Food Composition and Taste

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A Cup of Coffee

The luring smell of coffee can drag sleepers from beds and pedestrians into cafés, but this seemingly everyday beverage is actually very chemically complex. The myriad of compounds that affect taste, smell, and texture have been researched greatly in order to produce the extremely popular coffee that so many people wake up to. Food scientists must study the reactions that occur in coffee beans and in the beverage itself, starting from the roasting of the beans and ending with the final concoction.

A thousand volatile compounds have been found in coffee, but only a few dozen were shown to contribute to the smell. They include β-damascenone (which has an aroma like cooked apples), 2-furfurylthiol (sulfury, roasty), 2-isobutyl-3-methoxypyrazine (earthy), guaiacol (spicy), 2,3-butanedione (buttery), and 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like). These chemicals react in a multitude of chemical reactions, including the Maillard reaction, caramelization, polyphenol degradation, polymerization of carbohydrates, and pyrolysis. The important thing to note here is that these compounds are necessarily volatile so that they can easily evaporate to quickly deliver the attractive aroma.

During roasting of the beans, the added heat input causes the decomposition of many compounds into more reactive subunits. Many of these contribute to the somewhat bitter taste of coffee. The aroma molecules in the beans are susceptible to degradation when exposed to heat, so fresh coffee that is heated for too long can have a noticeably different smell and taste only minutes after initial preparation. Some of the flavoring compounds are shown in the following diagram:

In expresso coffee, there is usually a reddish-brown froth on the surface called crema. These tiny gas bubbles are encased in thin films, and they keep in much of the flavor and aroma. These important substances are actually dispersed in emulsions, a type of solution composition, of tiny oil droplets.

The following diagram displays the optimal extraction time of expresso that will lead to the most aromatic coffee. Happy coffee making!

coffee

Author: Jonathan Yu

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Ever tried Coconut Milk?

In modern Western civilizations, almost everyone drinks dairy milk their entire life after possibly relying on their mother’s milk during infancy. Surprisingly, this is actually strange in the rest of the world; coconut milk is a very popular food ingredient in Southeast Asia, especially in Thailand, Malaysia, Indonesia, Singapore, and the Philippines. In China and Taiwan, sweetened coconut milk is common drink during the spring and summer. In this post, we’ll go over some different to things you should know about coconut milk.

Fresh coconut milk derived from the meat.

First, let’s discuss coconut milk, since it is much less well-known than is dairy milk. Coconut milk is derived from the meat of coconuts, and its color and rich taste are the results of a high oil content. The meat is soaked in warm water and then squeezed through a cheese cloth to extract what is known as thin coconut milk. In addition, coconut milk contains a very wide range of minerals and vitamins, and is also a good source of protein. You can look at a more detailed nutrition breakdown in the following table:

Nutritional value per 100 g
Carbohydrates 2.81 g
Fat 21.33 g
-saturated 18.915 g
-mononunsaturated 0.907 g
-polyunsaturated 0.233 g
Protein 2.02 g
Water 72.88 g
Vitamin C 1 mg (1%)
Calcium 18 mg (2%)
Iron 3.30 mg (25%)
Magnesium 46 mg (13%)
Phosphorus 96 mg (14%)
Potassium 220 mg (5%)
Sodium 13 mg (1%)
Zinc 0.56 mg (6%

Even better, coconut milk can be an alternative for people with lactose intolerance because, unlike dairy milk, coconut milk does not contain lactose. Furthermore, it is unique in that it contains lauric acid, which is anti-microbial, anti-fungal, and anti-protozoal. Therefore, drinking coconut  milk can be another everyday way to protect against infections and viruses. Lastly, coconut milk has many antioxidant properties, which also means that it takes longer to go bad.

While both dairy milk and coconut milk are high in saturated fats, there is a key difference in this comparison. Coconut milk mostly contains medium-chained fatty acids, which are easier for the body to metabolize quickly. Dairy milk, on the other hand, contains a lot of long-chained fatty acids, which are more difficult for the body to break down.

By Jonathan Yu


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Pineapple Jello

At the supermarket, you can find all sorts of delicious Jell-O and even ones with bits of fruit in them. Jell-O is simply a brand name for gelatin, so we’ll try to refer to it as gelatin from now on. The chemical structure of gelatin is basically a mixture of peptides, or short polymers of amino acids, and proteins. Gelatin is a processed version of the protein collagen, which is found in the hooves, bones, and connective tissue of cows, horses, and pigs. The 3-D protein mesh that composes gelatin is fairly organized, and the holes of the mesh are filled with water. The result is a “jelly”-like substance, or something that is soft enough to cut yet rigid enough to hold its shape.

For desert, you might happen to want to eat a gelatin dessert with some fresh fruit. This certainly sounds delicious, but be careful about what fruit(s) you use! If you don’t want your gelatin dessert to dissolve into a puddle of colored water, don’t use pineapples, mango, or kiwi. All of these fruit contain the enzyme bromelain, which is a member of a special class of proteins called proteases. These proteins essentially eat through other proteins. Because the structure of gelatin is a protein mesh, mixing gelatin and bromelain leaves you with a soupy mess instead of an enjoyable treat.

You might even see this warning on Jell-O packets, such as the one below:

Fresh pineapples will certainly have bromelain proteins to break down the proteins in gelatin. Why not frozen ones though? Fresh frozen pineapples never lost their proteolytic abilities; the bromelain was still active, just frozen away temporarily. Canned pineapples are good though! Why? Because in their manufacturing preparation, the enzymes were denatured in order to enhance preservation. With these tips in mind, have fun making a successful and delicious jello!

Enjoy your dessert! (:

Author: Jonathan Yu


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Soft-serve or Hard-serve Ice cream?

Have you ever wondered what the difference is between soft-serve and hard-serve ice cream (other than the obvious soft vs hard)? It seems as if outdoor ice cream stands only sell soft-serve ice cream, while supermarkets and ice cream shops tend to sell hard-serve ice cream. And if both are ice cream, why is one soft enough to lick with your tongue while the other is hard enough to require metal ice cream scoopers?

Both choices are delicious, so what’s the difference?

Ice cream can be categorized as different types of colloids. It is an emulsion, which is when both the dispersed substance and the dispersing medium is liquid. Molecules of fat are suspended in a water-sugar-ice structure. Hard-serve ice cream is generally more of an emulsion than is soft-serve ice cream because it has many more dissolved substances. For example, hard-serve ice cream has 10 to 18% fat content while soft-serve only has 3 to 6%. It is also a foam, which is when the dispersed substance is gas while the dispersing medium is liquid. The most important foam property of ice cream is the air content. Soft-serve ice cream is much lighter and softer than hard-serve ice cream because its air content is significantly higher.

Soft-serve ice cream is much more convenient when you’re on-the-go since it melts very fast, due to the high air content, so that you can eat it quickly. Hard-serve ice cream, on the other hand, can hold those small bits of dried fruit or chocolate to make this more expensive option even tastier! There’s no clear winner on which to choose, so we’ll leave it up to you depending on the occasion (:

Author: Jonathan Yu


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Well-done? Medium rare? Rare?

Eating steak at a high-end restaurant isn’t something many people do on a regular basis, so when you do get the opportunity, you should be ready. In this post, we’ll go over the differences between the levels that the meat can be cooked: from very rare to medium rare, to well done.

We’ll start off with a table that shows the basics of each level:

Term (French) Description Temperature range
Extra-rare or Blue (bleu) very red and cold 46–49 °C 115–120 °F
Rare (saignant) cold red center; soft 52–55 °C 125–130 °F
Medium rare (à point) warm red center; firmer 55–60 °C 130–140 °F
Medium (demi-anglais) pink and firm 60–65 °C 140–150 °F
Medium well (cuit) small amount of pink in the center 65–69 °C 150–155 °F
Well done (bien cuit) gray-brown throughout; firm 71–100 °C 160–212 °F

Chemical composition:

Meat is generally made up of proteins, fats, carbohydrates, minerals, and water (moisture). On average, three-quarters of meat is water. However, this number can change significantly after cooking; the more the moisture is evaporated off, the harder the meat will become. The rest is mostly protein and fat, while carbohydrates make up only a small percentage. At around 140 °F, fat in the steak begins to dissolve and dissipate. This is crucial because it releases the flavor into the meat.

Meat Texture:

Meat texture is highly dependent on the extent of proteolytic changes, or changes related to breakdown of protein, that occur during cooking. High cooking temperatures, as used for well done steak, can reduce tenderness. Long cooking times with a slightly lower temperature can tenderize meat that contain large amounts of connective tissue by converting them into gelatin. The chemical composition of meat is also extremely important in determining texture; large amounts of fat will make the meat more tender since fat is softer than muscle. The pH of the environment in which the meat is cooked in has also been determined to have a clear relationship with meat texture. High pH values favor proteolysis, helping to break down the muscle fibers in the meat and making it more tender.

Meat Quality:

Meat quality is a subjective topic, but it can assessed objectively in a few ways. For example, tenderness is usually the most desirable. This is reflected in the fact that fillet steak is both the most tender and the most expensive cut of beef. In addition, juiciness can range from dryness to succulence. Dry meat can be the result of reduced “water-holding capacity” as a result of chemical changes due to heating or low  levels of fat. Flavor is usually determined by the water-soluble constituents of the meat, while the odor is usually determined by the fat-soluble, volatile components.

To finish off this post, we’ll present a picture of the different cooking levels of steak to help you decide which one you’ll order:

Author: Jonathan Yu


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What makes bread become stale?

Rock-hard cookies are not only hard to eat, but are usually associated with staleness. While cookies can be intentionally baked to be harder than usual, a cookie that has lost its original softness is likely to be stale. Stale bread gives similar tell-tale signs. What makes bread go stale though? It’s not mold or oxidation, which are common suspects of food gone bad.

The staleness of bread actually arises from the presence, or rather the lack of the presence of water. Bread is mostly made up of starch molecules, hence it being a significant source of carbohydrates. When you bake bread, the starch molecules weaken and allow moisture to enter the complex of chemical structures that make up bread. Water molecules are able to squeeze between the starch molecules. Starch granules weaken, giving most bread a soft and fluffy texture.

Once the bread is taken out of the oven, the slow journey to staleness begins, although it won’t be noticed for a while yet. The most significant factor in staleness is the recrystallization, also called retrogradation, of starch molecules; once cooling begins, which is the moment you take the bread out of the oven, the process that was used to bake the bread essentially reverses itself. The bread slowly drys itself out; as water molecules detach themselves from the network of starch molecules, the starch begins to recrystallize into their original shape.

Let’s go into a more detailed analysis of the chemistry behind all of this. Amylose and amylopecton are the two types of starch that are found in bread. When bread is baked in the oven, a process called gelatinization occurs. This occurs when starch is heated in water; hydrogen bonds in the starch granules break, allowing water to enter the granule and causing the granule to swell. Amylose molecules leave the starch granules, while the water molecules form hydrogen bonds with the amylopectin molecules. The swelling of the starch granules is the reason why bread “rises” in the oven.

Let’s see this in action: (the swelling becomes visible at around the 00:12 mark)

However, as stated before, this entire process begins to reverse itself once the bread is taken out of the oven. First, synersis causes amylose molecules to pull back together into the starch granule, squeezing out the water molecules previously inside. Then, retrogradation allows the amylose molecules to realign in a linear-chain pattern. This structure is kept rigid because hydrogen bonding occurs between the chains of amylose. As a result, the bread feels hard and is now stale. The following diagram should help in visualizing all this chemistry.

Figure 1: Gelatinization and Retrogradation of Starch Molecules

On a less chemistry-intense note, there is something pretty surprising about preventing bread from becoming stale: bread goes stale about 6 times faster in the refrigerator than at room temperature. You might ask – Isn’t the fridge supposed to keep things from going bad? Well, for most things, yes. However, the temperature inside the fridge is actually near the optimal temperature for retrogradation of starch molecules. So how you keep bread from going stale? If you do leave it on the kitchen counter at room temperature, mold growth will occur extremely fast, as you probably already know. What you can actually do is put the bread in the freezer. Freezing temperatures are too low for starch molecules to want to recrystallize, and mold growth is basically nonexistent. Just reheat the bread when you want to eat it, and it’ll be good as new!

So should you throw out any stale slices of bread you already have? No! Unless the bread feels like a piece of rock, it is highly likely that you can make it edible again.  Just bake the bread in the oven again with water, and the effects of retrogradation should be almost completely reversed. Enjoy!

Author: Jonathan Yu

Sources:

http://www.thaiscience.info/Article%20for%20ThaiScience/Article/1/Ts-1%20a%20fresh%20perspective%20on%20staling%20the%20significance%20of%20starch%20recrystallization%20on%20the%20firming%20of%20bread.pdf

http://pubs.acs.org/doi/abs/10.1021/ma00158a016

http://www.todayifoundout.com/index.php/2011/08/bread-goes-stale-about-six-times-faster-in-the-refrigerator-than-at-room-temperature/


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What’s Wrong With Burnt Food?

Almost everyone has accidentally burnt food before, and many of them still eat the burnt areas either because they especially enjoy the crispy parts or because it is just unavoidable. However, people are often warned against doing so and are told that burnt food is carcinogenic. What’s the truth behind this? Does burnt food actually increase cancer risk? In order to have a better understanding, we need to consider the chemistry in burnt food.

Carcinogenic Compounds

One solid reason to not eat burnt food is that it contains polycyclic aromatic hydrocarbons (PAHs), which are a class of air pollutants. Some of these chemicals have been proven to be carcinogenic, and some are even found in coal tar and cigarette smoke. The toxicity of PAHs depends heavily on its structure; while many PAHs may have the same chemical formula and same number of rings, different isomers can vary from being nontoxic to being extremely toxic.

The most well-known of PAHs is benzo(a)pyrene (shown below in Figure 1), which damages DNA, which in turn can possibly cause cancer. Cooked meat products contain up to 4 ng/g of benzo(a)pyrenes and up to 5.5 ng/g in fried chicken. However, in overcooked beef, the amount of benzo(a)pyrene can reach over 60 ng/g.

The mechanism of action of DNA damage from benzo(a)pyrenes is relatively simple. The benzo(a)pyrene molecules (shown in Figure 1) intercalate themselves into DNA strands. This means that they fit between base pairs, as shown in the Figure 2, thereby interfering with transcription and possibly causing mutations.

Figure 1. Molecule of benzo(a)pyrene

Figure 2: Benzopyrenes intercalated into DNA.

The smell of smoke from burning food is not only acrid, but also contains these benzo(a)pyrenes. Therefore, in addition to avoiding eating burnt food, we should avoid burning food in the first place. While accidentally leaving the bread in the toaster for too long is sometimes hard to avoid during the morning rush, at least we now have a good reason to not eat burnt food.

Author: Jonathan Yu

Sources:

http://www.healthy-food-site.com/burnt-food.html

http://www.abc.net.au/health/talkinghealth/factbuster/stories/2011/01/25/3093063.htm#.UICQSsXyp8F