casein – Science and Food

One of the most aggressive flavors we can experience is spiciness. Imagine a bright red chili pepper whose color gives us fair warning of its propensity to ignite a fire. In fact, a common physiological response to eating spicy food is analogous to the way our body responds to an elevation in internal body temperature. You can feel the burn. The consumption of spicy ingredients triggers our exocrine (sweat) glands to secrete fluid at the skin’s surface to promote cooling through the evaporation of our perspiration.

So how is it that spicy food can make us sweat?

Red Chili Peppers [Photo Credit: The Paleo Diet]

Chemical Structure of Capsaicin [Photo Credit: Dartmouth]

The answer lies in our head. Our body’s reaction to spiciness is a result of input delivered from receptors in our mouth to our central nervous system. A compound found in many spicy ingredients such as chili peppers is called capsaicin. This molecule activates sensory neurons in our mouth called thermal nociceptors [1,2]. The activation of these thermal pain receptors in turn stimulates our sympathetic nervous system [3], which is associated with our body’s “flight-or-flight” responses. Therefore, the characteristic increase in heart rate and sweat production due to the consumption of capsaicin-rich ingredients is due in part to the capsaicin’s role in adrenaline secretion [4].

If you decide to brave the spice, here’s some advice: When it comes to relieving the pain of the capsaicin burn, your best bet is to have a glass of cold milk or better yet, a bowl of ice cream. The compound, casein, is a hydrophobic substance found in milk that operates to “absorb” the lipid-rich capsaicin molecules making it easier to cleanse your mouth of the spicy chemical [5].

However, capsaicin isn’t the only fiery compound that can elicit unique physiological responses. On the other end of the spectrum is a compound known as hydroxyl-alpha sanshool, commonly found in Z. Bungaenum peppercorns. This variety of peppercorn is commonly grown in the Sichuan province of China, and is referred to as the Sichuan peppercorn. The characteristic “flavor sensation” one might experience when eating Sichuan cuisine is a mind-numbing spiciness.

Here’s how that happens.

Sichuan Peppercorns [Photo Credit: Serious Eats]

Chemical Structure of Hydroxy-Alpha Sanshool [Photo Credit: Hong Kong University]

Hydro-alpha sanshool activates somatosensory neurons that are responsible for detecting innocuous stimuli such as a gentle touch. This is opposite of the nociceptors that detect capsaicin which primarily detect noxious or painful stimuli. The stimulation of somatosensory neurons by hydro-alpha sanshool produces a similar effect to local anesthetics used to moderate pain in surgery. The compound found in Sichuan peppercorns trigger somatosensory neurons to prevent the influx and efflux of electrolytes, Na⁺ and K⁺, through ion channels [6,7]. The resulting effect of hydro-alpha sanshool is to cease the propagation of neuronal action potential through the nervous system’s pain pathways. So, when you enjoy a delicious dish of Liang Fen (Cold Mung Bean Noodles with Sichuan Peppercorn/Chili Vinegar), or Suan Cai Yu (Poached Fish in Green Sichuan Peppercorn Sauce) you can experience thrilling mouth numbness. Your first instinct after eating these dishes will be to grab the closest glass of water available to you. Interestingly, a sip of flat water after eating these dishes will produce a surprising fizziness like you might experience while drinking sparkling water.

Liang Fen: Cold Mung Bean Noodles with Sichuan Peppercorn/Chili Vinegar [Photo Credit: Serious Eats]

References cited

Caterina, M., Schumacher, M., Tominaga, M., Rosen, T., Levine, J., Julius, D. “The capsaicin receptor: a heat-activated ion channel in the pain pathway”. Nature. 389 (1997): 816-824.
Liu, M., Max, M., Parada, S., Rowan, J., Bennett, G. “The Sympathetic Nervous System Contributes to Capsaicin-Evoked Mechanical Allodynia But Not Pinprick Hyperalgesia in Humans” The Journal of Neuroscience. 16.22 (1996): 7331-7335.
Ohnuki, K., Moritani, T., Ishihara, K., Fushiki, T. “Capsaicin Increases Modulation of Sympathetic Nerve Activity in Rats: Measurement Using Power Spectral Analysis of Heart Rate Fluctuations”. Bioscience, Biotechnology, and Biochemistry. 65.3 (2001): 638-643.
Watanabe, T., Sakurada, N., Kobata, K. “Capsaicin-, Resiniferatoxin-, and Olvanil-Induced Adrenaline Secretions in Rats Via the Vanilloid Receptor” Bioscience, Biotechnology, and Biochemistry. 65.11 (2001): 2443-2447.
Fire and Spice. General Chemistry Online!.
Tsunozaki, M., Lennertz, RC., Vilceanu, D., Katta, S., Stucky, CL., Bautista, DM. “A ‘Toothache Tree’ Alkylamide Inhibits AD Mechanonociceptors to Alleviate Mechanical Pain” Journal of Physiology. 591 (2013) 3325-3340.
Bautista, DM., Sigal, YM., Milstein, AD., Garrison, JL., Zorn, JA., Tsuruda, PR., Nicoll, RA., Julius, D. “Pungent Agents from Szechuan Peppers Excite Sensory Neurons by Inhibiting Two-Pore Potassium Channels” Nature Neuroscience. 11.7 (2011): 772-779.

About the author: Anthony Martin received his Ph.D. in Genetic, Cellular and Molecular Biology at USC and is self-publishing a cookbook of his favorite Filipino dishes.

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Melted Cheese Frize [Photo Credit: Pittaya Sroilong]

Whether you are making cheese fries, grilled cheese sandwiches, quesadillas, baked cheese bites, or homemade mac and cheese, choosing the right type of cheese can make or break these comfort foods. The key to all of these dishes is cheese that produces an even and homogenous melt. Cheeses like Cheddar, Mozzarella, and Gruyere are used often. If you aren’t feeling adventurous, you could just memorize the names of these greatest hits. However, if you want to experiment and change the melty cheese game, you’re going to have to understand why these cheeses work.

Let’s first examine what happens to cheese as it melts. The interactions of casein (milk proteins) and calcium help define its solid structure. When solid, caseins are bound together in large branching porous protein networks that entrap milkfat and water. Calcium (as calcium phosphate) acts as a bridge to stabilize these networks. When you apply heat to a cheese, melting occurs in two stages. First, at around 90 ˚F, milkfat is released¹. This is because hydrophobic (water-repulsive) interactions between casein molecules increase under heat². These interactions force out water molecules and the space between casein molecules increases allowing milkfat, which melts at this temperature, to escape. If you’ve put cheese on a burger that’s being grilled, you may see little sweat beads of liquid form on the cheese in the early stages of melting. The second stage happens at about 40 to 90 degrees higher, at around 130 – 180˚F³. At this point, the casein proteins do not break down, but rather, the increased movement of the proteins, resulting from the heat, allows for the proteins to act more fluid-like and the cheese melts.

There are many factors that control melting and explain why melting temperatures vary by as much as 50 degrees. No one factor defines a cheese’s melting properties as these factors can interact.

Moisture and Fat

Cheese with higher moisture and fat content tends to have lower melting points. For example, high moisture cheeses like Mozzarella melt around 130 ˚F and low moisture cheeses like Swiss melt at 150 ˚F ². First, as previously highlighted, the milkfat and water portion of the cheese react to heat at lower temperatures than the proteins. Accordingly, with more moisture and fat present in a cheese, greater proportions of the cheese are susceptible to melting at lower temperatures. When the fat becomes liquid, it can no long provide support for the protein networks. Secondly, increased moisture and fat means that the casein proteins are more spread out and the mesh size (gap between proteins) is larger. This means there are fewer connections (bound calcium bridges) between proteins networks making melting more likely to occur at lower temperatures.

You may not know the exact moisture and fat content of every cheese variety without looking at a label, but intuitively, softer cheeses have more moisture and fat. Additionally, younger cheeses generally have more moisture so they also tend to melt more uniformly and evenly.

Acid Content

Chesses typically melt homogenously and evenly around a pH of 5.0 – 5.4⁴. This is related to the calcium bridges. At too high a pH (pH > 6), too much calcium is present as bound calcium phosphate and the protein is too tightly bound to melt. With lowered pH, the calcium phosphate bound to the casein is replaced by hydrogen (H+), allowing for more movement among proteins.² At around a pH of 5.0 – 5.4, there is a sufficient number of calcium present as bridges to allow for melting. At too low a pH (pH < 4.6), too many calcium bridges are lost and proteins aggregate and are unable to flow and melt evenly.

Lastly as a caveat, the factors being highlighted are specific to rennet-set cheeses, and not acid-set cheeses. Acid-set cheeses like queso fresco, paneer, and ricotta are not generally used, as they don’t produce even melts⁴. This results from the way they were made. In cheese making, you have two options for separating the solid curds (primarily casein proteins) and the liquid whey; Use rennet (an enzyme derived from the intestines or baby goats and cows) or use an acid (like vinegar or lemon juice).

When they are free floating in liquid milk, casein proteins have a slightly different molecular structure than when they are in cheese. In milk, caseins stick together in small clusters (micelles) that have negative charges on their surface. Since negative charges repel each other, these micelles won’t combine. Adding acid to heated milk lowers the pH, which neutralizes the negative charges on the micelles; therefore the casein micelles can aggregate. In contrast, using rennet to set cheese is a more targeted approach. In this process, an enzyme contained in rennet called chymosin, selectively removes negatively charged portions of the casein micelles and allows the micelles to clump.

In an acid-set cheese, calcium bridges are never formed as a result of the acidic environment used to generate the cheese⁵. These cheeses are only held together in protein aggregates rather than protein networks with calcium bridges and don’t produce the even melt desired.

Bottom Line:

Rennet-set cheeses with high moisture and fat are the best cheeses for melting as they melt evenly and consistently.

But don’t fret if you still want to harness the flavor of other cheeses (especially older or drier cheeses)! You have options: Try using a cheese blend with a higher proportion of the better melting cheeses and a small proportion of the other cheeses. For example, this recipe uses a 1:4 ratio. Experiment! You now know the keys for melty cheese!

References cited

Schloss, Andrew and David Joachim. “The Science of Melting Cheese” http://www.finecooking.com/item/64019/the-science-of-melting-cheese
Johnson, Mark. “The Melt and Stretch of Cheese” https://www.cdr.wisc.edu/sites/default/files/pipelines/2000/pipeline_2000_vol12_01.pdf
Mcgee, Harold. On Food and Cooking. 2004 “Cheese” (57 – 67).
Tunick, Michael. The Science of Cheese. 2013 “Stretched Curd Cheeses, Alcohols, and Melting” (82 – 91).
Sargento Food Service. “Cheese Melt Meter” http://www.sargentofoodservice.com/trends-innovation/cheese-melt-meter/
Achitoff-Grey, Niki. “The Science of Melting Cheese” http://www.seriouseats.com/2015/08/the-science-of-melting-cheese.html

About the author: Vince Reyes earned his Ph.D. in Environmental Engineering at UCLA. Vince loves to explore the deliciousness of all things edible.

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Tag Archive for: casein

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