Chlorogenic acids are an important compound group in coffee chemistry, with caffeoylquinic acids commonly tracked by chromatography. When coffee liquid is concentrated, the first question isn't “does CGA increase or decrease” but whether you're talking about concentration or total amount. Removing water from a CGA-containing liquid can certainly increase concentration if the compound is retained.

Distinguishing increased concentration from increased chlorogenic-acid amount

In coffee, CGA composition is strongly influenced by raw material and roast level. A study on commercial coffee found CGA levels vary significantly between samples, with roast level being an important factor. So when assessing concentration, the input sample needs to be clearly defined. If two liquids come from different raw-material sources, the final difference can't be entirely attributed to the concentration process.

How can concentration change apparent content?

So a sample after concentration can show higher CGA content per unit mass while the total CGA amount in the whole stream hasn't increased. These two ways of expressing the result need to be kept separate.

Concentration usually relies on removing water, whether by vacuum evaporation or other techniques. Temperature and the liquid's residence time are two notable variables since CGA isn't a single compound and its isomers can have different stability. A low-temperature process with a very long time can still create a different effect than a high-heat, short-time process.

To distinguish the concentration effect from chemical change, you need to calculate absolute CGA amount before and after concentration. If concentration increases in proportion to the volume-reduction factor while total amount stays nearly unchanged, this can mostly be explained by concentration alone. If total amount decreases or the ratio between isomers changes, loss, decomposition, or chemical transformation needs consideration.

On what basis should chlorogenic acids be measured?

The analytical method also determines the quality of the conclusion. High-performance liquid chromatography or liquid chromatography-mass spectrometry can track 5-CQA and other CQA forms, rather than just measuring a total absorbance index. A study on commercial coffee quantified multiple CQA groups and showed 5-CQA is the dominant form in the samples surveyed. This shows “CGA” as a general term can hide meaningful differences between isomers.

During concentration, the liquid's viscosity can also increase. That changes heat and mass transfer, making operating conditions at the end of the process different from the beginning. If the equipment wall temperature is higher than the liquid's average temperature, the risk of creating overheated zones needs consideration. So residence time and heat-transfer method need to be recorded alongside chemical data.

A good research design should sample before concentration, during the process, and after concentration. Each sample needs recorded stream mass, solids content, liquid volume or mass, and each CGA concentration. From there, you can calculate a mass balance and determine whether the change comes from water loss or matter loss.

Concentration shouldn't be seen as a step that only increases CGA content per gram of product. The value of the data lies in determining how much compound is retained, which structure changes, and what effect the thermal conditions created. When concentration and total amount are clearly distinguished, the concentration process can be assessed on a more solid scientific basis.

A point worth noting is that CGA can be expressed on multiple bases: per volume of liquid, per mass of dry matter, or as absolute amount for the whole stream. If you only look at mg/L, a heavily concentrated liquid will always tend to show a higher value. So before-after studies must report at least both concentration and absolute amount.

Equipment surface temperature and residence time may not be uniform throughout the whole system. If the liquid passes through a strongly heated zone, part of the chemical change can happen locally. Position-based temperature data helps explain chemical results better than a single average batch temperature.

You also need to consider the possibility that CGA forms convert relative to each other. If total amount stays nearly stable but the ratio between 3-CQA, 4-CQA, and 5-CQA changes, reporting only total CGA misses composition information. Chromatographic methods capable of separating each form are therefore valuable for mechanistic research.

Designing a comparison to properly assess concentration effectiveness

When the goal is optimizing concentration, you shouldn't simply aim for maximum CGA retention. Conditions must simultaneously meet final concentration, energy, pumpability, and sensory quality. A liquid with excellent chemical retention but that's too thick or has major aroma loss still isn't the best solution.

Another way to check is comparing absolute chlorogenic-acid amount before and after concentration against the amount of solids recovered. If the compound amount decreases while solids are still fully recovered, that data shows you need to look for a cause beyond the concentration-driven concentration increase. This distinguishing step is necessary before attributing the change to a chemical reaction.

Some notes when reading the data

When comparing concentration conditions, you should keep the same input solids level and the same stream mass where possible. If input amounts differ, the final concentration result isn't enough to determine the process's effectiveness.

To avoid misinterpretation, the analysis should clearly state whether the sample is expressed relative to initial liquid mass, dry-matter mass, or volume. The same sample can give different numbers depending on the basis of expression. Only when the calculation basis is held fixed can the change in chlorogenic acids be compared across concentration points.

References

• Correa et al. (2018), Effect of falling-film freeze concentration on bioactive compounds in aqueous coffee extract, Journal of Food Process Engineering. DOI: 10.1111/jfpe.12606.
• Influence of drying techniques on sensory profile and chlorogenic acid content of instant coffee powders (2022), Measurement: Food, 6, 100030.
• Mills et al. (2013), The effect of processing on chlorogenic acid content of commercially available coffee, Food Chemistry. DOI: 10.1016/j.foodchem.2013.06.014.
• Development of an instant coffee enriched with chlorogenic acids (2016), Journal of Food Science and Technology.