Human beings eat food for both nutritional value and some due to medicinal components.  One of the most important plant species that has been consumed by human beings due to its nutritional and medical components is garlic (Fulder). This study will ensure that usage of garlic as an anti-bacterium is maximized by determining the most effective form in which garlic is in killing bactericidal. The study involved introducing 0.5ml of homogenized cooked garlic paste into agar plates containing bacterial lawns of Escherichia coli, Bacillus subtilis, Erwinia caratovora, Staphylococcus aureus and Sacchromyces cerevisiae. The garlic had been pre-boiled for 10mins, 15mins, 30mins and 60mins. The agar plates were placed in the incubator at 37C for a week.  The agar plates were then measured for areas of bacterial growth inhibition. The findings were that longer cooked garlic showed more areas of inhibition for all the bacterial species studied. The results were analyzed using 2 WAY ANOVA Tests and indicated that there was no significant difference in the level of bacteria inhibition between the various bacterial lawns. The homogenized garlic paste inhibited growth of all bacteria species in the study. Additionally, the longer cooked garlic seemed to be more potent than the garlic cooked for a shorter time.

Human beings eat food for both nutritional value and medicinal components.  One of the most important plant species that has been consumed by human beings due to its nutritional and medicinal components is garlic (Fulder).  Garlic is commonly available in most parts of the world and it is cheap to get it in areas where it does not grow due to it being “extensively domesticated over so many thousands of years that there no longer are wild forms found anywhere in nature”(Huber). Unlike other medicinal foods that have been used by man, garlic is known to have the ability to kill and inhibit parasites, bacteria, fungi, lower blood sugar, lower blood cholesterol, prevent tumor, prevent blood clotting in veins, boost immune system and improve the general body health among other benefits that are not yet scientifically proven (Bergner). A study funded by the Ontario Ministry of Innovation and the Natural Sciences and Engineering Research Council of Canada (NSERC) involving a research team from Queen’s Chemistry and the National Research Council of Canada are reported to have found the reason why garlic is so effective. Apparently, experiments using synthetically-produced allicin showed allicin decomposes rapidly producing an acid that reacts with radicals, giving garlic its medicinal properties (Science Daily).

Purpose of the research

Garlic has been proven to be a very important food to man in terms of nutrition and medicinal purposes, as Huber reports that “garlic has been employed for medicinal purposes by more cultures over more millennia than any other plant product or substance”.

Tattleman noted that despite extensive studies on the medicinal and nutritional value of garlic, comparisons between these studies and subsequent conclusions are difficult since the studies have been variable. There is also variability in the garlic preparations used for the various studies thus there exists a controversy on which is the best state in which garlic should be used. Some advocate for raw garlic while others advocate cooked garlic.

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The proponents of raw garlic cite the fact that health benefits of garlic are attributed to allicin which is formed when raw garlic is “cut, crushed or chewed” (Stone). However, heating or cooking garlic prevents the formation of allicin (Stone).  Proponents of cooked garlic cite its reduced pungency as a reason to advocate cooking, but advise the crushed or chopped garlic be allowed to stand for at least 10minutes to release as much allicin from it before cooking (Penn State).

Another hotly debated question is the use of cooked versus raw garlic as an antibiotic. Johnston argues that garlic is more effective in killing bacteria when taken raw rather than cooked since cooking garlic in oil destroys allicin unless crushed garlic is left standing for a while before cooking, in which case it will be effective too. This study aims at determining whether cooked garlic is as effective in killing bacteria as raw garlic. The study will also ensure that usage of garlic as an anti-bacterium is maximized by determining the form in which garlic is much effective in killing bacteria. The study also aims at establishing how garlic efficiency in killing bacteria is affected by heat in order to determine whether too much heating of garlic while cooking can lower its ant-bacterium components.  By completing this study, the researcher aims at making an inclusive conclusion on whether garlic is effective as an anti-bacterium when cooked.

Findings from the literature review

Garlic is scientifically known as Allium sativum. It is a species in the onion genus allium. It is known for its use as a natural antibiotic and for its use as an antibacterial agent and Science Daily states that due to the organic compound allicin, garlic “acts as the world’s most powerful antioxidant”. Historically, it was used to cure diseases caused by bacteria, virus and fungi (O’Mathúna & Larimore). It is also one of the most used species in cooking in many societies as it makes dishes more delicious. –ref. Over and above its nutritional value, garlic is important as it has impressive medicinal and health benefits as Ahmed  reports that garlic, as well as other allium plants has been proven to potent against “bacteria, fungus, viruses, protozoa, helminthes, tumors and thrombosis”. Garlic’s substances can be used for both curative and preventive infections (Fulder).

Other than serving as an antibiotic, garlic also provides people with potassium, vitamin C, vitamin B6, manganese and Selenium (Fulder).  Garlic also contains sulphur compounds such as allicin which is responsible for giving garlic its notable smell and flavor (Garlic and Cancer Prevention). When garlic is added to a diet, in cooked form, it will not have the same health properties as raw garlic due to the aforementioned destruction of allicin by heat. This is normally usual to the nutritional value of any food substance when cooked.

Garlic is known to contain many different medicinal benefits. One of the best known is its ability to kill and prevent bacteria that affect human beings when used as a natural antibiotic against Escherichia coli (Sohn et al).

Garlic is believed to be a broad spectrum antibiotic with the ability to inhibit and kill majority of the bacteria that affect human beings (O’Mathúna & Larimore).  According to a report by a prominent garlic researcher, Dr. Tariq Abdullah, “garlic has the broadest spectrum of antimicrobial substance that we know of; it is antibacterial, antifungal, anti-parasitic, antiviral and anti-protozoa”.  It is therefore proven beyond any doubt that garlic is among the leading medicinal plants consumed by man. Garlic can be consumed in different forms ranging from raw, cooked, as juice and capsules depending on the purpose, and taste of the user (Fulder).

Garlic is also used to fight cancer as it contains components that block the ability of carcinogens to mutate healthy cells into affected ones, is anti-toxic as it stimulates cellular detoxification (Meredith). Additionally, it is also used as decongestant to get rid of the respiratory tract of mucus (Meredith). Other medicinal properties of garlic include its ability to purify blood, to control blood glucose, ability to treat blood pressure, swellings and inflammatory infections, as anti-parasitic, an anticoagulant, prevents blood clots in veins and to help the body’s immune system (Fulder).  Crushing garlic releases an ointment that can be used for curing wounds, cuts, athlete’s foot, or any other external skin irritation, fungus, or infection. In case of a tooth ache, the application of a few drops of oil will help in easing the pain (Bakhru).

Major forms that garlic is used include capsules, in form of juice, fresh rub, aged oil and in form of Maceration (O’Mathúna & Larimore). Another form that garlic is used includes garlic infused in wine, vinegar, honey and other drinks. Although there is no précised storage measures for garlic, fresh garlic should be stored in a cool, dry place away from light (Fulder).

Ankri and Mirelman stated that allicin has a “chemcial reaction with thiol groups of various enzymes, e.g. alcohol dehydrogenase, thioredoxin reductase, and RNA polymerase, which can affect essential metabolism of cysteine proteinase activity involved in the virulence of E. histolytica”. Moreover, in their study they found that allicin exhibited antibacterial activity against a wide range of bacteria, antifungal activity, antiparasitic activity, and antiviral activity (Ankri and Mirelman)

Laboratory Research

Objectives: The experiment was conducted to determine whether cooked garlic will have any effect on the growth of bacteria. The intention is to determine the ability of cooked garlic to serve as anti- bacteria inhibitor.  If cooked garlic supports growth of bacteria, it means that subjecting garlic to high temperature damages it medicinal components and therefore cooked garlic is not effective in killing bacteria. On the other hand, if bacteria fails to grow in presence of cooked garlic it means that cooked garlic is still effective in killing bacteria. The experiment is based on the knowledge that garlic contains allicin, that stimulates the immune system of the body thereby increasing the number of white blood cells (Meredith). This component is believed to be less effective after being subjected to heat as heat prevents the allicin from reacting with the thiol enzymes produced by microorganisms.

Hypothesis – Garlic cloves were cooked for 3, 10, 15, 30 and 60mins will inhibit bacterial growth.

Fresh garlic will be the most effective. Cooked garlic and sterile water will be effective even when cooked at 3, 10, 15, 30 and 60mins at 37C temperature range.

Methods and Materials

28 bacterial lawns (Escherichia coli, Bacillus subtilis, Erwinia caratovora, Staphylococcus aureus and Sacchromyces cerevisiae were placed in 28 sterile nutrient agar plates, and 7 sterile malt agar plates. –confusing, did  you produce 28 lawns of mixed bacterial lawns. Sterile water and homogenized garlic paste were used.

Two fresh garlic cloves were homogenised to form a paste.

Two garlic cloves were   boiled for 3 minutes and then homogenised to form a paste. This method was repeated with 2 fresh garlic cloves at each of the following temperatures  10mins, 15mins, 30mins and 60mins.

Bacterial lawns of the bacteria listed (Escherichia coli, Bacillus subtilis, Erwinia caratovora, Staphylococcus aureus and Sacchromyces cerevisiae (fungi) were produced following aseptic technique. The well method was used to add 0.5ml of paste to each four well in each agar plate. The size of the well was 25mm.

The procedure was repeated for control but instead of paste, 0.5ml of water was added to each of the four well in each agar plate.

The agar plates were labelled and taped.  They were put in the incubator at 37C  for a week.


Table 1: The zones of inhibition showing effect of incubator heat on the activity of garlic on various microorganisms using the well method.

Are these mean results?

Time Garlic is Cooked
Bacteria organisms 3mins 10mins 15mins 30mins 60mins Fresh Control
S. cerevisiae 34mm Clear 17mm Clear Clear 30.5 0
E.coli 18.25mm 18mm 25.75mm 23.75mm 21.21mm 16mm 0
Staphylo aureus 7.25mm 5.75mm 4.75mm 21.25mm 20.5mm 9.5mm 0
Bacillus subtilis 13.5mm 18mm 4.25mm 19.5mm 23.5mm 15.25mm 0
Erwinia caratovora 15mm 16.5mm 5.5mm 24.75mm 11.5mm 15.75mm 0


The results indicated that bacterial inhibition was less than in fresh garlic in all experiments. However the garlic cooked for 15 minutes showed more effectiveness than fresh garlic. This could be the result of an error in the procedure.

Additionally, the other cooked garlic trials showed inhibition of presence of bacteria, indicating that cooked garlic still retains the ability to inhibit bacterial activity. Cooked garlic released polysulfide compounds that break down into Di Ally Tri-Sulfide that breaks down into the much more stable Di Ally Di- Sulfide (Pommerville). Therefore the allicin component in garlic could have weakened due to garlic being cooked.

The results are in agreement with research conducted by other researchers indicating that the heat reduces the potency of the allicin compound to inhibit bacterial growth. However, research showed that the medicinal value of garlic is hardly affected by cooking especially when the crushed or chopped garlic is left to stand for a while. There has been some proven evidence that some antibacterial components of garlic are not affected by heat (Bergner).

The use of the cooked garlic for killing bacteria is less effective as compared to the use of the raw ones (Johnston). The effect of raw garlic has been tested on the most common types of food borne bacteria, such as Salmonella, Shigella and Listeria. Raw garlic extracts were created and then boiled for about 15 minutes. The boiled extracts were added to the dishes that had the bacteria Salmonella, Shigella and Listeria. After about a day, bacteria inhibited zones that were free of bacteria surrounded either the spots that had the raw garlic or the cooked ones. But comparisons between the zones constituting the raw garlic and the cooked ones showed in confirming that the dead zones for the raw extracts was about twice as large as that of the cooked ones.

Medical point of view

For medicinal purposes, these results indicate that cooking garlic reduces its potency on bacteria thus it is best eaten raw. Bioessential Nutrition recommends that “raw garlic or garlic supplements are more effective than cooked garlic, because heat quickly destroys the alliinase, stunting the conversion of allicin”.  As previously stated, chopped or crushed garlic releases the enzyme alliinase that generates allicin, garlic’s most important ingredient. However, Blaylock notes that “heat destroys the enzyme and prevents the formation of allicin”. Therefore, the effect of heat is to destroy the enzyme making it impossible for allicin to be produced, thus when such garlic is eaten, it cannot yield the medicinal benefits. This is a rational conclusion since it is well known that enzymes are destroyed by heat. Since garlic’s allicin is made from the enzyme alliinase, it is therefore likely that destruction of alliinase will result in non-formation of allicin, reducing the potency of the garlic.

Moreover, further research indicating that heat destroys garlic’s medicinal properties was conducted by Galmarini et al showed that “crushing garlic before cooking can reduce the loss of garlic’s healthful properties”. The study suggested that cooking “inactivates the enzyme alliinase which catalyses the formation of allicin (allyl 2-propenylthiosulfinate), which then breaks down to form a variety of healthful organosulfur compound”. Therefore, they concluded that crushing or chopping garlic will release the alliinase that triggers the formation of allicin before being inactivated by heat. Verma et al, reported that “boiling unpeeled whole clove for 15minutes completely inactivates alliinase. However,

before the alliinase is inactivated, about 0.5-1% of the alliin is converted to allicin, possibly due to the cloves bumping into each other, which is than rapidly converted mainly to diallyltrisulphide and to smaller amounts of the di and tetra sulfides.”

It has been noted that letting crushed garlic stand for a period of time before cooking it makes it more potent than garlic cooked immediately after crushing. With more time, this study would have explored the potency of garlic that has been crushed and left standing for various times. This would help establish how long the garlic should be left standing in order to give maximum effect on fighting the microorganisms.   Although cooking garlic may lower its nutritional value, it does not completely damage its antibacterial properties (Meredith). It is therefore evident from the study that cooked garlic is still effective in killing bacteria although too much cooking may lower its effectiveness.

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