Alessandro Gianfrancesco

Generation of Product Structures During Drying of Food Products

The sensorial profile, nutritional quality, and rehydration properties of dried food depend on the structure of the dehydrated material. The molecular, supramolecular, micro-, and macrostructure is influenced by the applied drying conditions. During drying of foods, specific product structures can be generated. For instance, during drying at elevated temperatures, Maillard reactions are accelerated. Thus, peptides and reducing sugar molecules are transformed into taste-active molecules. During drying, proteins are also denatured, and their three-dimensional structure changes accordingly. Following this denaturing, proteins can coagulate. Furthermore, gelling of starch is observed during drying of food. In addition to these reactions, isomerization, oxidation, and various other reactions are accelerated during drying at higher temperatures. Thus, the molecular structure of food products changes significantly during most drying processes. Depending on the drying conditions, different supramolecular structures of solid food products are generated during dehydration of solutions. The drying velocity has a significant impact on the characteristics of the generated supramolecular structure. Fast dehydration of liquid products leads to amorphous structures, whereas slow drying allows substances with low molecular weight to crystallize. Furthermore, the chosen drying technology, solid content of the wet product, composition, pressure fluctuations during drying, and the kinetics of mass transfer influence the generation of microstructures. In addition, the liquids can be enriched with gas before drying in order to increase the products porosity. Finally, the macrostructure and the optical appearance of the dry product are affected by the drying technology applied and the chosen drying conditions.

Kinetics of Lysine Loss in an Infant Formula Model System at Conditions Applicable to Spray Drying

The kinetics of lysine loss at conditions applicable to spray drying were established for an infant formula model system. The composition of the model system was derived from the typical composition of milk-based infant formulas. The model system contained skim milk powder, lactose, and whey protein isolate as reactive compounds. The impacts of the water activity, temperature, time, and physical state on lysine loss were evaluated. The samples were equilibrated at water activities of 0.06–0.75 and then heated at 60–90°C for 30 s to 30 min. A maximum in lysine loss of 81% was observed at a water activity of 0.17 at 90°C and 30 min of heating. The water activity range of maximal lysine loss was shifted to higher water activities for lower heating temperatures. In order to study the impact of the physical state on lysine losses, the physical state of the model system was determined across the whole parameter range. For a water activity a w ≤ 0.17 the lactose contained in the model system did not crystallize at any experimental condition. Between water activities of 0.23 and 0.43 the lactose crystallized in the course of heating and the delay of crystallization was influenced by the temperature and the water activity. At water activities of 0.53 and 0.75 the lactose crystallized during the equilibration of the model system, i.e., before heating. The highest lysine losses were determined in the transition zone between the rubbery and the crystalline state.

Developing Supra-Molecular Structures During Freeze-Drying of Food

In this contribution, we discuss a scientific approach to controlling the freeze-drying process in order to obtain products with desired attributes, using two examples. In the first part, the freeze-drying kinetics of a model food are recorded and represented using a state diagram, which was then used to optimize a strategy to preserve the physical structure and minimize the drying time of the product. The second part focuses on the application of freeze drying to produce amorphous sucrose coatings. In this study, model food materials have been coated with a liquid sucrose solution, and freeze-dried on a laboratory scale. The drying kinetics have been measured and plotted on a sucrose state diagram in order to optimize the process conditions and to minimize possible crystallization. Analysis of the physical state of the sucrose (using DSC, NIR, and microscopy) showed that a dominantly amorphous coating was obtained. Compared to a crystalline reference, the amorphous sucrose dissolved more quickly in a liquid, inducing a faster sweetness perception. This result also opens new opportunities for sugar reduction in food due to an enhanced sweetness perception.

Impact of Spray-Drying Process Parameters on Dairy Powder Surface Composition and Properties

To design dairy powder properties according to their future use, it is necessary to know how to identify the main process parameters that influence the drying mechanisms and to optimize them on the basis of the desired powder properties. The impacts of atomization process parameters on powder physicochemical properties and surface composition were investigated and correlated. The analytical results permitted us to sort the produced powders into groups according to their dependency to process parameters. This study shows the feasibility of developing a model for the prediction of the physicochemical properties of dairy powders via a reverse engineering approach.

Influence of Feed Composition and Drying Parameters on the Surface Composition of a Spray-Dried Multicomponent Particle

Surface properties of multicomponent particles produced in spray drying can be controlled by selective accumulation of specific components, which are present in the liquid feed, on the particle surface. Such modification of the surface composition can take place only before a solid shell forms on the particle surface. In this contribution, the influence of the concentration of surface active component on modifications of the surface composition is discussed. Based on results of single-droplet drying simulations, changes in the concentration of the surface active component at the solution-air interface are related to the composition of spray-dried particles.

Impact of Dehydration on Lysine Loss in a Model Dairy Formulation

The essential amino acid lysine can be damaged due to exposure to elevated temperatures and due to dehydration during drying of dairy products. In this study, the impact of dehydration on the loss of available lysine was elaborated using vacuum dehydration at moderate temperatures at timescales that allow recording the degradation kinetics. The content of available lysine decreased during dehydration. The drying rate as well as the thermal conditions during drying had no significant impact on the loss of available lysine above a water content of 10%. A sharp increase in lysine loss was observed at water contents below 10% when thermal stresses become important. In conclusion, the dynamics during water removal seem to provoke lysine losses during drying.