The removal of water from food leads to a decrease in its activity, as well as to an inhibition of enzymatic and non-enzymatic reactions, resulting in a persistent product. Despite severe dehydration of food, adverse changes can still occur, such as vitamin C oxidation, fat auto-oxidation, starch retrogradation or colour change. Adverse processes occurring in dried food can be reduced by appropriate selection of process parameters, such as temperature, humidity and air velocity, and by using drying methods that do not cause aeration of the dried material.
Comparing Drying Processes: Freeze-Drying, Spray-Drying and Air-Drying
To better illustrate the difference between the processes, all the necessary information has been gathered in a table:
Effect of different drying methods on colour, water solubility and presence of particles using beetroot powder as an example.
FD – freeze-dried beetroot powder partially dissolves in the water, you can see the beetroot particles floating on the surface, as well as those that have fallen to the bottom in small amounts. The solution has an intense red colour and a sweet beetroot taste. This solution has the most intense beetroot flavour of the others.
SD – spray-dried beetroot powder, very good soluble in water. The beetroot or carrier particles are not visible in the solution, as in the other samples. The solution obtained is clear. Characterised by its red colour and not very intense beetroot flavour compared to FD and AD.
AD – air-dried (convective) beetroot powder, partially dissolves in water, a large amount of beet particles fall to the bottom to form a clear solution in the middle. A smaller proportion of beet particles remains on the surface. The solution has a less intense colour and beetroot flavour compared to FD.
In summary, choosing the right drying method is crucial to the quality of the final product. Freeze-drying, although expensive and time-consuming, best preserves the nutritional value, taste, colour and texture of the food. Spray drying is faster but can lead to a loss of bioactive ingredients and a change in flavour, and also requires the addition of carriers, which affects product purity. Convection drying, on the other hand, although the cheapest, often leads to quality deterioration, taste alteration and loss of nutritional value. The choice of method should depend on the specifics of the product and expectations of final quality.
Reference:
Calín-Sánchez Á, Lipan L, Cano-Lamadrid M, Kharaghani A, Masztalerz K, Carbonell-Barrachina ÁA, Figiel A. Comparison of Traditional and Novel Drying Techniques and Its Effect on Quality of Fruits, Vegetables and Aromatic Herbs. Foods. 2020; 9(9):1261. https://doi.org/10.3390/foods9091261