P – as in PPM analysis

What does PPM stand for?

PPM is an abbreviation for "parts per million" and literally means "parts per million".

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What is behind PPM?

Parts per million means, for example, that 1 ppm corresponds to one milligram per kilogram. It becomes clearer if you look at a million small styrofoam balls. If you paint one of them red, then the proportion of this red ball among the other 999,999 white balls is exactly 1 ppm. Or let's consider a 50-liter oak barrel filled to the brim with whisky. One drop from this 50-liter barrel is in the order of 1 ppm. Another example: If we consider a period of 11.6 days, then a single, tiny second in this very long period corresponds to 1 ppm.

What can be measured in ppm?

The proportion of phenols in malt, which has been kilned over peat smoke, is given in ppm. We source this peat-smoked malt directly from Scotland from our supplier Glenesk Maltings in the eastern Highlands. Of course, the phenols in New Make or in bottled whisky can also be determined. However, the PPM value usually refers to the amount of phenols in the peat-smoked malt.

How do phenols get into the malt?

When dried peat is burned, chemical compounds, including phenols, are formed. During the malting process, these highly aromatic molecules rise with the smoke in the kiln and pass through the moist barley malt. Some of the phenols then adhere to the outer shell of the malt grains, the husks, through adsorption.

What kind of phenols are these?

There are about 11 phenolic compounds that play an important role in peat-smoked malt. These include the basic substance phenol, which gave the entire class of substances its name. Furthermore, three methylphenols, the so-called cresols (ortho-, meta-, and para-cresol), are included. Alkylphenols, such as 4-ethyl- and 4-vinylphenol, also play a major role, as does 2,4-xylenol. Finally, the group of guaiacols is important, including guaiacol, 4-methyl- and 4-ethylguaiacol, and syringol.

What properties do these phenols have?

In pure form, phenols are solid, crystalline or liquid compounds that can have a characteristic, smoky, phenolic, pungent, medicinal smell reminiscent of gauze bandages or hospitals. However, the individual phenols have different properties. While phenol and 4-ethylphenol are described as medicinal, guaiacol is considered smoky, tarry, 4-ethylguaiacol as spicy and sweet, and syringol as spicy and sweet-spicy. Among the three different cresols, ortho-cresol is described as medicinal, meta-cresol as smoky, rubbery, and para-cresol as sulfury with a smell of sewage. In addition, there is a certain degree of overlap in the respective aromas. A number of phenolic compounds contribute both smoky and medicinal notes to the aroma, for example.

Does the phenol content change during whisky production?

Yes! Although phenols are extremely heat-resistant, their content decreases during the production and maturation of whisky. This is because the second distillation in particular represents a very critical step for phenols. Depending on the alcohol content at which the second cut point is set, which separates the middle cut from the feints, a (large) part of these aromatic, late-boiling molecules does not end up in the heart of the distillate, but in the low wines and feints receiver. Depending on the cut point, the fresh distillate then only has about 30 to 70% of the ppm value in the peat-smoked malt. In addition, some of the volatile phenols escape from the inside of the cask into the open air during cask maturation through evaporation ("angel's share"). However, some phenols, which are formed as a result of the heat treatment of the cask (toasting, charring) by the thermal degradation of the wood component lignin in the wooden staves, can also be leached out of these by the distillate during cask maturation and thus enter the whisky. Compared to the phenols from the peat-smoked malt, these quantities are relatively small.

How is the peat content in malt measured?

There are various analytical methods for the quantitative determination of the phenol content in peat-smoked malt in ppm. The most established include the colorimetric analysis method and the HPLC method.

What is the colorimetric method?

The colorimetric method, or colorimetry for short, is the longest established method for determining phenol content. It is based on the chemical reaction between individual phenols and special chemicals, which lead to colored reaction products.

How does colorimetry work?

The phenols adhering to the husks of a weighed amount of peat-smoked malt grains are dissolved using a suitable solvent and, in the presence of added reagents (mild oxidizing agent, base), mixed with a special chemical, a color reagent. Its reaction with the various phenols develops a colored solution, which is then analyzed with a special device, the colorimeter.

How do you get from color to phenol content in malt?

Since the intensity of the color formed is directly proportional to the amount of phenols in the peat-smoked malt sample, the phenol content can be determined in this way. The colorimeter is designed to send light through the colored solution and then measure how much light of a very specific wavelength is absorbed by the colored solution. Based on a previously created calibration curve, which was created with color samples of known phenol quantities under the same conditions, the phenol content in the unknown malt sample can be determined in ppm.

Which color reagents are used?

The so-called Folin-Ciocalteu and Emerson reagents are mostly used as color reagents. The former leads to a blue color after reaction with phenols, the latter to a red color. The Emerson reagent is a chemical compound named 4-aminoantipyrine. It was first described by Edgar Emerson in 1943 as a reactant for novel color tests for phenols. The Emerson reagent was also used in the colorimetric method for the analytical determination of phenols in Glenesk peat-smoked malt.

What are the advantages of colorimetry?

The colorimetric analysis method is easy to perform and cost-effective. It delivers relatively fast results and is also suitable for analyzing a large number of samples.

What are the disadvantages of the colorimetric analysis method?

The colorimetric method is only limitedly specific. The widely used Emerson reagent does not react with all phenols. Those that carry special chemical groups at a certain position in the phenol molecule do not undergo a color reaction with the Emerson reagent, including para-cresol and 4-ethylphenol. Therefore, these specific phenols are not recorded in the overall result and are thus not taken into account. In addition, only the total phenol content in peat-smoked malt can be determined by colorimetry. However, this analytical method is not able to determine individual phenols and their individual quantities.

How can individual phenols and their quantities be determined?

This is achieved by a more modern analytical method compared to colorimetry, HPLC. It is the more precise and specific method for determining phenols.

What does HPLC mean?

HPLC is an abbreviation for "High Performance Liquid Chromatography".

How does the HPLC method work?

HPLC is based on the principle of chromatographic separation. In this process, the phenols adhering to the husks of the malt grains from a previously weighed amount of peat-smoked malt are also dissolved with a solvent. A certain amount of this liquid is then injected into the HPLC device. The heart of the HPLC device is a thin stainless steel column. The inside is hollow and filled with a special packing material that has different chemical groups. After injecting the liquid containing the phenols into the column, it is transported through the entire column with a suitable liquid, the mobile phase, and with the help of a pump. The phenols contained in the sample interact more or less intensely with the groups of the column packing material depending on their chemical properties. Depending on their own chemical properties, the phenols adhere very little, little, strongly or very strongly to these groups. Further propelled by the mobile phase flow, the individual phenols appear at the column outlet at different times, each according to their individual contact times with the groups of the column material, are detected by another device, the so-called detector, and are graphically displayed by it using suitable software.

What can be seen on a graphical HPLC printout?

The profile of an HPLC printout, the so-called chromatogram, resembles a mountain and valley ride and looks like the profile of a cycling stage. The individual phenols can each be recognized as a mountain or peak, which has the shape of a Gaussian curve. With the recorded times that the respective phenol needed to reach the end of the column and by comparison with data from a standard solution under the same conditions, the corresponding phenol can be uniquely identified. In addition, the area under the peak, i.e. the Gaussian curve in the chromatogram, gives the quantity of the respective phenol.

What are the advantages of the HPLC method?

This analytical method is very precise and extremely specific, as individual phenols can be identified and their individual quantities determined, in contrast to colorimetry. The only exception: the quantities of the two cresols meta- and para-cresol can only be determined together, as they leave the HPLC column together in most cases. In addition, HPLC is less prone to interference.

Does the HPLC method also have disadvantages?

Yes, because the acquisition and maintenance of the HPLC system are expensive. In addition, the HPLC system can only be operated by specially trained and experienced personnel, which also makes the analyses expensive. Compared to colorimetry, the analysis times per sample are longer with HPLC.

What conclusion can be drawn from the colorimetric method?

Colorimetry is a simple and cost-effective method to determine the total amount of phenols in peat-smoked malt. Due to the lack of specificity of the colorimetric method, the determined PPM values do not match the actual amounts in the sample, but are lower with the Emerson color reagent. Colorimetry is therefore only suitable if a rough idea of the phenol content in peat-smoked malt is sufficient.

What conclusion can be drawn from the HPLC analysis method?

This analytical method identifies the individual phenols in the peat-smoked malt and determines their individual quantity. HPLC is very precise and is the analytical method of choice when an exact characterization of the aroma profiles in peat-smoked malt is required.

Which method is used to determine the phenols in St. Kilian's peat-smoked malt?

Our peat-smoked malt from Glenesk Maltings has so far been analyzed using the colorimetric method with the Emerson color reagent, resulting in a peating level of 54 ppm. Recently, the analysis method was changed. Glenesk now exclusively analyzes the phenol content of their malts using the HPLC method. This is, by the way, also the standard analysis method in Scotland. The more precise HPLC analysis method has now determined a total phenol content of 80 ppm for the same peat-smoked malt.

What phenol content will St. Kilian state in the future?

For the peat-smoked malt that we receive from Glenesk in Scotland to Rüdenau, we will from now on state a peat content of 80 ppm instead of the previous 54 ppm, as this new value is determined with the more precise standard HPLC analysis method.

What are the effects of the change from 54 ppm to 80 ppm?

None! The new, more accurate value of 80 ppm in our peat-smoked malt does not change the taste of our smoky whiskies. Because the peat-smoked malt from the Scottish Highlands is the same and the production conditions in Rüdenau are the same. Only the ppm value is different, because it is more accurate. And we are happy to pass this information on to our customers.