Sunday, 12 January 2014

Talk : The Flavour of Food

A recent UoN Public Science lecture featured a fascinating talk by Dr Ian Fisk (Lecturer in Food Chemistry, University of Nottingham). The talk was entitled "The Flavour of Food" and gave an insight into how flavour is affected by the combination, level and physical form of the ingredients used in foods. This post is based on the talk, with a few extra links etc thrown in.

Dr Fisk began be pointing out that flavour was, to a large extent, a function of aroma and that humans were sensitive to some 5,000 to 10,000 different volatile compounds. As well as the basic tastes of sweet, salt, sour, bitter and umamni there were also a number of other factors at play. For example, people can taste the difference between low and high fat contents in foods, and between food from an iron and a steel spoon!

It was also pointed out that the amount of a flavour compound could have a big effect on the way it was perceived. For example, the compound Trans-non-2-enal is perceived as woody at a level of ~1ppb but if the level increases it is perceived as fatty, unpleasant and finally cucumber at 1000ppb!

Wood and Cucumber aroma - from the same chemical !

Surprisingly, a number of culinary flavours are actually sulphur based compounds, which is not an element generally associated with nice aromas. For example there is 3-mercapto-3-methylbutyl acetate (in coffee); Allyl isothiocyanate (in mustard); allyl disulphide (in garlic) and Dimethyl sulfide (in cabbage).

The human aroma detection system is so subtle that, in some cases, it can even detect different isomers of a chemical, with the classic example being that of Carvone, which is perceived as mint or as caraway, depending on whether the molecule is left or right handed.

Dr Fisk also talked about some of the aromas developed during the cooking process, perhaps the most interesting of which was the complex Maillard reaction that occurs at high temperatures (>140C), often in conjunction with the browning process, and which gives roasted food its characteristic flavours. (see also here, here and here)

That crust is the Maillard Reaction, oh yes!

At these high temperatures, of course, one is also on the verge of burning food, which is a process known as Pyrolysis

The talk also included a "practical" in which the attendees were presented with cottom buds soaked in one of three different chemicals and asked to identify the aroma.

The three chemicals turned out to be Ethyl Butyrate (Pineapple aroma); cis-3-hexanol (Grass aroma) and Butyric Acid (Butter aroma)

It was noticeable that, when presented without any context on a cotton bud, some people found it difficult to identify the aromas. But when told what the aroma "should" be (ie. given a context) if became difficult to imagine NOT recognising the aromas.

As a punchline, Dr Fisk explained that, when the three aromas were combined, they produced a combined smell similar to strawberries!

One approach used by the food industry to keep volatile aromas locked away until required is to place aromatic compounds in other ingredients that are glassy at room temperature, but become rubbery on heating - which then allows the aromatic compounds to be released.

Salt Particle Size
Sodium (usually taken up as salt) is a significant problem in Western diets, so food scientists are trying to reduce the amount of salt used in foods as far as possible.

In some foods, such as bread and cheese, salt has a "structural" function and is difficult to remove. But in other foods, such as crisps, there is more room for manoeuvre.

Dr Fisk pointed out that there is a complex pathway between eating a crisp and perceiving its flavour. First the crisp has to be chewed to form a bolus, then the salt has to pass into the saliva, then diffuse to the taste receptors, and finally to actually be perceived as a taste in the brain.

The talk then looked at some interesting recent research that has been performed by the department on the effect of size of salt crystals on saltiness perception. The work is reported in Impact of salt crystal size on in-mouth delivery of Sodium and saltiness perception from snack foods

(NB: another, related, paper from the team can be found here and the work is also reported in a trade journal here.)

It is perhaps worth looking at the work in a little a little detail.

The salt (which, heartwarmingly, was from the longstanding SAXA salt company) was ground with a mortar and pestle and then sieved to produce three grades of salt powder:

S1 : less than 106micron
S2 : 106-425micron
S3 : 425-710micron

The unsalted crisps that the salt was to be applied to came from a supermarket and were sorted with only those crisps weighing 0.7 to 0.9g and being 45-55mm in diameter were used.

[NB imagines that selection process going something like this: That ones is too big [munch], this one is too small [munch], too big [munch], ok, too small [munch], just right, oh dear it appears to have broken [munch]...[munch]...]

Be that as it may, three packets of 100g crisps were prepared, to which were added 2.5g of the S1, S2 and S3 salt samples respectively

For each crisp type, some trained test subjects chewed the crisps three times and then kept the bolus of chewed crisp and saliva in their mouths for 65 seconds, during which time samples of saliva were taken every 5 seconds.

The results, which are summarised in the chart below, clearly show that the fine grained salt sample produced more saltiness, more quickly - meaning that less salt is required to achieve a given level of flavour. Given that most people do not chew longer than a few seconds, any salt released after this time is simple wasted as flavouring - but will still contribute to adverse health effects.

Get more Saltiness, more quickly, by using finer salt grains
(after Impact of salt crystal size on in-mouth delivery of
Sodium and saltiness perception from snack foods, 2012)

Dr Fisk also discussed some of the technology used by the Food Science Group at their home in the Samworth Flavour Laboratory. The key pieces of equipment is the Atmospheric Pressure Chemical Ionisation - Mass Spectroscopy and the Proton Transfer Reaction Mass Spectroscopy machines, which are used to measure aromas in the air as it is inhaled and exhaled from the nose. Interestingly, this will also include chemicals such as Acetone which are formed by the human body and released into the exhaled air

Winningly, the group also has access to Elecroglottography, which is a non-invasive method of investigating the process of swallowing, as shown in this rather interesting paper by an equipment manufacturer.

Not to be confused with...
Despite his name, award winning 1980s rap star Flavor Flav is not a professional Food Scientist.

Flavor Flav - award winning musician,
but unlikely to be able to provide Food Science advice

Related Links
List of Aroma Compounds (seriously, check this out !)

Image Sources
Wood, Cucumber, Bread, Flavor Flav

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