Friday, December 20, 2013

Chemistry of Beer: Carbon Dioxide vs Nitrogen

There's a lot of interesting science behind beer and brewing.  I'm not just saying that because I happen to like beer;  it's actually true.  Besides the interesting biology, chemistry, and physics behind the creation of beer, there is also an emerging field of research studying various health benefits of certain chemicals found in beer.

Throughout history, beer was often considered more than a nutritional or recreational beverage.  It was also considered a medicine that was, for example, used as a mouthwash and applied to wounds as a disinfectant.  The ancient Egyptians held beer in such high regard that they believed it was a gift from their god Osiris.  In fact, some breweries were able to stay in business during the US Prohibition era by producing "near beers," or very low alcohol beers, as well as a small amount of real beer that was available with a doctor's prescription.  Even as late as the 1930s, a daily glass of beer was thought by some doctors to be nutritious, particularly  for pregnant or nursing women.  

We now know that over-consumption of alcoholic beverages, including beer, can be very detrimental to health, and doctors also now discourage alcohol consumption during pregnancy.   However, many studies have supported the idea that there are some potential health benefits of some of the compounds found in beer, particularly compounds called polyphenols.  This remains an active area of research as people try to discover new chemical compounds to treat diseases like cancer and heart disease.    

I thought it would be entertaining to create another post on the science of beer.  In the previous beer post, we talked about the biology of beer and the role that microorganisms play in fermenting as well as spoiling beer.  Today, I wanted to put up a post about another interesting scientific aspect of beer: the gas dissolved inside of it.  It might sound like a kind of boring topic at first, but it's actually very important to the flavor of beer, and the topic will allow us to examine a little bit about how your body detects chemicals and how you perceive flavors in foods and beverages.

Beer Contains Dissolved Gasses, Including CO2 From  Fermentation

You'll often hear the term "head" used when people talk about beer and carbonation.  The beer's "head" is the foamy froth that comes to the top after the beer is poured.  The head is created by the dissolved gasses escaping from the beer liquid.  Various types of beer are expected to have various levels of head, which is thought to play an important role in the aesthetics of the beer.  Volatile organic compounds can escape along with the gas and create a pleasing aroma.  The gas molecules that remain in solution also play an important role in the actual taste of the beer. 

For the most beers, that gas that makes the fizz and gives the beer its head is carbon dioxide (CO2).  CO2 is created during the fermentation of sugar by yeast cells that also produces the alcohol.  More CO2 is also sometimes added the beer by the brewers during bottling, canning, or kegging.  That's because a lot of the yeast-produced CO2 can escape during the brewing process.  The brewers want to make sure that there is enough CO2 in the beer when they bottle, can, or keg it, because CO2 is not just a byproduct of the fermentation process but also a critical component of the beer's flavor.  Before we can talk about CO2 and flavor, let's take a moment to distinguish taste from flavor.

Taste vs Flavor

Taste is made up of five types of chemicals that are recognized by cells in the taste buds of your tongue, which then talk to neurons also in your tongue which then send signals to your brain.  The five types of taste are
  • Sweet, which is the taste of sugars and artificial sweeteners (which we discussed in a previous post
  • Sour, which is the taste of acids.  It is thought that sour evolved as a "bad" taste to prevent you from consuming harmful things like spoiled milk. 
  • Salty, which is predominately the taste of sodium chloride (table salt; NaCl).
  • Umami, which is the "savory" taste stimulated by certain types of amino acids, including glutamate. Mono-sodium glutamate (MSG) is added to some canned foods to make them taste more savory.   
  • Bitter, which is also a "bad" taste thought to protect you against eating foods containing harmful chemicals.  Bitter taste receptors are activated by numerous different chemical structures including many toxic plant and bacterial products.  There is an enormous genetic variability in the ability of different people to taste certain bitter chemicals, which underlies some of the complex variations in individual taste preferences for certain bitter foods and beverages, like coffee. 
Flavor, on the other hand, is more complicated.  Flavor is a complex sensation that is made up not only of taste, but also of smell (olfaction).  Smell contributes quite a lot to how we perceive flavor.  Often, when people who are getting older complain about a loss of taste, it is really a loss of smell that is causing a loss of flavor, not taste.  Another important component of flavor is the touch or feel of food, sometimes called the mouth-feel.  Mouth-feel is a major component of why people like fatty foods, as there is something about the texture of fats that we are hard-wired to like.

The flavor of a beer is made up of many different components, including taste, smell, and mouth-feel.  Taste comes from the chemical components of beer, including naturally-occurring unfermented sugars in beer or added sugars that can create a sweet taste.  There are bitter chemicals, too, such as humulone (also known as α-lupulic acid) that comes from hops (Humulus lupulis).  Humulone is a hops α-acid (we previously discussed α-acids from hops in the last post on beer) that is broken down into numerous chemical derivatives (iso-α-acids) during the boiling and fermetation process.  These chemicals then contribute to the bitter taste of beer, and this is why hoppier beers taste often taste more bitter.  The genetic variability in bitter taste creates a lot of variability in individual preference for beers with different amounts of hops.  People that are less sensitive to bitter taste typically like hoppier beers.    

Carbon Dioxide and Nitrogen Have Different Effects on Flavor

As we mentioned above, another important part of the beer flavor comes from the dissolved gases, usually CO2.  However, sometimes, you may see a beer on tap advertised as "nitro" or "nitrogenated."  Nitrogenated beers are usually gassed with about 70% nitrogen (N2) and about 30% CO2, unlike fully carbonated beers which contain 100% CO2.  The N2 is always put into the beer by the brewer before bottling or kegging.  It is not a natural part of the fermentation process. 

Nitrogenated beers have been around for a long time.  Guinness was the first brewery to patent a design for a nitrogenated keg in 1932, but it wasn't until 1964 that they released nitrogenated Guinness Stout in bottles.  Many beer fans will know that the same beer can taste very different if it is kegged, bottled, or canned with pure CO2 vs a N2/CO2 mix.  Why is this?

To start answering this question, let's start with some general ideas about how carbonation in beverages affects flavor.  Carbonated beverages have been popular for thousands of years.  The ancient Greeks and Romans drank naturally carbonated waters that came from bubbling mineral springs.  They developed a taste for this naturally sparkling water.  Today, we consume large quantities of mass-produced carbonated beverages--approximately 200 billions liters worldwide in 2006.  This equates to approximately 30 liters per person per year.

Most carbonated beers usually contain anywhere from 5 to 10 grams per liter of CO2 when they are kegged, bottled, or canned.  A lot of this is produced by the yeast during fermentation, but often the brewer needs to add CO2 to supplement or make up for gas lost from leakage during the fermentation process.  

When you drink a carbonated beverage, most of this CO2 does not actually reach your stomach.  A lot is lose in the initial fizz when the bottle or can is opened.  Temperature will influence the amount of gas that remains in the beverage after the initial fizz.  While the solubility of solids in liquids increases with the temperature of the solution (think about heating water to dissolve sugar to make syrup), the solubility of gasses actually decreases as temperature goes up and increases as temperature goes down (exactly the opposite as with solids in water).  This is why warm soda or beer will fizz more when it is opened compared with cooled soda or beer.  More gas will be immediately released at warmer temperatures and more will be retained in solution at cooler temperatures.

While you do swallow some of the CO2, some of which is released through belching (burping), the most important effects of the CO2 occur in your mouth on your tongue.  The fizzy flavor of carbonated beverages is caused by two components.  The first is carbonic acid, which is formed when CO2 dissolves in water (H2O).  CO2 reacts with water molecules (in a hydration reaction) to create carbonic acid (H2CO3).
Carbonic acid can then can dissociate to form bicarbonate (HCO3-) and hydrogen (H+).  This reaction will increasing the H+ in solution and thus decreases the pH of the solution.  In other words, it makes the solution  more acidic.   This acidity is detected by the acid sensing ion channels (ASICs) in taste cells on your tongue that signal neurons which transmit the sensation of sour taste to your brain.

CO2 Also Contributes to Flavor by Activating Pain-Sensing Nerves That Cause Tingling or Burning

Carbonated beverages also create a slight burning or tingling sensation in addition to the actual acidic taste.  This burning sensation is not technically classified as a "taste," but rather as a pain sensation as it is detected directly through pain receptor nerves (also called nociceptor nerves).  This process is a part of what is sometimes called chemesthesis or the "common chemical sense," terms used to describe a process by which the body detects chemical irritants.  This is called the "common" chemical sense, because this is a sensory mechanism that is present in many different tissues of the body.

The burning sensation from the capsaicin in chili peppers, the cooling effects of menthol, and the tingling of CO2 are all sensed through chemesthesis.  Chemesthesis is designed to detect irritants, and thus at high concentrations, these sensations can be very painful or at least very unpleasant (think of too much hot sauce on your chicken wings).  However, at lower levels, many people find these sensations to be quite pleasant.  While chemesthesis is not technically a "taste," it is still an important contributor to flavor.

Capsaicin and menthol each activate receptors (TRPV1 and TRPM8, respectively) on the surface of the nociceptor nerves that tricks the nerves into thinking they are sensing heat or cold.  CO2 causes sensations in these nerves through a much different mechanism.  CO2 diffuses directly inside the nerves to alter the intracellular pH through the hydration reaction that we drew above.  How does it do this?

As we previously discussed, cells are surrounded by a membrane that is composed of a bilayer (a double layer) of molecules called phospholipids.  This bilayer has both hydrophobic (water fearing or nonpolar) and hydrophillic (water loving or polar) regions.

Generally, polar or charged molecules (which are hydrophillic), like sugars or ions, can't get through the membrane because of the hydrophobic layer.  Hydrophillic molecules interact favorably with water, and they won't let go of the water to cross the hydrophobic part of the membrane, so they don't cross unless there is a specific channel or transporter (usually a protein) that can transport them.  

However, small uncharged nonpolar molecules like CO2 and other dissolved gasses can easily go right through the membrane.

This means that CO2 can diretly enter the cells, and once inside, it can alter intracellular pH directly by reacting with intracellular water and releasing hydrogen ions (H+).  

Part of the evidence that we have for the direct action of CO2 on the nerves is that the fizzy sensation requires an enzyme called carbonic anhydrase.  An enzyme is a protein that increases the rate (speed) of a chemical reaction.  In chemistry and biology, we say that an enzyme catalyzes a particular reaction, which basically means it makes the reaction go faster.  The first part of the hydration reaction of CO2 with water is catalyzed inside the cells of your body by the enzyme carbonic anhydrase.

When carbonic anhydrase is blocked (inhibited) with particular types of drugs, the fizzy/tingly sensation of soda, beer, or other carbonated beverages goes away.    Mountain climbers often take a carbonic anhydrase inhibitor (usually a chemical called acetazolamide) to combat the effects of altitude sickness (maybe in another blog post we'll talk about why).  Climbers that take acetazolamide often disappointingly find that the "victory beer" they bring along with them to drink on the mountain top tastes flat.  The fact that the burning/biting/tingling sensation of carbonated beverages like beer is blocked by carbonic anhydrase inhibitors suggests that this sensation requires movement of COinto the nociceptor neurons themselves.  

Unlike CO2, when nitrogen (N2) dissolves in water, it doesn't react.  It stays in its pure elemental N2 form. 

A chemist would probably balk at the reaction equation that I drew above, because it isn't really a valid equation because nothing happens.  However, you get the point: N2 is very inert.  It doesn't react with much, including water.

Now, as nitrogenated beers are typically pressurized with 70% N2 and 30% CO2, these beers will have much less CO2 in solution to react with water, creating fewer H+ ions and making the beer have a less acidic pH which will lessen activation of acid taste receptors.  There's also much less COto go into the nociceptor neurons and create the fizzy tingling pain sensation that is part of the flavor.  This dramatically changes the flavor of the beer by making it feel less tingly and more smooth.  Nitrogenated beers are often characterized as more "soothing" on the palate, while carbonated beers are more "crisp."    

Nitrogen Also Affects Bubble Size and Foam Stability, Which Alter Mouth Feel and Flavor

Stout foam (image from Wikimedia Commons)
Two other major differences between CO2- and N2-tapped beers are the bubble size and the foam stability.  Nitrogenated beers have much smaller bubbles and much more stable foam.  Let's talk about why this is and how both of these phenomenon can drastically change the "mouth feel" of a nitrogenated beer compared with a carbonated beer.

When gasses are dissolved in solutions, the formation of a bubble (a pocket of gas surrounded by fluid) requires a "nucleation" or initial formation step.  Spontaneous bubble formation out of the bulk fluid is called homogenous or de novo nucleation.  Homogenous nucleation requires extremely large differences in pressure between the dissolved gas and the atmosphere, usually greater than 100 atmospheres of pressure (100 times gas pressure inside the solution vs gas pressure outside the solution in the atmosphere).  This is much greater than the pressure in your typical beer.  Most often, the bubbles in your beer form from pre-existing pockets of gas that get trapped in surface cracks and imperfections of the glass or bottle as well as left-over fibers floating in the beer from dust or from the cloth used to dry the glass.  Bubble nucleation from pre-existing gas pockets is called heterogenous nucleation.  Gas diffuses out of the solution and into the gas pockets, the bubble grows, and when the bubble gets big enough, it detaches from the glass and floats to the top.  

It is thought that, because N2 approximately 50x less soluble in water than CO2, bubble detachment may occur before the bubbles can reach a large size.  The lower solubility of N2 also means that the concentration of dissolved N2 is about 50x lower than the concentration of CO2 would be if both gasses were at the same pressure (based on a law of physics called Henry's law).  The rate of gas diffusion and bubble nucleation is related to the concentration of gas in the solution, and there is less N2 floating around in nitrogenated beers to form bubbles compared with the amount of CO2 in fully carbonated beers. 

Thus, the lower solubility of nitrogen and lower gas concentration slows the bubble growth in nitrogenated beers.  This is somewhat of a Catch-22 disadvantage to nitrogenated beers: even though many people prefer the foaming and bubble size of nitrogenated beers, it actually much harder to get them to foam.  When nitrogenated stouts are poured from a tap, the beer is usually forced through a plate with tiny holes that creates turbulence to help force bubbles to form.  Nitrogenated beers in cans typically have a small plastic device called a widget that creates turbulence to help bubble nucleation.

Bubble nucleation and coalescence (the joining together of bubbles to make larger bubbles) in fluids also appears to be highly dependent on the type of gas as well as the pH of the solution.  The unique reaction of CO2 with water to produce H+ and decrease pH plays an important role in bubble size differences between CO2 and N2 bubbles.  As nitrogenated beers are less acidic, this also likely affects bubble nucleation and coalescence.  

Bubble coalescence
Another influence on the "mouth feel" of the beer that depends on the dissolved gas is the foam (head) stability.  Foam is actually very unstable in pure water, because the formation of bubbles increases the surface area of the water, which is counteracted by pressure created by the surface tension of the water.  Thus, when bubbles form, they typically reach the surface and the gas quickly disappears into the atmosphere.  This is why carbonated waters and sodas don't have foam the way that beer does.  The stable foam in beer requires other molecules in the beer that come from grains such as wheat or barley.   These are mainly amphipathic polypeptides (small chains of amino acids) called "surface active substances."  Amphipathic means that these peptides contain both hydrophobic and hydrophilic regions.  The more hydrophobic these polypeptides, the more they can stick together and stabilize the foam against the surface tension of the liquid.  The iso-α-acids from hops can also increase foam stability by interacting and stabilizing these polypeptides. 

A Guinness (image from Wikimedia Commons)
However, the gas itself also has an important effect on foam stability in beer.  The head on a Guinness or other nitrogenated stout lasts much longer than most carbonated beers.  The creamy smooth foam is a large part of the appeal of nitrogenated beers. Again, the lower solubility of N2 plays a large role in why nitrogenated beers have a longer-lasting head. 

The process of the decaying foam in solution is called disproportionation: larger bubbles get bigger while smaller bubbles get smaller.  This is driven by something called the Laplace pressure, which is the pressure difference between the inside and outside of a curved surface, like a bubble.  The surface tension of the surrounding fluid creates pressure against the gas inside of the bubble.  The Laplace pressure is inversely related to the size of the bubble, which means that the pressure gets greater as the bubble get smaller.  This drives the movement of gas from smaller bubbles, which are under higher external pressure, to lower bubbles, which are under less external pressure.  This process is strongly affected by the solubility of the gas inside the bubble, as it requires the diffusion of gas between bubbles, which means the gas inside of a bubble must briefly go back into solution as it moves from one bubble to another.  Because CO2 is much more soluble than N2, it can move between bubbles faster and create a much faster decaying head foam.

Disproportionation is also related to temperature and atmospheric pressure.  Higher temperatures increase the rate of disproportionation while higher atmospheric pressure decreases the rate of disproportionation.  This is part of the reasoning behind the cover over a traditional German beer stein.  Covering the beer creates higher pressure inside the container and preserves foam.

Foam stability is also related to the rate at which the liquid drains from in between the bubbles of the foam that rises to the top of the beer.  The smaller N2 bubbles result in slower drainage, which is yet another stabilizing effect that N2 has on the head of a beer.

People Seem to Like Small Bubbles

In at least one taste test of carbonated sodas, where CO2 bubble size was controlled by etching patterns into glasses that affected bubble detachment, people preferred the flavor of smaller bubbles.  So, there appears to be something more pleasing in terms of mouth feel with smaller bubbles.

Studies have shown that even the sight of bubbles in a carbonated beverage can induce a pleasing neural sensation before the beverage is even tasted.  Taste is intimately linked to memory and pleasure centers in the brain.  Thus, many beer drinkers who like nitrogenated beers find the sight of the small nitrogen bubbles in a nitrogenated beer's head to be particularly pleasant and inviting.  The milky color of a nitrogenated beer is created by the large amount of tiny bubbles that reflect and refract light.  You'll often see some bubbles sink along the side of the glass when a nitrogenated stout or porter is poured.  This is partly due to the shape of the glass and the fact that smaller bubbles are more easily carried along in waves by currents as the beer settles after pouring.  In carbonated beers, the bubbles are much larger and thus float immediately upward when they detach from the side of the glass. 

By just taking a look at the gasses dissolved in beer, we were able to learn something about taste and flavor, we went over a little bit about how COinteracts with cells in your body, and we talked about the physics of bubbles and foam.  Hopefully, I've convinced you by now that there really is a lot of interesting science to be found inside your beer.      

© 2013 TheMadScienceBlog. 

Sources and Further Reading
  • S. Arranz, G. Chiva-Blanch, P. Valderas-Martinez, A. Medina-Remon, R. M. Lemuela-Raventos, and R. Estruch.  "Wine, Beer, Alcohol, and Polyphenols on Cardiovascular Disease and Cancer."  Nutrients.  2012.  4:759-791.  Available here.
  • A.S. Attwood, N.E. Scott-Samuel, G. Stothart, and M.R. Munafo.  "Glass Shape Influences Consumption Rate for Alcoholic Beverages."  PLOS One.  2012.  Available here.
  • R. Liu, X. Guo, Y. Park, J. Wang, X. Huang, A. Hollenbeck, A. Blair, and H. Chen.  "Alcohol Consumption, Types of Alcohol, and Parkinson's Disease."  PLOS One.  2013.  Available here.
  • T. Lefevre, L.-C. Gouagna, K.R. Dabire, E. Elguero, D. Fontenille, F. Renaud, C. Costantini, and F. Thomas.  "Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes."  PLOS One.  Available here.
  • W. T. Lee.  University of Limerick.  "Bubble Nucleation, Questions and Answers" and "Stout Beers"
  • P.M. Wise, M. Wolf, S.R. Thom, and B. Bryant.  2013.  "The Influence of Bubbles on the Perception Carbonation Bite."  PLOS One.  2013.  Available here.
Other Sources
  • C.W. Bamforth.  "Nutritional Aspects of Beer -- A Review."  Nutrition Research.  2002.  22: 227-237.
  • C.W. Bamforth.  "Beer and Cider" in Physico-Chemical Aspects of Food Processing.  Ed. S.T. Beckett.  Springer.  1995.  
  • G.S. Barker, B. Jefferson, and S.J. Judd.  "The Control of Bubble Size in Carbonated Beverages."  Chemical Engineering Science.  2002.  57:565-573.  
  • E.S. Benilov, C.P. Cummins, and W.T. Lee.  "Why Do Bubbles in Guiness Sink?"  American Journal of Physics.  2013.  81:88.  Available here.
  • W.F. Boron.  "Regulation of Intracellular pH."  Advances in Physiology Education.  28:160-179.  Available here.
  • R. Cuomo, G. Sarnelli, M.F. Savarese, and M. Buyckx.  "Carbonated Beverages and Gastrointestinal System: Between Myth and Reality."  Nutrition, Metabolism, and Cardiovascular Disease.  2009.  19:683-689. 
  • C. Gerhauser.  "Beer Constituents as Potential Cancer Chemopreventive Agents."  European Journal of Cancer.  2005.  41:1941-1954.    
  • J.B. German, M.J. McCarthy.  "Stability of Aqueous Foams: Analysis Using Magnetic Resonance Imaging."  J. Agricultural and Food Chem.  1989.  37: 1321-1324. 
  • W.T. Lee and M.G. Devereux.  "Foaming in Stout Beers."  American Journal of Physics.  2011.  79:991.  Available here.
  • W.T. Lee, J.S. McKechnie, and M.G. Devereux.  "Bubble Nucleation in Stout Beers."  Phys. Rev. E.  2011.  83:051609.  Available here.
  • D.R. Reed and A. Knaapila.  "Genetics of Taste and Smell: Poisons and Pleasures."  Progress in Molecular Biology and Translational Science.  2010.  94:213-240.  
  • L.B. Sørensen, P. Møller, A. Flint, M. Martens, A. Raben.  "Effect of Sensory Perception of Foods on Appetite and Food Intake: A Review of Studies on Humans."  Int. J. Obes. Relat. Metab. Disord.  2003.  27:1152e66.  
  • R.F. Tabor, D.Y.C. Chan, F. Grieser, and R. R. Dagastine.  "Anomalous Stability of Carbon Dioxide in pH-Controlled Bubble Coalescence."  Angew. Chem. Int. Ed.  2011.  50:3454-3456
  • L.C. Verhagen.  "Beer Flavor."  In Comprehensive Natural Products II: Chemistry and Biology. Eds., L. Mander and H.W. Lui.  Elsevier: Oxford.  2010.  3:967-997 
  • J.V. Verhagen and L. Engelen.  "The Neurocognitive Bases of Human Multimodal Food Perception: Sensory Integration."  Neurosci Biobehav Rev.  2006.  30:613e50. 
  • M.E. Wijnen and A. Prins.  "Disproportionation in Aerosol Whipped Cream."  in Food Macromolecules and Colloids.  Eds. E. Dickinson and D. Lorient.   


    1. Chemistry of beer, using the carbon dioxide and the nitrogen will make it more stronger as it first used to make it to the way, now let's see how it make an impact on the other side of the lab.

    2. How do I go about getting a job researching this type of stuff? That would be an exciting, fun and rewarding career.

    3. You guys should try NitroBrew! Infuse nitrogen into beer, coffee, cocktail or whatever you fancy! Its a great product and I use it for beer, coffee and black tea! Just love the nitro taste.

    4. I take a carbonic anhydrase inhibitor daily and it's unfortunately changed my whole relationship with beer. I never really liked nitro beers before and now they're all I can drink. The original Guinness is too dry for me (I prefer a good imperial oatmeal, chocolate or coffee stout) but I'm enjoying some craft stouts on tap where I can find them. The nightmare is finding anything nitro off tap but blessed be Sam Adams for being the first to get a nitro wheat in cans out to the masses. I could kiss them!

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    10. I'm still struggling to understand how much nitrogen actually ends up in nitro beers. My research indicates that a beer can be carbonated with pure CO2 to around 1.8 vols then dispensed with a CO2/N2 blend and you'll still get the effect. I've also done this at home with my homebrew keg setup. To me this indicates that nitrogen itself is not what makes the mouthfeel different, but the lower initial carbonation combined with the restrictor plate in the faucet. The blended gas is really only needed to be able to push the beer at a pressure high enough to get it through the restrictor plate without overcarbonating it since it will be chilled and CO2 will be more soluble. You said that nitrogen is 50x less soluble than CO2. Have there been any studies done on the actual amount of nitrogen in beer before and after carbonating with blended gas? At 50x less, how much nitrogen would one expect to get into a 34 degree F beer carbonating at 25 PSI with a 30/70 mix of CO2/N2? What am I missing that's making me so confused?

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    16. Mi nombre es Carlos Camejo soy de Inglaterra. Quiero compartir un testimonio de cómo el aceite de hierbas del Dr. Twins me salva de la vergüenza y la desgracia, mi pene era un gran problema para mí, ya que el tamaño era realmente tan vergonzoso, no pude satisfacer m esposa más y se convirtió en Cansada de los problemas de mi vida sexual, y se estaba enojando cada vez que hacemos el amor, porque siempre cum rápidamente en 10 minutos y fue muy frustrante, pero nunca perder la esperanza, salí a buscar en una manera de hacer mi pene Grande y por último en la cama. Milagrosamente vi un testimonio de cómo el Dr. Twins ha ayudado a mis compañeros de trabajo, en obtener ayuda y hacer su pene más grande y más fuerte, escribí a los gemelos para su aceite de hierbas y me dijo una vez que usé su aceite en 5 días mi pene Ser 9 pulgadas más grande ser capaz de satisfacer a mi esposa más tiempo en la cama y también ser capaz de controlar mi eyaculación a cum siempre que quiero. A mi sorprendente sorpresa después de usar el aceite de hierbas 5 días después de que me sorprendió cuando mi pene crecer más grande a 9 pulgadas exactamente como quería y ahora mi esposa disfrutar de mí en la cama e incluso anhelan más. Mis amigos no sufre más, si usted necesita su problema de pene para ser resuelto puede ponerse en contacto con el Dr. Twins hoy y ahora mismo en su correo electrónico: PEACEHELPMEDICALCENTRE@HOTMAIL.COM

    17. It is fairly rare to see the science of beer written by a beer enthusiast and that is what makes this piece so interesting. This article cleared up many of my misconceptions about nitrogenated beer and the tiny bubbles in beer in general. Thank you for writing it.

    18. Read this today - great article! Did want to ask: if a beer or coffee (or whatever) is kept cold in a keg and has pure nitrogen on it, is there a chart somewhere that generally explains the volumes of dissolved nitrogen gas you get at certain pressures (assuming you reach equilibrium) and temperatures? Also, any rough estimates on how long it'd take to reach equilibrium in those instances?

      Heads up: if you want to get rid of the spammers on your blog, you can introduce a Captcha or other human verification mechanism - I'd be happy to help you set up as a thank you for the wonderful article :)

    19. I should add: I'm also curious to know what volume(s) of nitrogen in a beverage are ideal for a good pour on tap with a restrictor plate faucet, and maybe what the ideal PSI at the restrictor plate would be. I realize those two are interrelated (i.e. more volume + less PSI at restrictor plate is probably roughly equivalent to less volume + more PSI at restrictor plate).

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