Coffee is one of the most common hot beverages consumed in various parts of the world. It is obtained from plants. Coffee cherry is first processed to make green coffee beans which are roasted to form brown coffee beans.
These brown coffee beans are available in the market and are brewed into our cup of coffee. Brewing is supposed to extract the soluble flavored compounds from the coffee beans into water.
There are many species of coffee, but the most useful ones are Coffea arabica L. and Coffea canephora Pierre. The flavor of coffee, which is prepared using Coffea arabica beans, is mild, and this coffee has a better aroma and quality. On the other hand, coffee from Coffea canephora has a strong and bitter flavor.
Coffee is a complex substance, and the exact composition of coffee depends on various factors like origin, variety, and species of coffee; processing, roasting, grinding, and brewing conditions. Coffee contains different bioactive compounds.
The most important chemicals present in all types of coffees are caffeine, multiple acids, diterpenes (cafestol and kahweol), melanoidins, and trigonelline.
So, is the pH of coffee acidic or basic? Coffee is weakly acidic and has a pH ranging from ~4.85 to ~5.10. Coffee contains over 30 organic and chlorogenic acids, and the acid content is different for different coffees. The acid content depends on various factors like roasting, grinding, and brewing conditions.
What is pH?
The potential of hydrogen or pH is used for measuring the concentration of H+ ions in a substance. pH indicates the nature of the substance (acidic, basic, or neutral) under study.
Concentration is measured in terms of molarity, i.e., the number of moles of H+ ions per litre of solution.
Thus, for pH to be measurable, the substance should be able to dissociate into H+ ions or H3O+ ions. That’s why dry HCl gas does not give pH value, but an aqueous solution of HCl does.
pH is calculated by taking the negative logarithmic value of H+ ions concentration.
pH= – log10[H+]
A standard pH scale range from 0 to 14.
For aqueous solutions at 25° C-
- Acidic substances have a high concentration of H+ ions, and hence the pH is low (below 7).
- Basic substances have a low concentration of H+ ions, and hence the pH is high (above 7).
- Neutral water has a pH of 7.
The pH of a 1 mM strong acid like H2SO4 is 3.01, and 1 mM weak acid like H2CO3 is 4.68.
The pH of a 1 mM strong base like NaOH is 10.98, and a 1 mM weak base like Cu(OH)2 is 7.69.
pH depends on temperature and solvent. At different temperatures, the acidic and basic ranges are different. At 100°C, the pH of pure water is 6.14.
In the organic compounds, pH is decreased by the presence of electron-withdrawing groups (like -Cl, -NO2) in the compound and increased by the presence of electron releasing groups ( like -CH3).
Why is Coffee Acidic?
Coffee is acidic because it contains more than 30 acids, and each acid plays a unique role in making coffee what it is.
The acidity of coffee in the human body is due to the donation of acidic protons to the receptors on the human tongue.
Acids present in coffee can be broadly divided into two categories- organic acids and chlorogenic acids.
Organic acids include acetic acid, tartaric acid, maleic acid, citric acid, succinic acid, etc.
The acidity of coffee is mainly due to chlorogenic acids, which are esters of caffeic acid and quinic acids.
Chlorogenic acids get broken down into these components on roasting. Some of the chlorogenic acids are shown in the figure.
All the chlorogenic acids have a -COOH and a phenol group. Both groups are acidic because the H+ can be extracted to form an anion stabilized by resonance.
In the carboxylic acid group, on the removal of a proton, a carboxylate ion is formed. Two equivalent resonance structures stabilize the carboxylate ion.
The negative charge is symmetrically delocalized between the two O-atoms, which are more electronegative than C-atoms.
In phenols, phenoxide ions are formed on the removal of a proton. The negative charge is delocalized on the benzene ring.
This delocalization is less effective as it is not symmetrical like than in carboxylate ions.
Also, the negative charge is delocalized on the less electronegative C-atom. Thus, carboxylic acids are more acidic than phenols.
At different stages of coffee, the acid content is different. In green beans, the acid content is very high. Some acids can survive the roasting process, while others do not.
During the roasting process, chlorogenic acids degrade to form quinic acid and caffeic acid, and the concentration of acidic content changes. The extent of the decrease depends on various factors.
Why is Coffee Bitter?
A general assumption is that acidic substances are sour to taste, and basic or alkaline substances are bitter.
Since coffee is acidic, it is not supposed to be bitter, but it is. The bitterness is generally caused at the stage of roasting.
Up to 8% chlorogenic acid is present in the unroasted coffee beans.
During the roasting process of coffee beans, the chlorogenic acid content decreases, and the bitterness increases.
Specifically, chlorogenic acids are converted to bitter-tasting lactones at bean temperature (~ 200° C).
Quinolactones, phenylindanes, and melanoidins formed on roasting are responsible for the bitterness and flavor of the coffee.
Caffeine content is reduced, sucrose is lost, and many other changes occur at roasting.
Thus, the lighter roasted coffee beans have more chlorogenic acid content and less content of bitter lactones, and dark coffee beans have less chlorogenic acid content and are more bitter because of more lactones.
Naturally, coffee is acidic. Some don’t prefer to have acidic coffee because of various health reasons.
Coffee can be treated with around 16000 phytonutrients which make the coffee alkaline. The powder has all the nutrients in a high amount.
It can neutralize the acid in coffee and can also absorb nutrients from food.
Alkaline coffee is preferred by many because of its low caffeine content.
It provides more sustainable energy as compared to regular coffee. The research about alkaline coffee is still not complete, and therefore, various aspects of the same are yet to be uncovered.
Factors Affecting the Acidity of Coffee
1. Variety of beans – Coffea arabica beans are less acidic than Coffea canephora.
2. Condition of the coffee plantation – Coffee grown in volcanic soil and high altitudes is more acidic than the ones grown at low altitudes and in normal soil.
3. Roasting Conditions – Shorter roasting time results in the most acidic, and longer roasting time results in the least acidic coffee.
4. Brewing conditions – Shorter brewing time results in more acidic coffee and moderate to long time results in less acidic coffee. Also, brewing in cold water reduces acidity.
5. Grinding of beans – Finer ground beans are more acidic due to more surface area exposure, while coarse beans are less acidic.
6. Additives – The addition of milk, eggshells, salt, etc. to coffee decreases the acidic content.
Caffeine in Coffee
Caffeine is present in many food items like tea, cocoa beans, coffee beans, etc. It is one of the major bioactive compounds present in coffee. It stimulates the central nervous system.
It is a bitter and white crystalline substance belonging to the class of alkaloids.
Alkaloids are naturally occurring organic compounds which are basic in nature. They have one or more N-atoms present.
Caffeine has positive as well as negative impacts on the human body. Almost all the harmful effects of coffee are associated with the presence of caffeine.
In 100 g of regular coffee, ~40mg of caffeine is present.
Despite the presence of bitter caffeine in coffee, it is not alkaline because acids are present in much more significant amounts, and hence coffee is weakly acidic.
Why do people use coffee as a wake-up drink?
Coffee is well known for its property of increasing alertness. It is commonly used as a pre-workout drink.
The caffeine in coffee binds to the adenosine receptors in the brain and does not allow our brain to get tired for some time.
Though people get all the more tired after the 5-6 hours of having coffee.
Thus, people commonly use coffee to tackle fatigue and jetlag.
Everybody reacts differently to different compounds and because of this reason coffee does not prove to have these effects for all.
Coffee is a weakly acidic beverage having pH in the range of ~4.85 to ~5.1.
The composition of coffee is quite complex, consisting of some acidic components and some alkaline components, but the coffee as a whole is weakly acidic.
Coffee is bitter mainly because of the chemical changes that occur on roasting.
Roasting decreases the acidic content of coffee.
The acidity of coffee is inevitable but can be controlled by modifying the roasting and grinding process.
I hope you will have fun while reading about the chemistry of the world’s second most common beverage.