After reading Salt, Fat, Acid, Heat I became a lot more aware of the importance of acid in food. Not just in a lemon meringue tart (even though those are delicious), in lemon bars or as a drizzle over a salad. Instead, acid can play a vital balancing role in most dishes and is even very important for foods you might not think of (most soft drinks, such as a coke, are very sour).
Within that realm of using acidity there is the realm of lemons. You can use lemons in so many ways and they really are quite a unique fruit.
Colour of lemons
Lemons are a citrus fruit and like most of their related species have a bright colour, in the case of lemon, it’s yellow. Colour of fruits & vegetables is very diverse and is all rooted in (somewhat) complex chemistry. A specific set of molecules give the fruit its colour.
Lemons are green when they’re unripe. This green is made up by chlorophyll as is most of the green in plants around us. Once the lemon ripens and turns yellow, the colour is mostly determined by carotenoids. Carotenoids are a wide group of molecules. They also make an orange orange and a tomato red (or yellow).
What is interesting is that a lemon actually contains a large amount of carotenoids that do not have a colour, which are quite rare. The molecules phytoene is one of those. Other carotenoids such as carotenes and cryptoxanthins do contribute to form that bright yellow colour.
Limes, a close relative
Limes are easily confused with lemons because of their very similar names. They are noticeably smaller though and have a bright green instead of yellow colour. Another important distinction is where they grow best. Lemons are suited for more mild climates, they don’t like extreme heat nor cold. Whereas limes are well adapted to higher temperatures.
Lemons and acidity
Apart from there bright yellow colour, lemons are probably best known for their acidity. A sip of lemon juice will make your mouth pucker and adding too much of it to any dish will throw off any balance that was in there and make everything taste sour.
Lemon juice has a pH-value (the measure for acidity) of only 2, whereas a neutral water would be as high as 7. This makes it very powerful from a flavour perspective, but also from a preservation perspective. At very low pH-values as these micro organisms will not be able to grow anymore. Lemon juice will have a hard time spoiling because of micro organisms.
The source of its acidity
Scientists have been trying to explain how lemons manage to be so sour for a long time and only recently (2019) did they explain how. It is not common in nature to store much acidity inside a plant but most citrus do it in some way or shape. It so happens that lemons have very special ‘proton pumps’. These pumps are very efficient in moving around protons (aka acidity) through a structure, allowing them to create this acidic environment.
Why lemons are as sour as they are is probably because of the way we humans have selected specific breeds and varieties over centuries. Selecting and growing those varieties we enjoyed best.
A good source of vitamin C (ascorbic acid)
One of the acids present in lemon juice is ascorbic acid, commonly called vitamin C. Several centuries ago, during the time of long travels at sea a lot of the crew on boats would get sick or even die because of scurvy. Scurvy is caused by a shortage of vitamin C and lemons were a known food to help solve this.
Acids can ‘cook’
A way to prepare fish is by making a ceviche. A ceviche is a fish that has been ‘cooked’ in acid. The acid denatures the protein in fish, resulting in a similar texture to a heat cooked piece of fish. Unfortunately, the acidity does not kill off any micro organisms.
Flavour of lemons
Lemons are very acidic but there’s a lot more going on. If not, you could always use a neutral distilled vinegar or something similar to get that acidity. Instead, lemons contains a bunch of other flavour molecules. The types of flavour molecules differ throughout the lemon though.
The juice in the center is mostly water and thus contains mostly water soluble flavour molecules. The yellow peel on the outside though contains a lot more oils (just like oranges). These oils contain a lot of oil soluble flavour molecules.
Because of these flavour differences and because you might not always want to add extra water (e.g. in a dough) recipes may call for the use of zest. Zest is the yellow outer part of the lemon (without the more bitter white inside!). Zest is also the part that you can use to make a lemon extract (which is actually quite similar to making a vanilla extract) using some strong liquors. You need the alcohol content to help extract those flavours.
There are several varieties of lemon, but probably the most well known less common one is the Meyer lemon. Meyer lemons pop up in recipes quite regularly. Meyer lemons are lemons as well and are that sma ebright yellow. However, they are slightly smaller in size the regular lemons, a bit more orange and they are sweeter. As a result, they give a slightly different flavour to your recipe than a regular lemon would.
Lemon are yellow, acidic and flavorful, but wait, there’s more! Lemons also contain a considerable amount of pectin. Pectin is a complex carbohydrate naturally present in a lot of fruits such as apples & cranberries, but low in fruits such as strawberries.
Pectin comes in handy when you’re mkaing jam. When you make jam, pectin in the fruit will help to form a gel and thus a firm sturdy jam instead of a runny one. Lemons (especially the seeds and peel) contain a considerable amount of pectin. Some of the commercially available pectin is actually made from lemons.
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