Garlic has an enzyme called alliinase that converts sulphur-containing compounds in the garlic into forms that your taste receptors recognise as hot or spicy (alliin > allicin & diallyl disulfide, other compounds).
The alliinase is released and becomes active when the cells in the bulb are disrupted by cutting, chopping, crushing, etc. Normally alliinase activity is reduced in an acidic environment like the kalamata olive brine - though since the cloves were left whole, the brine didn't fully penetrate and interact with the alliinase, allowing it to form its products inside the cloves.
The same principle is used for making aioli, where garlic is mashed with a mortar and pestle. Time affects how quickly it reacts - in dry garlic powder, you can get full alliin > allicin conversion in 30 seconds [i] after adding water.
You can easily replicate this by crushing and rolling the cloves to break up cells and let them sit in plain water in the fridge, or crush and paste them and add vinegar to stop the enzyme activity once the spice reaches your preference. Use fresher garlic for faster/more consistent activity, as the amount of time affects how much alliinase remains active; conversely, use older garlic for slower or reduced spice.
You can easily replicate this by doing exactly what you did the first time - put peeled garlic in the olive brine. If the olives were originally displayed at room temperature when you bought them, then the brine is formulated with enough added salt and vinegar for osmotic stress and acidity to inhibit C. botulinum, and little enough oil to not form a gas barrier at the top.
Add refrigeration temperatures and free oxygen exchange with the aqueous part of the brine after opening, and the risk profile for C. botulinum toxin formation is similar to jarred capers - negliglble.
In the comments below:
Wow thanks for the quick reply Borky. So what caused the allicin to form since I put the cloves in whole and didn't crush or slice? Was it the salt in the brine?
That's a good question, and one that I don't know for sure, but can guess at:
- Like you mentioned, the salt in the brine could be the cause - the osmotic stress could cause the cells themselves to leak while the cell wall structure remains intact due to the acidity strengthening the pectins present;
- The olive oil could have interfered with the cell phospholipid layer, also causing leakage. This can be seen with some herb-in-oil mixes where internal components end up in the oil carrier solution.
- Any slight bruising or damage to the garlic during peeling and handling could trigger alliin release - the allicin is part of garlic's natural defense response to pests.
The acidity from the vinegar affects the cell membranes, releasing the alliinase and precursor compounds. This mechanism is used in a northern Chinese pickled garlic food, "Laba" garlic [ii]. Laba has a milder spice due to the use of undiluted vinegar with a pH around 2 - a relatively small amount of allicin is immediately produced before enzymatic activity halts, then breaks down into diallyl disulfide and other less reactive compounds.
[...]As compared to the unprocessed garlic (0 day), most organosulfur compounds showed a significant change from day 3. This change could be
explained with the damage of garlic cell membrane by vinegar.
According to the report of Bing et al.,8,9 the permeability of both
plasma membrane and intracellular membrane of garlic are improved
after the garlic being soaked in vinegar. Thus, alliin and alliinase
in garlic react rapidly to form allicin. -Liu et al. [ii]
In contrast, the kalamata brine likely has vinegar at <4% formulation, yielding a brine pH roughly between 4-5. This is still a range where alliinase is active, though less optimally.
Fig 3. Effect of pH (A) on the alliinase activity at 25°C.[iii]
Further reading and references:
Garlic and Organosulfur Compounds. Jane Higdon, Ph.D., Victoria J. Drake, Ph.D., Barbara Delage, Ph.D., Karin Ried, Ph.D., MSc. Linus Pauling Institute, Oregon State University. https://lpi.oregonstate.edu/mic/food-beverages/garlic
Pungent products from garlic activate the sensory ion channel TRPA1. Diana M. Bautista, Pouya Movahed, Andrew Hinman, Helena E. Axelsson, Olov Sterner, Edward D. Högestätt, David Julius, Sven-Eric Jordt, and Peter M. Zygmunt.
[i] Allicin Bioavailability and Bioequivalence from Garlic Supplements and Garlic Foods.
Larry D. Lawson, Scott M. Hunsaker.
Storage time and temperature affects volatile organic compound profile, alliinase activity and postharvest quality of garlic.
Richard A. Ludlow, Marianna Pacenza, Adriana Chiappetta, Sarah R. Christofides, GarethEvanscMichaelGrazcGraciaMarticHilary J.RogersaCarsten T.Müllera
[ii] Investigation of the dynamic changes in the chemical constituents of Chinese “Laba” garlic during traditional processing.
Jian Liu, Wei Guo, Minli Yang, Lixia Liu, Shengxiong Huang, Liang Tao, Feng Zhang, and Yongsheng Liu.
[iii] Effect of physicochemical parameters on the stability and activity of garlic alliinase and its use for in-situ allicin synthesis.
Petra Janská, Zdeněk Knejzlík, Ayyappasamy Sudalaiyadum Perumal, Radek Jurok, Viola Tokárová, Dan V. Nicolau, František Štěpánek, Ondřej Kašpar.