A glass of sparkling water please!

Carbonated water is known by many names- soda water, club soda, seltzer water, fizzy water, and sparkling water. It forms the backbone of all the carbonated beverages, several cocktails, and is becoming increasingly popular in cooking where it is used to provide a lighter texture to doughs and batters. The uses of club soda also extend beyond gluttony; it is used as a stain remover despite there being no scientific basis for why it would be superior to ordinary water. So what is carbonated water and how was it discovered?

Naturally carbonated waters are a common occurrence in springs when carbon dioxide (CO₂) bubbles through the water (H₂O). Artificially carbonated water, however, involves dissolving low concentrations of carbon dioxide in water. The reaction generates carbonic acid (H₂CO₃), which gives a slightly tart flavor to the carbonated water:

                                                   H₂O + CO₂ ⇌ H₂CO₃     

    

The above reaction is reversible- carbonic acid can easily dissociate to release carbon dioxide and water. Henry's Law states that the amount of a gas that can be dissolved in water depends on the temperature of the water and the pressure of the gas. Cold water will permit the maximum amount of carbon dioxide to dissolve in it. If the temperature of the water is increased or if the pressure of the gas is decreased, which happens when the container of carbonated water is opened, the carbon dioxide escapes in the form of bubbles. 

Figure 1: Bubbles of carbon dioxide in sparkling water. Source.

Artificially carbonated water was first made in 1750, by a French chemist Gabriel François Venel. He had observed the effervescent water of the Selz river in Germany. He attributed the effervescence to the escape of common air, which he called "superabundant air". In an attempt to replicate this phenomenon, he combined hydrochloric acid (HCl) and soda (NaHCO₃) to produce carbonated water resulting the following reaction:

                                                HCl + NaHCO₃  →  NaCl + H₂O + CO₂     

        

The other byproduct of the reaction is sodium chloride (NaCl) that we know as table salt.

It was only in 1754 that a Scottish chemist Joseph Black characterized the air bubbles as carbon dioxide. He passed the gas through lime (Ca(OH)₂), which produced a precipitate of calcium carbonate (CaCO₃):

                                               CO₂ + Ca(OH)₂   →  CaCO₃ + H₂O 

Figure 2: Test for carbon dioxide. From left to right: test tube containing calcium hydroxide, bubbling carbon dioxide through the calcium hydroxide solution, formation of calcium carbonate. Source.

In 1767, an English chemist Joseph Priestly discovered how to produce drinkable carbonated water. He utilized the fact that carbon dioxide is released as a byproduct during the production of beer; he  suspended a bowl of water over a beer vat at a local brewery, which impregnated the water with carbon dioxide. Priestly realized that the resulting water had a pleasant taste and could be offered as a refreshing drink. In 1772 he developed an apparatus to do the same. He used sulfuric acid (H₂SO₄) and chalk (CaCO₃) to generate carbon dioxide which was then dissolved in an agitated bowl of water:

                                             H₂SO₄ + CaCO₃   →   CaSO₄ + H₂O + CO₂     

Figure 3: Directions for producing carbonated water in Joseph Priestly's book Impregnating Water with Fixed Air. The procedure involved using a glass vessel (labelled "a" in Figure 1) that was inverted into a bowl of water "b". Pipe "c" connected the bowl to a bladder "d" which was fit into a phial "e". Initially the phial was filled with chalk and water. Sulfuric acid was then added to this mixture in small amounts. Once the effervescence started, the phial was stoppered, and the carbon dioxide released passed through the pipe and into the water in the inverted glass vessel. Source. 

J. J. Schweppe, a Swiss watchmaker, improved on Priestley's method and developed the first practical method to manufacture carbonated water on a large scale. This led to the founding of the Schweppes company in 1783. He also tried to move the business to London but was unsuccessful. However, the drink was later popularized by Erasmus Darwin, the grandfather of Charles Darwin, and by King William IV of the United Kingdom who adopted the beverage with the Royal Warrant of Appointment.

Packaging woes

The bottles containing carbonated beverages were usually sealed with a cork. The main disadvantage of using a cork was that the bottles could not be stored upright- the corks had a tendency to dry out and shrink thereby allowing the gas to be released and causing the bottle to "pop". Therefore these bottles were stored on their side, which prevented the corks from drying out. Furthermore, it was difficult to open the bottles by hand; a fact that was wonderfully depicted by the British painter William H.H. Trood.

Figure 4: The paintings "Uncorking Bottle" and "Surprising Result". Source.

To circumvent the problem of using corks, in 1872 a British soft drink maker Hiram Codd designed the Codd-neck bottle to store carbonated drinks. The neck of the bottle enclosed a marble and a rubber gasket. These bottles were then filled upside down with the carbonated beverage and the pressure of the gas forced the marble up, thereby sealing in the carbonation. The bottle also contained a chamber into which the marble was pushed to open the bottle. This prevented the marble from blocking the neck of the bottle as the drink was poured. However, the design had an unsanitary disadvantage that lead to a decline in it's popularity- people would generally use their fingers to push down the marble.

Figure 5: The Codd-neck bottle. Source.

Another improvement in bottle design took place in 1874 when an American inventor Charles de Quillfeldt invented the flip-top bottle. The mouth of the bottle was sealed by a stopper, fitted with a rubber gasket, and held in place by a set of wires. This allowed the bottle to be opened and resealed repeatedly without the use of a bottle opener. 

Figure 6: The top of a flip-top bottle. Source. 

The pièce de résistance in the manufacturing of bottle caps was the invention of the crown cork in 1892 by the American engineer William Painter. He also invented bottle openers to enable the removal of these metal bottle caps. The crown cork bottle stopper allowed the soda bottles to be stored standing upright.

Figure 7: The "Bottle Sealing Device" patents issued in 1892. Source. 

What's in a name?                    

If you want to order carbonated water at a restaurant, what should you ask for? As previously mentioned, there are several types of carbonated water. Soda water, sparkling water, and fizzy water are used to describe any type of carbonated water. However, club soda and seltzer water do have a few defining characteristics. Club soda refers to artificially carbonated water to which sodium salts and/or potassium salts have been added. In contrast, seltzer water is artificially carbonated water which does not contain any added ingredients. Seltzer water gets it name from the town of Selters in Germany, which was renowned for its natural springs. Cheers!

A cheesy beginning

What is the common theme of this list: Reggianito, Etzy Ketzy, Catupiry, Kanafeh, Blu Dealla Casera, Brunost, and Chhurpi? Intriguing names? Yes. Types of cheese? Bingo. 

So what is cheese? Cheese is derived from milk by converting milk into a solid or semi-solid state (a process also known as coagulation). This process is achieved by breaking down the milk protein (casein) which causes it to clump together and form cheese. Coagulation can be carried out by two ways- adding an acid (such as lime juice) or using rennet which is a complex of enzymes that is found in the stomach of ruminant animals such as cows. Both these processes cause the casein to lose its structure and form aggregates, which results in the formation of curd. The curd is heated, pressed to remove the excess moisture, salted, and ripened (which may or may not include the addition of micro-organisms). 

Where did cheese originate? In 2012, Salque et al discovered that the earliest evidence of cheese production dates back to the Neolithic period (between 5400-4800 B.C.). It had been previously recognized that several pottery vessels resembled sieves or cheese strainers. Chemical analysis of the organic residue that coated these vessel walls confirmed that these vessels were used to contain cheese. The making of cheese seems to be accidental; nomadic tribes used to carry milk in bags made of the lining from animal stomachs. This practice, coupled with the churning that occurred due to the galloping motion of the horse, would have caused the curdling of milk which could then be sieved to make cheese.

Figure 1: Cheese making (14th century). Source.

Figure 2: Fragment of a clay sieve used to separate cheese curds from whey. Source.

The role of microbiology in diversifying the types of cheese is particularly intriguing. Although large scale manufacturing of cheese can be achieved by using acids such as vinegar (for example manufacturing paneer and queso fresco), it is more common to use starter cultures of lactic acid bacteria (LAB). These bacteria (usually belonging to Lactobacillus, Lactococcus, and Streptococcus families) are capable of breaking down the milk sugars to form lactic acid. This process is then coupled with the addition of rennet to maximize the coagulation of milk proteins. The starter cultures also contribute to the flavor of aged cheese and prevent the growth of undesirable spoilage organisms and pathogens.

Figure 3: Lactobacillus delbrueckii- one of the LAB species that is used as a starter culture. It is commonly used in yogurt, Italian-type cheese such as Romano and Mozzarella, and Swiss-type cheese. Source.

In addition to starter cultures, adjunct cultures are used to provide additional flavors and textures to cheese. These organisms can be bacteria, yeast, and mold. Some of them are listed below.

• Lactobacillus casei and Lactobacillus plantarum are used as secondary cultures in Cheddar cheese. These bacteria contribute to the flavor and texture by breaking down the proteins, fats, and sugars in milk to release volatile compounds. (Fun fact: these bacteria do not contribute to the typical orange color of Cheddar; that color comes from the flavorless Annatto seed that is added to dye the cheese.)

Figure 4: Annatto seeds. These seeds are also used as coloring agents for Gloucester cheese, Chesire, Red Leicester, and processed cheese products such as American cheese. Source.

• Propionibacterium freudenreichii is used to create "eyes" in Swiss cheese. During the cheese production, this bacterium uses the generated lactic acid to produce carbon dioxide, acetate, and propionic acid. The carbon dioxide forms bubbles that riddles the cheese with holes. Acetate and propionic acid give Swiss cheese its distinctive nutty flavor. 

Figure 5: Propionibacterium freudenreichii when viewed under a microscope. A relative of this bacterium, Propionibacterium acnes, is the principle cause of acne. Source.

• Brevibacterium linens (also responsible for foot odor) is used to smear cheeses such as Limburger and Münster. Such surface-ripened cheeses (which ripen from the rind to the inward paste) have a complex and transient microflora; they are initially dominated by yeast and then by bacteria such as Brevibacterium. The yeast break down the lactate present in curd, thereby raising the pH, which allows bacteria to grow. The growth of the bacteria is also facilitated by brine-washing the cheese surface. The bacteria produces pungent odors by releasing sulfur compounds and contributes to the firm rind around the cheese. 

Figure 6: Limburger cheese with the outer rind. Source. 

Fun fact: In 2006, a study showed that the malaria mosquito is attracted equally to the smell of Limburger and the smell of human feet. Cheese eaters beware!

• Penicillium camemberti and Penicillium roqueforti are used to make Camembert and blue cheese respectively. P. camemberti forms a distinctive hard, white crust and can be either mixed with the other ingredients or added to the cheese afterwards. Blue cheese is characterized by the spotted blue color of the P. roqueforti cultures. Blue cheese was believed to have been discovered by accident when cheeses were stored in caves; the moist and temperature-controlled environments were ideal for fungal growth. An interesting legend claims that Roquefort was discovered when a youth left his meal of ewe's milk cheese in a cave (after seeing a beautiful girl in the distance) and returned several months later to discover that his cheese had turned into Roquefort. 

Figure 7: Stilton, a blue cheese from England. When the cheese is still in its curd form, it is inoculated with Penicillium roqueforti. Subsequently, the fungi grows within the cheese as it ages imparting the characteristic appearance and taste. Source.

There are around 900 known types of cheeses in the world. The exact number varies based on how one categorizes them: moisture content, source of milk, types of starter cultures and adjunct cultures, and processing techniques. This article highlights the most common techniques and cultures employed to make cheese. Understanding the science behind cheese-making will hopefully make the eating experience more enjoyable. Bon appetit!