Learn the science behind:
After reading Salt, Fat, Acid, Heat (a great book by Samin Nosrat by the way) my eyes were opened to the importance of acid in food. Not just in a lemon meringue tart, or as a drizzle over a salad. Instead, acid can play a vital role in balancing dishes, aside from being crucial for baking soda to work, and for ceviche to be cooked.
One of the most powerful and easily accessible, sources of acid in foods are lemons! So, about time we dig a little deeper in the science behind this powerful fruit.
- 1: Balancing a dish and adding flavor
- 2: Leavening with baking soda
- 3: Impacting proteins
- Bonus: preventing scurvy
1: Balancing a dish and adding flavor
The simplest, and very common, way to use lemon juice is to add some brightness to a dish. A lot of foods do well with a little bit of acid to add some ‘zing’. Sweet dishes are perceived as less sweet if they also contain acid, balancing them out, and hearty, dense meals become lighter with a little acid.
Lemon juice is an acid
The reason lemon juice is so well suited for this is twofold. First of, it contains some flavor molecules, that contribute to the overall profile. But, most importantly, lemon juice is an acid. As a matter of fact, it’s quite a strong acid.
The strength of an acid is measured on the pH-scale which runs from 0-14. A food is acidic when it has a pH below 7. Values closer to zero indicate strong acids. It is alkaline, that is, the opposite of acidic, if it has a value >7. Lemon juice has a value of approximately 2-2.5, the exact value depends on the individual lemon. As a reference, orange juice has a value closer to 4, avocados have a pH over 6, whereas that of vinegar is within 2-3. Lemon juice is one of the most acidic natural foods out there and it’s why your mouth puckers when you drink pure lemon juice!
Citric acid is its ‘culprit’
To be acidic, a food needs to contain acids, a special group of molecules that lower the pH-value. The main acid in lemon juice is citric acid, which has chemical formula C6H8O7.
2: Leavening with baking soda
Acids don’t just add taste, brightness, and ‘pucker’ to a dish. Acids can be very useful for a range of other applications, the first of those being to help leaven cookies, pancakes, cakes, and many other baked goods with baking soda HCO3–. Baking soda leavens foods by reacting in a chemical reaction during which carbon dioxide (CO2), a gas, is formed. When you trap this gas within a batter or dough, it will cause the batter or dough to expand. Tiny pockets of gas are formed, which ensure your product isn’t too dense.
Baking soda needs an acid to react
In order for baking soda to react, it does need an acid. Baking soda itself is alkaline, and when and alkaline and acid molecule meet, they’ll react. Below, the reaction of baking soda with an acid is given. Notice that H+ represents the acid. It’s the excess of H+, also referred to as protons, that actually make something acidic. Acids release these protons in the liquid, making the overall liquid acidic.
Balancing lemon juice and baking soda
Lemon juice is one of the most powerful acids you can add to activate baking soda. And as such, it can be a tricky one to use. You might overshoot and get too much of a rise of your baked good, causing it to collapse again at the end. It’s also why you can’t just replace one acid for the other. Yogurt is another example of a commonly used acid. However, it has a pH of ‘only’ 4-4.5. As such, if you replace yogurt with the same amount of lemon juice, you’ve added a lot more protons! This may skew the recipe and cause it to fail.
Lemon zest does not activate baking soda
As an aside, lemon zest can be used in cakes without any interference with the leavening agents. It’s why most lemon cakes contain lemon zest, but not lemon juice. The zest adds lemon flavor (thanks to powerful oils) and a hint of color, but without the acidity that would interfere with the baking powder or baking soda!
3: Impacting proteins
Proteins are a special group of molecules. They consist of long chains that curl and fold up. This curling and folding up is heavily influenced by the environment. The surrounding pH can have a big impact on a protein’s behavior and you can use that to your advantage. Add a little lemon juice to a fish and it ‘cooks’, or to hot milk and it curdles!
Lemon juice can ‘cook’ fish in ceviche
The proteins in a white piece of fish are very sensitive to acidity. They will denature, aka ‘cook’ when sumerged in lemon juice for some time. The proteins deform in a similar way as they would when heated. But, keep in mind that the acidity does not kill off any micro organisms, which would have been killed by heat. So working cleanly is crucial.
Lemon juice turns milk into paneer
The proteins in milk are also very sensitive to acid, and thus lemon juice, even more so when the milk is hot. It’s how paneer is made. Bring milk to a boil and acid some lemon juice. The lemon juice will cause the casein proteins in milk to form large curds. These can then be pressed into paneer, a cheese that’s ready to use right after.
Fun fact, acids, including lemon juice, alsom impact the way an egg fries!
Slow down enzymatic browning of apples
Enzymes are just a special type of protein. Enzymes catalyze chemical reactions, helping them take place. Without enzymes, more specifically the PPO enzyme, apples, pears, and bananas wouldn’t turn brown after peeling. This enzyme is responsible for the process also referred to as enzymatic browning. This enzyme is also impacted by acidity. It can’t do its job as efficiently anymore when the environment is too acidic. It is why recipes may recommend sprinkling some lemon juice on freshly cut apples for an apple pie. The lemon juice keeps the apples light and bright.
Bonus: preventing scurvy
Lemon juice doesn’t just contain citric acid, it also contains ascorbic acid, also known as vitamin C. Vitamin C is crucial for humans, without it we’d get sick. We’d get scurvy. It’s why it was so beneficial to take lemons on board during long sea voyages several centuries ago. It would ensure the crew got enough vitamin C and wouldn’t get sick during the journey.
M. Satyanarayana, Lemons are sour. Here’s why, March 1, 2019, link
G.B. Seymour, et. al., Biochemistry of fruit ripening, 1993, p.118, 110, link
C. Stange, Carotenoids in nature: biosynthesis, regulation and function, 2016, Springer, p. 175 (fig 6.2), link
Clemson university, pH Values of Common Foods and Ingredients, link