Microcapsules are tiny particles that contain an active agent or core material surrounded by a shell or coating, and are now increasingly being used in food ingredients preparation.
Encapsulation of important essential oils, like mint, citrus and garlic, have been well studied, but research into other essential oils is rarer.
According to Frost and Sullivan, the demand for encapsulation technologies is estimated to be rising at a rate of about 10 per cent every year, with new markets and opportunities opening.
The scientists, from Kaumas University of Technology in Lithuania and Ghent University in Belgium, investigated the effects of microencapsulating oregano, marjoram and citronella in both skimmed milk powder (SMP) and whey protein concentrate (WPC).
The study, published in the journal Food Research International (Vol. 39, pp. 413-425), used WPC and SMP obtained from the Belgina-based Joint Stock Company Belgamilk, and marjoram and citronella were obtained from Wild (Germany). Oregano essential oil was purchased from Germany-based Frey and Lau GmbH.
Using the spray-drying technique of micro-encapsulation, the most commonly used technology in the food industry because of the low costs and availability of equipment, the researchers found that the SMP was the most effective food matrix for encapsulation.
The total oil content of the particles was about eight per cent higher in the SMP for both oregano and marjoram, with over 80 per cent total oil content for both in this matrix. Citronella content was approximately the same for both SMP and WPC.
Encapsulation efficiency, defined as a percentage of flavouring entrapped into the microcapsules, however varied significantly for each of the essential oils. The highest efficiency was for oregano in the skimmed milk powder, with a value of 80.2 per cent efficiency, compared to 71.8 in the WPC.
Citronella encapsulation efficiency was less than 70 per cent for both matrices, while marjoram encapsulation efficiency was only 54.3 per cent in the why protein concentrate.
The composition of the flavourings after micro-encapsulation was also investigated.
When natural flavourings are encapsulated in the different matrices it is important to know the changes in the composition taking place during emulsification and especially spray-drying, explained lead author Renata Baranauskiene.
Encapsulation of the oregano in the SMP did not change significantly the composition of the essential oil. However, significant changes in the composition of citronella and marjoram aroma extracts were determined after processing, said Baranauskiene.
The reason for these discrepancies, said Baranauskiene, was possibly due to the losses of the aroma compounds during encapsulation.
Scanning electron microscopy of the particles showed that encapsulation in SMP produced crack- and pore-free spherical particles with sizes varying between eight and 224 micrometres.
The WPC capsules had a wider range of particle sizes, from two to 556 micrometres, and the researchers also found the presence of holes, which may explain the more rapid loss of volatile compounds from this matrix.
Research into micro-encapsulation is continuing with the technology applicable to delivering a host of ingredients - flavours, oils, peptides, amino acids, enzymes, acidulants, colours and sweeteners - in a range of food formulations, from functional foods to ice cream.
A recently launched research programme in Finland hailed enzymatic micro-encapsulation as one of the major food technology tools of the future.