao trees; these seeds are better known as cocoa beans.
Chocolate is a complex flavor, containing over 200 different flavor compounds [3]. While the type and mixture of cocoa beans that go into a chocolate bar play a role in determining the final flavor, chocolate is the kind of food where its taste is influenced by how it’s made rather than what it’s made of [4]. The chocolate-making process varies among types of chocolate (milk, dark, bittersweet, etc.), but also depends on the style of the chocolate maker. So while the general principles and chemical processes at each step remain the same, chocolate-making is a delicious art form.
Straight off the trees, cocoa beans are bitter. When cacao pods are harvested, they are cracked open and left to sit for a couple of days, depending on the tree varietal. (5–6 days for forastero versus 1-3 days for criollo [2].) This allows the cocoa beans to undergo fermentation, a process that is carried out by naturally occurring yeast and bacteria. During fermentation, the microorganisms digest the pulp in the pods, which aids in converting the sugars in cocoa beans into acids. These acids decrease the overall bitterness of the beans. Notable flavor compounds, such as pyrazines, are also generated during fermentation, making the beans slightly more floral in aroma [2]. After fermentation, the beans are scraped from the pods to dry. Drying releases certain molecules from the beans that would otherwise make chocolate taste smoky and sour [2].
Roasted cocoa beans. Photo credit: AnubisAbyss/Flickr
The dried cocoa beans now taste nutty, bitter, and acidic; to drive out volatile (easily evaporating) acidic molecules, the dried beans are further processed by roasting. The elevated temperatures of roasting (120–150°C) also facilitate Maillard reactions that yield flavor molecules that are distinct to chocolate [2]. These reactions are sensitive to both temperature and pH, so both the roasting temperature and bean acidity contribute to the final composition of flavor molecules that form during these Maillard reactions. Typically, milk and certain dark chocolates are made from beans that have been roasted at lower temperatures [2]. The shells of roasted beans are then removed, leaving behind pieces called cocoa nibs. Depending on the chocolate-maker, cocoa nibs may undergo alkalization, whereby they are treated with an alkaline solution in order to further decrease their acidity. Alkalization also causes flavonoids to polymerize (link together), which reduces the astringency of the nibs [2].
The final phase in chocolate manufacturing is a two-step process known as conching. At this stage, the nibs have a gritty texture; the first step in conching turns this into a paste through grinding and heating. Acidic compounds and water are evaporated in this process. More importantly, many flavor compounds formed during fermentation and roasting that are responsible for astringent and acidic notes become oxidized during conching, which mellows the flavor of the final product [2]. In the second step, cocoa butter and soy lecithin are added, decreasing the viscosity of the chocolate mixture to make it flow more easily.
Cocoa beans go through quite a long journey, from the cacao tree to the candy wrapper, where each step plays a role in producing the final combination of flavor molecules that makes chocolate such a beloved treat. This is just one of many reasons to savor your next taste of chocolate.
References Cited
About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.
Photo credit: Chris Battaglia (photog63/Flickr)
Hazelnuts may not be as popular as other nuts in the U.S., but they have quite the culinary versatility, enjoyed in pralines, Nutella, and even as themselves. These nuts grow on hazel trees, of the genus Corylus. Depending on the plant species and nut shape, hazelnut also refers to the filbert nut or cobnut. Filbert nuts have an elongated shape that tapers into a “beak”, and are found on the Filbert (C. maxima), Colchican Filbert (C. colchica), and Turkish Hazel (C. colurna). Cobnuts are generally rounder, and grow on the American Hazelnut (C. americana) and the more commercially recognized Common Hazel (C. avellana) [1].
Whether in the form of a nut, essence, or oil, hazelnuts owe their sweet, buttery flavor profile to the molecule filbertone. Interestingly, filbertone can be used to test for the authenticity of olive oil. Olive oils are sometimes cheapened by mixing in hazelnut oil [2]. As filbertone is one of the components of hazelnut oil, testing for its presence can determine whether or not a sample of olive oil is impure [3]. Although hazelnut oil is less expensive compared to olive oil, it has a strong, robust flavor that makes it a great substitute in salad dressings and baked goods.
Like many nuts, hazelnuts are a good source of protein and monounsaturated fats. Further, they contain a significant amount of thiamine, various B vitamins, and especially vitamin E [4]. Need another reason to try out hazelnuts this month? The warm, rich, velvety taste of roasted hazelnuts in decadent truffles or comforting lattes has a way of slowing down time. Try it for yourself.
References Cited
About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.
The holidays are the perfect time for some festive science fun. To help you get into the holiday spirit, here are a few chemistry-themed party tricks and a science-powered holiday cocktail. Have a wonderful holiday season and a happy new year! Read more
Photo credit: Lisa Bunchofpants (bunchofpants/Flickr)
Seasonal winter treats somehow seem incomplete unless they are imbued with a frosty, peppermint flavor. This is easily accomplished by enhancing the recipe with peppermint oil or peppermint extract, cultivated from the leaves of the peppermint plant. This plant’s scientific name is Mentha x piperita, the “x” indicating that it is a hybrid mint, formed by crossing watermint (Mentha aquatica) and spearmint (Mentha spicata). As a hybrid plant, peppermint is sterile, unable to produce seeds. Instead, it reproduces via rhizomes, bulbous plant masses found underground that are very similar to ginger and turmeric roots. Like many rhizomes, peppermint rhizomes can be planted almost anywhere, growing quickly once sprouted. For this reason, the peppermint plant is listed as invasive in Australia, the Galapagos Islands, New Zealand, and the Great Lakes region of the U.S. [1].
Peppermint oils and extracts get their characteristic Christmas-in-your-mouth flavor from their two main constituents, menthol and menthone. Of the two, menthol may be the more recognizable: When ingested, applied topically, or inhaled, menthol triggers cold-response sensory receptors, which cause that familiar cooling sensation [2]. You may have experienced this from chewing minty gums, using toothpaste, or applying Bengay to sore muscles.
Menthone is structurally related to menthol, but it affects a different sense in peppermint-flavored treats. This molecule gives rise to the icy, minty scent reminiscent of evergreen winters. Its distinctive fragrant property makes it popular in perfumes, cosmetics, and scented oils.
If you indulge in something peppermint this month, take some time to appreciate the menthol and menthone that makes this essence a holiday classic. Feel the sharp chill in your mouth while you bask in the warmth of a heated room. Take in the scent of cool mint while the winter wind outside whirls away. ‘Tis the season.
References Cited
About the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.