Garlic: An Exploration of Allicin and Antioxidant Activity

Allicin is one of the active chemicals found in plants of the Allium family. Garlic is by far the most concentrated source of this valuable nutrient, with lesser quantities found in onions, leeks, ramps, shallots, and scallions. There are many ways to prepare garlic. Raw vs. roasted, powdered vs. pickled; which form contains the most allicin? Furthermore, allicin is a harsh, oxidative compound that can cause damage to human cells, so how exactly does garlic work to our benefit?

In this article, we will explore how allicin is derived from garlic and how the different preparations of garlic compare in terms of allicin content. We will learn about how allicin boosts our innate antioxidant system to improve overall health.

How is allicin formed in garlic?

An intact garlic clove contains two components that exists separately:

• Alliin is an alpha amino acid that is similar to cysteine. It is the precursor to allicin and other sulfur-containing compounds formed in garlic.
• Alliinase is an enzyme, or a protein that speeds up a chemical reaction. Alliinase facilitates the transformation of alliin to allicin. Alliinase is inactive when exposed to acid and heat, therefore it will not work after cooking or entering an acidic environment such as the stomach or vinegar preparations.

When you crush, smash, chop, mince, or chew a clove of raw garlic, these two components intermingle resulting in the production of allicin, also known as allyl thiosulfinate. Once the cells of the intact garlic glove are disrupted, this reaction takes place very quickly with much of the allicin forming in under a minute. (1) This is the reason for the advice to chop your garlic and let it sit for a few minutes prior to cooking. Once you allow allicin to form, some of it may remain stable during cooking at low temperatures, but much of the allicin degrades in the presence of heat, resulting in the mellowing of flavor and pungency of cooked vs raw garlic.

Properties of Allicin

• Allicin is an oily, yellow liquid that is responsible for garlic’s characteristic odor. It is soluble in fats, which means that it can penetrate the cell’s membrane (which is composed of phospholipids, or fatty substances).
• It is chemically unstable, which means that it readily transforms into other sulfur-containing compounds spontaneously during cooking, aging, or as they are metabolized in the body.
• Allicin is a reactive sulfur species. It oxidizes thiols in the body, which are indicated by sulfur-hydrogen bond, -SH. (Think of an alcohol, -OH, but with a sulfur atom instead of the oxygen.) The reactivity of allicin can make consuming raw garlic particularly harsh, producing a burning sensation in the throat or stomach for some people.

What is the concentration of allicin in various forms of garlic?

Now that we know how allicin is formed and a little bit about its properties, let’s compare the allicin content of various garlic preparations and supplements. This study determined the allicin equivalence of a variety of products based on breath measurements of allyl methyl sulfide (AMS), which is a known breakdown product of allicin in the body. Allicin equivalence is not a perfect measure of allicin alone because other sulfur containing compounds also break down to AMS, many of which have similar therapeutic effects as allicin. Nonetheless, AMS measured in breath is a useful tool for determining the amount of allicin (or allicin equivalence) that is ingested in various garlic products.

• Raw minced garlic contains about 2.5-4.5 mg of allicin per gram. A medium sized garlic clove weighs about 4g, or about 10-18 mg of allicin per clove.
• Roasted garlic contains less allicin per gram relative to raw garlic because the roasting process inhibits the activity of alliinase. To get the same allicin equivalence as raw garlic, you need to consume 3 times as much roasted garlic. Roasting mellows out the sharp, burning intensity of raw garlic, so it’s very feasible to eat more of it.
• Garlic powder is simply dehydrated, ground garlic. The heat used to dehydrate the garlic is low enough not to destroy the alliinase, but inactivate it. As this interesting experiment concludes, adding water to garlic powder for about a minute allows the enzyme to activate and form allicin. For the equivalence of one medium clove of garlic, use roughly ¼ tsp of garlic powder, hydrated prior to heating for maximum allicin potential.
• Pre-chopped jars of garlic typically contain citric acid or phosphoric acid to stabilize the product for its shelf life. The acid in these foods will render alliinase ineffective, however, the rough chopping in industrial production would probably lead to a robust allicin production (and more intense flavor) if carried out prior to the addition of acid. The allicin content of acid-preserved, pre-chopped garlic would be difficult to determine without knowing the procedure of the manufacturer and is likely to be quite variable.
• Aged garlic, whole-pickled garlic, and black garlic use ethanol, acid, and low heat, respectively, sustained over long periods of time to transform garlic into a mellow extract with dramatically different smell and flavor as compared to raw garlic. As expected, these preparations result in a much lower allicin content than garlic prepared in the ways previously discussed. In aged garlic products, the alliin converts to other sulfur compounds by different mechanisms than alliinase. These products provide larger amounts of other sulfur derivatives such as γ-glutamyl-S-allylcysteine (GSAC) and S-allylcysteine (SAC), which show similar antioxidant effects as allicin.
• Garlic supplements are typically provided as garlic powder in capsule form in doses equivalent to 1-2 grams of fresh raw garlic, either with or without an enteric coating. Some are standardized by allicin content.
  • Some garlic supplements are given an enteric-coating in order to protect the alliinase from damage by stomach acid. However, the study concluded that when the enteric-coated supplement was consumed with a high protein meal, alliinase was actually more susceptible to inhibition by stomach acid because the pill remains in the stomach longer. Consuming a high-protein meal raises stomach pH to a level where the alliinase is active, therefore for maximum allicin effectiveness take a non-enteric garlic supplement with a high-protein meal.

*It is always advised to check with your doctor before consuming supplements of any kind, especially if you are on any medications. Although it is generally considered to be safe, garlic may cause a burning sensation, gas, nausea, upset stomach, and may increase the risk of bleeding.

What are the benefits of allicin?

Allicin is capable of entering cells easily due to its lipophilic nature where it can oxidize proteins and other compounds like glutathione. So why exactly is garlic beneficial?

Before we can answer that question, we need to understand what glutathione is. Glutathione is a tripeptide, or three amino acids bound together. It is composed of glutamic acid, cysteine, and glycine. The cysteine portion contains the sulfhydryl group (-SH), which makes this molecule a thiol.

Glutathione is the body’s major innate antioxidant. It exists in two forms:

• Reduced glutathione (GSH) is the more active antioxidant form.
• Oxidized glutathione (GSSG) is two glutathione molecules bound together.

In resting, healthy cells, GSH is over 100 times more concentrated than the oxidized form. When cells are exposed to oxidative stress, GSSG can actually outnumber GSH by about 10 to 1.

Allicin can oxidize glutathione, lowering cellular concentration of GSH. The results of this study confirmed this effect. However, this result is only temporary and what happens next will have garlic lovers rejoicing. After the initial decrease in reduced glutathione, cellular glutathione levels actually increased up to 4 times the amount of the normal level. Furthermore, the study found that allicin enhanced gene expression related to stress response, synthesis of glutathione, and production of phase II detoxification enzymes, which help the body neutralize and remove toxic compounds from cells.

The byproducts of allicin’s reaction with thiols such as glutathione and cysteine were also found to have antioxidant properties themselves!

• Glutathione oxidized by allicin forms S-allylmercaptoglutathione (GSSA)
• Cysteine oxidized by allicin forms S-allylmercaptocysteine (CSSA)

GSSA and CSSA were found to increase overall cellular glutathione levels by a measure of 15 times the amount of allicin itself, a finding that suggests that allicin’s reactivity with cellular thiols is one of the main drivers of its antioxidant boosting activity.

Finally, this research showed that repeated additions of allicin were able to raise the baseline glutathione levels by a factor of 5, making a great case for regular garlic consumption.

Garlic is a Superfood That Lives Up to its Reputation

In conclusion, it’s no coincidence that the culinary and medicinal use of garlic has stood the test of time. From ancient Greece to modern kitchens, garlic is a widely available food that is well-loved for its flavor and therapeutic benefits. Allicin’s antioxidant effect is reminiscent of exercise. After an initial increase in oxidative stress, cells respond by boosting metabolic function that actually increases cellular antioxidant activity, reducing inflammation and oxidative damage overall. Whether you enjoy it raw, roasted, powdered, or aged, garlic is a delicious way to boost your body’s innate antioxidant potential.

Can’t wait to incorporate more garlic into your life? Try these delicious NTI recipes: 3 Sisters Mexican Stew, Sunflower Pâté & Simple Mint Pea Dip, or get into the fall spirit with this Autumn Harvest Soup.

Want to take your garlic game beyond the kitchen? Check out these 7 Surprising Uses for Garlic.

• “Allicin Bioavailability and Bioequivalence from Garlic Supplements and Garlic Foods”

About the author: Karyn Lane is working towards her holistic nutrition certification in NTI’s Nutrition Therapist Master Program. She finds her chemistry degree a useful tool in her study of holistic nutrition and loves to treat herself as a laboratory for new recipes and cooking techniques. You can follow her on Instagram @feel.alive.nourishment.

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References:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6073756/

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