Breathe deep, tea drinkers.

Sorry for my lack of posting over the last few weeks, but I have had the amazing opportunity to go to the UK for 10 weeks for research, and spent last week recovering from jet lag, learning English slang (like ‘having a nosey’) and setting myself up! Anywho, it’s QJART time!

Anyone who is a lover of tea, or has visited the tea shop T2, would recognise that different teas have different characteristic aromas, which depend on the leaf type and manufacturing process. But what makes up the smell of green tea?

green tea

A nice green tea in a green cup with a green teapot… So green, must be environmentally friendly! 😉

Three cultivars of Chinese green tea (Longjing, Maofeng, and Biluochun) were analysed by researchers in Japan to identify which volatile compounds make up the characteristic aroma of green teas. They began by first extracting the volatiles from the tea infusions, using a method called ‘SAFE’ (Solvent Assisted Flavour Evaporation).

SAFE unit

The distillation unit used in SAFE is quite complex!

This method involves first the solvent extraction of the volatiles from the teas, followed by the use of a specialised glassware and high vacuum pump system to extract the tea volatiles from the solvent. The volatiles are then concentrated to give the SAFE extract, which can be used in gas chromatography-olfactory (GC-O) (an instrument which first separates the volatile components and then allows you to sniff these volatiles through an attached ‘nose’).

GCO

See that in front of her nose? That’s the ‘nose’ where you smell the separated volatiles!

To identify which volatiles contributed the most to the green tea aroma, the authors used Aroma Extract Dilution Analysis (AEDA). In AEDA, the SAFE extract is diluted a number of times, to give different Flavour Dilution (FD) factors. For example, diluting the original sample by four in solvent will give an FD of 4. Diluting again will give an FD of 16, and continuing on will give 64, 256, 1024, and so on, multiplying by four each time. The whole idea behind these dilutions is that the concentration of the individual components should become weaker and weaker with each dilution. Therefore, if we can still smell a particular volatile at the highest FD factors using GC-O, then it is associated with being a major component of the aroma.

Fifty eight odour-active peaks (separated volatile components which had a smell) were identified in the teas, at different concentrations in the different cultivars. Of these, seven had the highest FD factors in all tea cultivars, and are therefore believed essential for the aroma of Chinese green tea, including vanillin (smells like vanilla), geraniol (smells ‘green’), and (E)-isoeugenol (smells floral or spicy). The authors further suggested that (E)-isoeugenol, which was newly identified in Chinese green tea, was a product of the manufacturing process rather than the leaves themselves.

E-isoeugenol

(E)-Isoeugenol smells floral or spicy, and is a volatile found in the aroma of green tea.

Next time you sit down with your cuppa, take in a deep breath through the nose, and admire those volatiles.

Baba R, Kumazawa K (2014) Characterization of the Potent Odorants Contributing to the Characteristic Aroma of Chinese Green Tea Infusions by Aromatic Extract Dilution Analysis. Journal of Agricultural and Food Chemistry 62: 8308-8313

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Can you smell that? Mmmmm, garlic.

A few nights back, I went to an Indian restaurant with my family. I do enjoy Indian food, but my favourite would have to be garlic naan! I also love garlic bread and garlic pizzas (are you sensing a theme here?).

garlic

Mmmmmm… But maybe not raw.

The garlic naan on this particular night, however, was a little bit tooooo garlicky. Some family members complained that it was spicy, as though pepper or chilli had accidentally been placed on top. However, this was just the effect of too much garlic! The chemical component of garlic which is responsible for this burning, spicy taste, is allicin.

Interestingly, allicin is not present naturally in garlic. When garlic is crushed or cut, enzymes then convert the compound alliin to allicin, which provides that well-known aroma. The use of heat with garlic will also cause this conversion, which explains why we smell that strong smell when cooking up garlic and onion for a Bolognese sauce, and not so much when the clove is sitting whole in our kitchen.

 800px-R-allicin-2D

Allicin is only found once garlic is chopped or cut!

As for the other effect of garlic, bad breath, that is actually due mainly to four different chemical compounds, which are once again only observable in garlic once we cut, crush or chop it: diallyl disulfide, allyl methyl sulfide, allyl mercaptan and allyl methyl disulfide. You can find the structures on this great infographic from Compound Interest.

 The-Chemistry-of-Garlic

Of course, one of my favourite sites ever has an infographic on one of my favourite spices ever!

One thing you will notice in these structures is that they each contain an ‘S’. This ‘S’ stands for sulphur (or sulfur depending on where you come from!). Other things that you may think about when you hear are rotten eggs (the smell of which is made of hydrogen sulphide, H2S) and fire/ volcanoes (‘brimstone’ is the ancient name for sulphur). Methanethiol, another sulphur compound, is believed to be one of those responsible for smelly urine after eating asparagus. But it’s not all bad news. Saccharin (an artificial sweetener), proteins, and penicillin all contain sulfur, and they’re not smelly… well at least not while they’re fresh.

It is thought that our dislike of sulfur-containing volatile compounds is due to our need to avoid unfresh foods. Some sulfur-containing compounds are produced when proteins in putrid food break down. Then the human body is able to detect these compounds, even at quite low concentrations, so that we know to avoid that particular food.

But (potentially) good news everyone. There are a couple of foods known to eliminate that lovely smell for when you want to kiss your loved ones or tone down the ‘spice’ of a meal. In cooking, you can use tricks such as roasting the garlic or leaving it to sit for ten minutes after crushing, or by adding potato or parsley to your meal. Parsley or milk, as well as as-much-gum-and-mouthwash-and-mints-as-you-can-handle are among your options for bad breath.

 parsley

Chow down on a sprig of parsley, it’s supposed to help!

As a side note, some of the researched effects of garlic include antibiotic, anticancer, blood thinning, antiviral, and antifungal effects. So, to stink or not to stink?

Please see this website from the Linus Pauling Institute if you would like some more detailed information on Garlic and it’s health effects.

Sniff Your Soy?

When was the last time you had a sniff of soy sauce? Personally, I don’t think I ever have. Sure, I love pouring soy sauce all over my dumplings when I visit my favourite Dumpling restaurant in Richmond, but I don’t hold the bottle up to my nose and take a whiff. Although the next time I visit XiaoTing Box, I may do just that.

Here is this week’s QJART!

Characterisation of aroma profiles of commercial soy sauce by odour activity value and omission test

Yunzi Feng, Guowan Su, Haifeng Zhao, Yu Cai, Chun Cui, Dongxiao Sun-Waterhouse, Mouming Zhao

Food Chemistry 167 (2015) 220–228

These researchers, from China and New Zealand, looked at the different aroma compounds which make up the smell of soy sauce. They investigated twenty-seven commercial soy sauces, produced through three different fermentation processes; high salt liquid state(HLFSS), low salt solid state (LSFSS) and Koikuchi (KSS).

Soy sauce manufacture

The three fermentation processes use different soybean:flour ratios, salt concentrations, microorganisms, and bacterial fermentation times.

When you are next at your local chinese restaurant, and smell the soy sauce at the table, I’d like you to guess how many different compounds make up that smell. One? No. Ten? No, higher. A billion? Okay, maybe not that many. But this research identified 129 compounds that were volatile (able to enter your nose because they are in their gas form), and of these, more than 41 compounds which produce a smell (“aroma active components”).

dumpling soy sauce

Mmmm, dumplings and soy sauce…

The aim of this research was to identify what impact, if any, the different fermentation processes have on the smell and flavour of soy sauce. The researchers were able to do this by sensory evaluation (where a panel of ten trained judges smell and describe the soy sauce), as well as by analysing the volatile components by Gas-Chromatography-Mass Spectrometry (GC-MS). A GC-MS deserves its own blog post, so while I have not yet described how a GCMS works, I will tell you now that it enables us to separate, identify and quantify these different volatile compounds.

PIC_0059

A photo of a GC-MS that I currently work with.

An interesting point that you will come across in many of my future blog posts is that once we separate out the different chemicals which contribute to the smell of something, we find that they each smell like something that in no way represents the smell of the food as a whole. For example, just a few of the chemical compounds in soy sauce include 3-(methylthio)propanal, guaiacol, benzeneacetaldehyde, and 3-methylbutanal, which on their own would smell like cooked potato, smoke, honey and malt. Another interesting point is that we may not be able to smell some of these volatiles until they are at a certain concentration (known as an odour threshold).

The panel of ten judges for sensory evaluation underwent ‘omission experiments’, where the panel were provided with an aroma ‘model’ (made up of most, but not all, of the aroma compounds of soy sauce) to compare against the complete aroma of soy sauce. In all cases, the panel were able to tell which sample was the complete aroma. This is helpful as it allows the researchers to see which particular aroma compounds (omitted from the aroma model’ make the most impact in the smell of soy sauce.

There were many differences in both the GC results and the sensory evaluation across the different soy sauces and fermentation processes, and the authors noted that the differences in the overall aroma of the soy  sauce was due more to the concentrations of the individual aroma compounds, than the variety of compounds.

I think I know what I want for dinner tonight…