1. Why a Whole, Undamaged Garlic Clove Has Hardly Any Smell
In the kitchen, if you place a single unpeeled garlic clove on the table and bring your nose close, there is hardly any smell. However, as soon as you use a knife, the smell spreads instantly. Was the source of the smell there before you cut it? The answer is that the materials are present first, and the smell is created later. Inside the cells of undamaged garlic, an odorless substance called alliin is stored. When the tissue is damaged, the enzyme alliinase works on the alliin, changing it into an unstable substance. From there, various sulfur-containing substances are made [1].
Even though the materials are all present, the change does not start until damage occurs. So, from the moment the garlic is cut, in what order is the smell assembled?
2. How Crushing Garlic Builds Its Smell in Three Stages
When you crush or chop garlic, the cells break and alliinase acts on the alliin. The alliin changes into a substance called allicin, producing the smell [2]. In reality, an unstable substance is involved in the middle of this process [1].
This is not the end. Allicin is also unstable and decomposes into substances such as diallyl disulfide. Diallyl disulfide is said to be one of the main components of garlic essential oil and has a strong garlic odor [5]. In other words, the smell is assembled in stages: "odorless alliin → allicin → diallyl disulfide, etc."
The first step in this chain is carried out by the enzyme alliinase. So, what happens if we apply heat to this enzyme first?
3. Why Heating Garlic Before Crushing Stops the Enzyme
If you heat garlic whole, the enzyme alliinase, which changes alliin into allicin, becomes inactive. Therefore, it is explained that if you crush the garlic after heating it, allicin is not produced [2].
In an experiment using garlic extracts, the activity of alliinase increased when kept at 40–50°C, decreased at 60°C or higher, and almost disappeared with treatment at 90°C and 100°C [3]. At lukewarm temperatures, the enzyme is rather active, but at hot temperatures, it cannot work.
However, these are laboratory treatment temperatures. They cannot be directly applied to the temperature inside a home pot or the time of heating. Also, note that the order is a condition. If you cut the garlic first and then heat it, the enzyme is already working at the moment of cutting, so the situation is different from heating the whole garlic first and then crushing it. If the enzyme stops, what happens to the pungency (the sharp sensation)? Let's look at how much it changes with temperature and time.
4. How a 15°C Difference in Heat Changes the Time for Garlic's Pungency to Drop
In a study using thin slices of garlic (about 3 mm), the time for the pungency indicator (pyruvic acid produced by the enzymatic reaction) to decrease to one-tenth (D-value) was reported as 36.35 minutes at 60°C, 13.68 minutes at 65°C, 3.42 minutes at 70°C, and 0.97 minutes at 75°C [4].
Comparing 60°C and 75°C, 36.35 ÷ 0.97 is about 37.5 times. With a difference of just 15°C, the time for the indicator to drop to one-tenth changes by about 37 times. From the numbers, we can read that raising the temperature slightly makes the time required for the same decrease suddenly shorter.
These are numbers from sliced samples, so they do not apply directly to whole garlic. A summary of another study states that heated garlic tends to have less thiosulfinate (substances like allicin, related to pungency and smell) than raw garlic [3]. Another study comparing steaming, boiling, frying, and baking reported that steamed garlic had the highest amount of aroma components. The strength of the smell for each cooking method cannot be determined by these numbers alone.
It was confirmed in the thin-slice experiment that heat reduces pungency. However, there is a type of garlic that uses heat over many days to change the smell and taste into something else.
5. How Black Garlic, Aged Over Several Days, Differs in Smell and Taste
Black garlic is made by aging raw garlic in a high-temperature, high-humidity environment and is known as a food with little of the characteristic garlic smell [6]. According to materials from Hirosaki University, mass production of black garlic from Aomori Prefecture began after 2006.
There is a range in how it is made. Studies have organized a range of 30–90°C, 50–90% humidity, and 10–90 days. As an example, there is a report that 70°C at 80% humidity for 12 days was optimal for increasing the amount of S-allyl cysteine (a component in garlic) [7]. Under the conditions of this study, moisture decreased to about half, the color changed from white to black, the texture became soft, and the taste became sweet and sour.
As aging progresses, Maillard reaction compounds and polyphenols, which are involved when foods turn brown, increase. Maillard reaction compounds are brown substances formed when heating foods; the brown color of soy sauce, miso, and sauces also belongs to this group. It is reported that 5-HMF (5-hydroxymethylfurfural) increases especially in black garlic [6].
The materials report that the smell is less and that brown substances increase. From the materials used here, it cannot be said whether the decrease in smell can be explained by the Maillard reaction alone. The experiment at 75°C where pungency disappears in minutes and the black garlic at 70°C that takes 12 days are very different. Even at similar temperatures, the way time is spent makes the final product completely different.
6. A Kitchen Activity: Comparing Cutting and Cooking Order with an Adult
With an adult, try comparing the following. Since fire and knives are used, an adult must always supervise.
① Smell an unpeeled garlic clove. ② Peel it and smell it again. ③ Ask an adult to cut it with a knife, and compare the smell right after cutting and after a few minutes. ④ Ask an adult to heat another clove whole, with the skin on, for a set amount of time (if using a microwave, for a short time), let it cool, then crush it and compare the smell. Ensure the heating time and method are kept the same each time.
In step ④, where fire is used, how does the way the smell comes out differ from the raw cut garlic? Write "apply heat before cutting" and "cut then crush" in your notebook, and compare the smells for each order.
Is the "toasty aroma" when frying carried by substances different from allicin? That is the next question you might want to investigate.