The shelf life of vegetable carotenoids when stored in powdered form is severely affected by oxygen in the storage environment, finds a study by the members of the University of Natural Resources and Life Sciences, Vienna.
Carrot concentrates are rich in valuable carotenoids and highly sought after in the food industry for use as colourings. But in order for the rich orange colour in carrots to be transferred to food and drink preparations, the structure of the natural pigments must be preserved, and contact with oxygen avoided until they are used.
The team of researchers led by professor Henry Jäger found that the shelf life of carotenoids in stored end products to be greatly increased. But it is important to take advantage of the opportunities to influence the structure of the material for storage right at the beginning of the production process for vegetable concentrate powders.
“The infiltration of oxygen present in the air into pigment-rich powder particles is actually the biggest factor influencing the shelf life of colouring that we identified,” explained Klara Haas, a researcher from the working group. “But with delicate management of the drying process in production, the shape of the powder particles can be determined in order to greatly reduce diffusion of oxygen into the stored goods.”
A key role is played here by so-called “carrier materials”, which are used in production to ensure that powders have a stable structure. In the study maltodextrin, which is often employed as a carrier material, was replaced by gum arabic or modified starch.
“We discovered that when maltodextrin is used, after one drying stage over a third of the sensitive carotene is found on the surface of the powder particles, unprotected. This is reduced to just a quarter or a fifth when modified starch or gum arabic is used. This means that both are carrier materials that provide better protection against oxygen,” explained Professor Jäger.
The method of powder manufacture also has an enormous influence on storability – with the physical barriers to oxygen again proving decisive. If the powder is produced by means of freeze-drying, powder particles contain micropores and nanopores that allow oxygen to penetrate quickly. But by using an optimised spray drying process, which produces denser particles with lower surface area, oxygen diffusion during storage is reduced and the stability of the plant-based colouring concentrate is massively increased. The ideal size for particles is up to a little over 90 micrometres.
The larger the particles the greater proportion of carotenoids inside the particles compared to that on the surface, so there is greater protection against oxygen, according to Prof Jäger.
Without these measures, in extreme cases almost 90 % of carotene is lost within three months; with optimised production, this was reduced to less than 10%.
To a certain extent, the oxidation of vegetable carotenoids can also be reduced by adding antioxidants such as tocopherol, ascorbyl palmitate and sodium ascorbate, however, their use is limited by costs, the researchers said,
The results are of major significance to the food industry, since colouring concentrates from fruits and vegetables are increasingly being used as an alternative to artificial colourings.
The results of the study have now been published in Food Research International.