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.
Ingredients to make Greek salad dressing. Photo credit: Julle Magro (magro-family/Flickr)
Homemade vinaigrettes are about as easy as they look: mix oil, vinegar, and spices; shake before pouring. For those who want vinaigrettes without the inelegant step of shaking before serving, the solution is simple; add an emulsifier.
Understanding the role of an emulsifier first requires some familiarity with the primary components in vinaigrette, vinegar and oil. Vinegar is composed of acetic acid and water, which are polar compounds. In a polar molecule, one or a group of atoms have a stronger pull on the electrons in the molecule. Due to this uneven share of electrons between the atoms, weak charges form on opposite ends of the molecule [Figure 1a]. The weakly positive and negative charges on the polar molecule are called dipoles. Oil, on the other hand, is a type of lipid, which is a nonpolar compound. Since the atoms within the lipid are largely identical, the electrons are evenly distributed across the lipid molecule [Figure 1b]. Therefore, nonpolar molecules do not have such well-developed dipoles.
Figure 1. a) Acetic acid and water are polar molecules. b) Lipids are nonpolar molecules.
In solutions, compounds follow the chemistry fiat, like dissolves like. Polar molecules only interact with other polar molecules. Likewise, nonpolar molecules prefer to be surrounded by other nonpolar molecules. When a polar solution, like vinegar, is vigorously mixed with a nonpolar solution, like oil, the two initially form an emulsion, a mixture of polar and nonpolar compounds. However, this emulsion is unstable and will very quickly form layers in what’s known as phase separation. The solutions separate into layers according to their respective densities due to an aversion to each other. (In this case, because oil has a lower density than vinegar, it happens to be the layer floating on top.)
Phase separation in vinaigrette. Photo credit: Jan Persiel (janpersiel/Flickr)
To prevent phase separation, an emulsifier can be added to the vinaigrette to stabilize the emulsion. Emulsifiers are amphipathic compounds, meaning the molecule has both a polar and nonpolar section [Figure 2]. Common food emulsifiers include egg yolk, soy lecithin, garlic, and mustard. Egg yolk contains the emulsifying agent lecithin. The vegan version is isolated from soy and is thus known as soy lecithin. Lecithin is a commonly used emulsifier in many other food products, such as chocolates, mayonnaise, and Hollandaise sauce. Amphipathic compounds found in garlic include diallyl sulfide, allyl methyl disulfide, and diallyl trisulfide [1]. Mustard, the condiment, is made from mustard seeds. Emulsifying agents in the condiment, such as the pectin rhamnogalacturonan, originate from the mucilage of mustard seeds, a thick, glutinous layer that surrounds the seed hull [2,3].
a) Lecithin is an example of an emulsifying agent. b) Emulsifying agents stabilize emulsions by interacting with both the polar and nonpolar compounds. (b) adapted from Ioana.Blog.
So with a helping hand from emulsifiers, homemade vinaigrettes can still be as simple yet elegant as they seem, and best of all, ready to serve whenever.
Greek Salad Vinaigrette (Recipe from Ina Garten’s Barefoot Contessa)
½ cup olive oil
¼ cup red wine vinegar
2 cloves garlic, minced
½ tsp Dijon mustard
½ tsp ground black pepper
1 tsp salt
1 tsp dried oregano
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.