Ever looked at a chocolate fountain and wondered why the flowing chocolate slopes inward, instead of falling straight down? Adam Townsend and Dr. Helen Wilson from the University College London developed mathematical equations to explain this sweet, physical phenomenon. If wobbly desserts are more up your alley, take a look at the ingredients list for Jell-O. You may be interested to know that Jell-O contains cowhide.
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Photo credit: Flickr/louish
Hot chocolate: it’s a winter staple. Amidst falling temperatures and dreary skies, there’s nothing quite like taking a swig of this sumptuous beverage and seeking warm refuge in the delights of a steaming mug. Hot chocolate is as straightforward as drinks go: at its core, it’s milk, cocoa powder, and sugar. Despite its simplicity, this cold-weather classic is swirling with science.
The backbone of any decent hot chocolate is milk. Beyond water, milk is perhaps the most basic and familiar substance to humans. We’re all born drinking some form of it, but how often do we stop and think about its underlying science? Milk is an emulsion, which is a mixture of two immiscible liquids—in this case, water and fat. The water-based component of milk is loaded with vitamins, minerals, and protein and contains immiscible fat globules suspended throughout. How do water and fat coexist peacefully in solution together? The answer lies in emulsifiers, which are molecules that are both water- and fat-soluble. Milk contains proteins, namely casein, that attract and unite the fluids that would otherwise separate. Rich, silky, and chemically intriguing, this dairy product serves as the perfect vehicle for chocolate (1).
Photo credit: Flickr/chocolatereviews
Chocolate serves as the heart of the beverage. Some recipes call for it in the form of cocoa powder. Cocoa powder mixed in with your milk is a colloid—a type of mixture in which solid particles are dispersed throughout a fluid. Another popular culinary colloid you may recognize is coffee, which contains small coffee particles dispersed in water.
Photo credit: Flickr/csb13
A glass of hot chocolate simply isn’t complete with a dollop of whipped cream plopped on top. Lauded for its decadent mouthfeel, cream is an emulsion of butterfat and water, similar to milk but with a higher fat content. Fresh milk left undisturbed will separate into two layers; the top becomes enriched with fat globules that can be skimmed off as cream, leaving behind a relatively fat-free layer—skim milk. Cream and milk have remarkably different fat contents, as cream is required to have at least 30% milk fat compared to whole milk which is a mere 3%.
With some simple agitation, willpower, and a whisk, we can transform heavy cream into whipped cream, a culinary foam. Similar to emulsions, foams combine two immiscible substances, but instead of water and fat, air or gas is entrapped within a fluid or solid. Whisking incorporates air into the cream, and the newly introduced bubbles are held captive by the structure of the foam. Fluids and gases have very different properties, so how does agitation keep them together? Agitation disorients the fat globules and strips away their protective membranes, forcing them to cling to other fat molecules or aggregate around air bubbles—anything to avoid having to be in contact with water. Agitate your cream enough and you’ll wind up with stiff peaks when these fat-encapsulated air bubbles begin to form a stable network (2).
Photo credit: Flickr/knitsteel
Whether they’re being roasted over a campfire or floating lazily on the surface of your hot chocolate, marshmallows are a surefire way to please and are another way to enhance your chocolate-drinking experience. Marshmallows were originally made as a meringue (yet another culinary foam!) consisting of whipped eggs and sugar flavored with the juice from roots of the marsh mallow plant. The making of marshmallows has since evolved so that now they are created by aerating a mixture of simple sugar syrup and gelatin to form a foam that stabilizes once the gelatin sets. Whipping incorporates air bubbles that are trapped in the solid matrix, forming these springy and sugary confections that pair exceptionally well with chocolate (1).
Hot chocolate is the ultimate winter beverage. It’s creamy, decadent and versatile. Drink it plain or spice it up with some chili powder, orange, or peppermint and you’ll surely find a style that will leave you positively foaming at the mouth.
References cited
About the author: Mai Nguyen is an aspiring food scientist who received her B.S. in biochemistry from the University of Virginia.
In the town of Reading, located in Berkshire, England, exists the International Cocoa Quarantine Centre, where tropical cacao plants are kept to prevent the spread of pests and diseases which threaten the world’s chocolate supply. Over at Technische Universität München, physicists have shown that molecular simulations can solve how the chocolate-making process turns bitter cacao to sweet, silky chocolate on a molecular level.
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Is there anything more disappointing than finding a chocolate bar in the back of the desk drawer, anticipating a tasty treat, then unwrapping the bar only to find a dull, grey haze has overtaken your dear candy? Seeing as bloomed chocolate is still edible, yes, there are many things more disappointing than that. But surely you’re curious about how chocolate that was once shiny and perfect came to be filmy and rough. Chocolate blooming, the process that produces the white-grey film that appears on the surface of an old chocolate, is due to molecular migration. More specifically, this imperfection is caused by the movement of fats to the surface of the chocolate followed by a subsequent recrystallization. In a paper published by Applied Materials & Interfaces, a team of researchers dedicated to keeping our chocolates blemish-free has clarified the precise mechanisms that cause chocolate blooming.
The main fat in chocolate is cocoa butter, which is solid at room temperature and melts at 37 degrees Celsius. The proportion of solid to liquid cocoa butter depends on the lipid composition, which depends on which specific triglycerides are present. The solid to liquid proportion also varies with the storage conditions of the chocolate.
As proposed by Aguilera et al, scientists who study this chocolate blooming, consider chocolate as a particulate medium of fat-coated particles such as cocoa solids, sucrose, and milk powder, all suspended in a fat phase with the aid of an emulsifier, which helps to mix fats and oils with water, which usually repel each other. There are six crystallographic polymorphs of cocoa butter molecules, that is, there are six ways the molecules can organize themselves. The structural stability of these polymorphs increases from 1- 6; form 1 is the best at forming solid butter at room temperature, while form 6 tends to arrange in the loose bonds of a liquid. Form 5 is the main form in chocolate, as it possesses the most aesthetically desirable properties. While the phenomenon of blooming is well known to result from melting and recrystallization of chocolate into a less desirable polymorph, it has been unclear how fat moves through the chocolate particle network: Does it move along the fat-particle interface? Does it diffuse through the fat phase (cocoa butter), or through the matrix of assorted particles?
Possible lipid migration pathways in chocolate – Reinke et al
In this experiment, researchers used synchrotron microfocus small-angle X-ray scattering to determine the preferential migration pathway of the cocoa butter molecules surrounded by three different soild components (cocoa solids, skim milk, and sucrose). This technique allows researchers to record the scattering of x-rays through a sample with defects in the nanometer range. They can then extrapolate information about the material’s macromolecules, their shapes and sizes up to 125 nanometers, and distances between partially ordered materials, such as pore sizes. For this experiment, this method is better than more traditional macroscopic techniques as the sample does not need to be dissected in order to examine it, therefore the same sample can be continually analyzed.
Sketch of the experimental setup – Reink et al
The researchers prepared and tempered four different chocolate samples. An initial scattering of x-rays and data collection was performed before the addition of sunflower oil, then 10 uL of oil was pipetted onto the chocolate surface, and a second scan was performed. Images of the droplet were captured through a high-speed camera. These scans were repeated at 5, 10, and 30 minutes after oil addition, and again after 1, 2, 5, and 24 hours.
The results obtained suggest that oil is migrating through pores and cracks in the solid structure driven by capillarity within seconds. This means that the oil can flow in narrow spaces in opposition to gravity. Then chemical migration through the fat phase occurs. The oil doesn’t traverse the fat-particle interface, nor does it move through the matrix of solid particles. This migration disrupts the crystalline cocoa butter, which induces softening.
Because the most immediate migration of oils occurs through the material porous structure, the formation of chocolate bloom could be prevented by minimizing pores and defects in the chocolate matrix. To prevent the longer-term effects of chemical migration of lipids, one must minimize the content of non-crystallized liquid cocoa butter. Tempering chocolate lends to crystalline structures that resist migration, as will reducing the liquid fat content. However, to ensure that you never encounter a sad hazy chocolate again, we recommend eating all chocolate goods expeditiously.
Works Cited
About the author: Elsbeth Sites received 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.
Guest post by Christina Jayson
Photo credit: Lisa Townley (left); Pyogenes Gruffer (right), Flickr.
When my organic chemistry professor told me that the main molecular component of chocolate, theobromine, differs from caffeine only by the absence of one methyl group I was delighted: I could skip an entire step in caffeine metabolism, avoid the bitter taste of coffee, and increase my chocolate consumption. It seemed to make sense that as the caffeine I drank was metabolized by removing the methyl group, caffeine would convert to theobromine (the main compound of chocolate) (Figure 1). At the molecular level, a methyl group is a carbon with three hydrogens attached. It may seem simple, but a methyl group is an integral part of chemistry, biology, and biochemistry. For example, additional methyl groups can help a molecule to cross the blood-brain barrier and enter our brain – this barrier protects our brain from foreign molecules traveling in the blood that can be harmful [1, 2]. In the case of caffeine, it turns out that the extra methyl group on the molecule is what makes coffee active on our central nervous systems and an “energy stimulator,” while chocolate functions as a sweet treat and smooth muscle stimulator.
![Figure 1: During the metabolism of caffeine in the body, the methyl group (highlighted by the yellow box) is removed from caffeine and it is converted to theobromine (Modified from Wolf LK, 2013) [9].](https://scienceandfooducla.files.wordpress.com/2015/09/2.jpg)
Figure 1: During the metabolism of caffeine in the body, the methyl group (highlighted by the yellow box) is removed from caffeine and it is converted to theobromine (Modified from Wolf LK, 2013) [9].
So how do these two molecules act on different parts of the body, making coffee the substance of choice over chocolate bars when midterm season hits?
Caffeine is mostly derived from Coffea Arabica, or coffee beans, and seeds [3]. It is predominantly a central nervous stimulant, though it also stimulates cardiac and skeletal muscles and relaxes smooth muscles. Chocolate, or theobromine, is found in products of Theobroma cacao, or cocoa plant seeds (Figure 2). Much like caffeine, theobromine is a diuretic; however it mainly acts as a smooth muscle relaxant and cardiac stimulant [3]. While these two compounds have similar effects, the key difference is that caffeine has an effect on the central nervous system and theobromine most significantly affects smooth muscle [4]. In behavioral studies, caffeine intake improves self-reported alertness and mood over a period of 24 hours [5]. Theobromine produces mild positive effects in pleasure, but does not affect attention or alertness in moderate doses compared to caffeine [6].
Figure 2: Chocolate (left) is made from Theobroma cacao, or cacao plant seeds and contains theobromine (PC: Nic Charalambous). Coffee (right) is made from Coffea Arabica, or coffee beans, and seeds and contains caffeine (Photo credit: JIhopgood/Flickr).
But the true difference in the compounds lies at the molecular level. Both caffeine and theobromine belong to the methylxanthine chemical family. These chemicals act as stimulants of the nervous system, most notably by binding to adenosine receptors in the brain and thereby blocking adenosine from binding to the receptors [7]. Adenosine binding to adenosine receptors normally reduces neural activity, so the antagonistic action of caffeine and theobromine prevents this activity reduction (Figure 3). The increased energy and alertness that we connect to massive coffee consumption is due to the caffeine preventing your body from responding to signals that tell it to slow down or de-stimulate. Ever felt your hands jitter uncontrollably after too many shots of espresso?
![Figure 3: Caffeine molecules (C) compete with adenosine molecules (A) to bind to the adenosine receptors in the brain (Schardt, 2012) [10].](https://scienceandfooducla.files.wordpress.com/2015/09/4.jpg)
Figure 3: Caffeine molecules (C) compete with adenosine molecules (A) to bind to the adenosine receptors in the brain (Schardt, 2012) [10].
Experiments show the activity of caffeine on the nervous system is stronger than theobromine [7]. Caffeine and theobromine compete with adenosine to bind to the same adenosine receptor. Studies have shown that caffeine molecules are better able to compete with adenosine to bind adenosine receptors than theobromine – caffeine binds these receptors with two to three times higher affinity than theobromine [8].
To gain access to the different locations of the adenosine receptors throughout the body, the extra methyl group on caffeine ends up coming in handy. Because caffeine has three methyl groups instead of two like theobromine, it more easily crosses the blood-brain barrier. In crossing the blood-brain barrier, caffeine can act on the central nervous system. So while theobromine can act as a heart stimulant and smooth muscle relaxant, caffeine – boasting its extra methyl group – has access to the neurons of the central nervous system and can consequently enhance physical performance and increase alertness.
![Photo credit: Chris Swift, Rogers Family Co [11]](https://scienceandfooducla.files.wordpress.com/2015/09/5.jpg)
Photo credit: Chris Swift, Rogers Family Co [11]
References cited
Christina Jayson is a recent UCLA Biochemistry graduate and currently a Ph.D. student in the Biological and Biomedical Sciences program at Harvard.
Our very own Amy Rowat explains how to use science to bake a better pie, and geneticists look at the DNA of cacao beans to breed better chocolate beans.
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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.
Ed Yong investigates the parasitic threat to Ghana’s cocoa trees, while SciShow lays down some tasty scientific facts about chocolate. Read more
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.
If you’ve ever wondered what 200 Calories look like on a plate, wiseGEEK has just the photo gallery for you! Meanwhile, scientists create a healthier chocolate by replacing fat with fruit juice. Read more