In the food and flavoring industries, micro-encapsulation of flavors has turned out to be highly significant. It involves the blending of liquid flavors using a carrier to obtain a dry flavor powder, which is easier to handle compared to the liquid form. This method allows controlled release of flavors during food processing and storage, offers protection against degradative reactions, and prevents the loss of flavor. To date, gas chromatographic (GC) techniques have been largely employed to determine individual components of essential oils.
Raman spectroscopy has turned out to be an increasingly significant instrument for characterizing encapsulated flavors. This article investigates the suitability of Raman spectroscopy for the distribution of limonene—the main component in citrus oils—in spray-dried particles, and also its quantification.
Distribution of the Flavor Within Micro-Particles
Spray Drying
This technique involves producing a dry powder from a slurry or liquid by quickly drying with a hot gas, which is the primary encapsulation process of flavors. Effective spray drying leads to a homogeneous distribution of the flavor inside the carrier, which is crucial for ensuring an ideally controlled release of the flavor. A 30-µm spray-dried particle comprising of limonene (flavor) and maltodextrin (carrier) was mapped and investigated by Raman micro-spectroscopy for analyzing the flavor distribution on the microscopic scale (Figure 1).
Figure 1. Microscopic image of spray-dried sample (a), distribution of limonene (b) and maltodextrin (c) according to DCLS model, spectra of limonene (d), and maltodextrin (e)
The formation of chemical images demonstrating the distribution of every compound allows monitoring of the spectral features of limonene and maltodextrin within the mapping dataset. The chemical image of maltodextrin depicts homogeneous coverage over the particle: this is logical as maltodextrin is the carrier. More informative is the relatively uniform distribution of limonene, indicating an effective spray-drying process.
Quantitative Determination of the Encapsulated Limonene
It is important to quantify the amount of flavor in the dried powder for monitoring the stability of flavors in micro-particles over time. Although conventional gas chromatography–flame ionization detector (GC-FID) methods are the common procedures for analyzing these contents, they are tedious compared to Raman spectroscopy. After a simple extraction of the limonene in cyclohexane, a calibration curve on the basis of Raman spectra of these solutions was created for the concentration range of 2.0%–16.7%, by relating the area of a limonene peak with the concentration of the standard samples (Figure 2).
Figure 2. Calibration curve for limonene in cyclohexane (Aaverage is the area under peak at 1678 cm-1).
A seven-sample test set investigated with the GC-FID reference technique and the Raman calibration provided the same results with the two methods (Figure 3). The concentration range of limonene in the spray-dried particles that were studied was found to be 18.5%-–23.0% (wlimonene/wpowder). The relative difference in the limonene content obtained with both techniques was within 5.0%.
The concentrations given by the Raman technique illustrated in the table are outside the calibration range as they were re-calculated to consider the dilution factor inserted during the preparation of the samples. When compared to current GC-FID technique, the primary benefit of RS technique is its ease of use, simplicity, speed, non-destructive nature.
| |
Raman |
GC-FID |
| Sample 1 |
22,6 |
23,7 |
| Sample 2 |
20,9 |
19,9 |
| Sample 3 |
21,1 |
21,9 |
| Sample 4 |
18,5 |
18,9 |
| Sample 5 |
19,5 |
18,5 |
| Sample 6 |
20,5 |
20,5 |
| Sample 7 |
21,3 |
20,7 |
| Average |
20,6 |
20,6 |
Figure 3. Limonene concentration determinations of a 7-sample test set using both GCFID and Raman techniques.
Summary
Raman spectroscopy has proven to be a reliable, fast, and robust method that can be employed for quantifying limonene and demonstrate its distribution in spray-dried samples. On the basis of these determinations, the use of Raman spectroscopy for monitoring the stability of flavors in microparticles can be envisioned.
This information has been sourced, reviewed and adapted from materials provided by HORIBA.
For more information on this source, please visit HORIBA.