Stefan Palzer

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.

The amorphous state of spray-dried maltodextrin: sub-sub-Tg enthalpy relaxation and impact of temperature and water annealing.

The annealing behaviour of a spray-dried maltodextrin was investigated by differential scanning calorimetry. Special attention was paid to the effect of temperature and humidity on the annealing process. Comparison was also made with the glassy state of the same compound prepared by various cooling processes. The presence of a very pronounced sub-T(g) peak upon ageing reveals the specificities of the glass and the complexity of the relaxation spectrum of the spray-dried material. This peak seems actually to correspond to a partial ergodicity recovery that may be attributed to onset of molecular mobility occurring below T(g). The position of the sub-T(g) peak with regard to the conventional T(g) was systematically studied. It clearly showed the difference between the effect of temperature and water plasticization on the relaxations occurring in the glassy state of materials prepared by spray-drying.

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.

Food structure engineering for nutrition, health and wellness

Many of today’s food products addressing specific nutritional, health and wellness needs of human and animal consumers are often very complex structures. Consequently, it is of utmost importance in food engineering research to develop a detailed understanding of the time-dependent transient changes in all of the structural aspects of food matrices from raw material harvesting, to product processing, to the point of breakdown during shelf-life, consumption and final digestion. Food structural understanding and control needs to be mastered on a broad range of length scales including: the molecular, supra-molecular, microand macro-structural level. At the same time, the mechanical, physical and chemical properties of the food need to be considered. Only in this manner can a tailormade build-up and controlled breakdown of food products be achieved, and subsequently, can specific nutritional, physical and sensorial properties be engineered. Reducing fat and sugar content in order to reduce the energy density of food requires a particular microand macro-structural design in order to compensate for resultant sensorial changes. Specific structures can improve the stability and bioavailability (ultimately bioefficacy) of bioactive compounds and probiotics. Improving the nutritional profile of food by increasing the overall content of (plant) proteins, dietary fibres or whole grains can be achieved by certain means of structuring. Specific health care products exhibiting a particular rheological behaviour at very high protein content, for example, can only be realised by targeted modification of their supra-molecular structure. Finally, food microstructures can be designed in such a way that their modulated digestion behaviour triggers different physiological responses.

Roller compaction: Effect of morphology and amorphous content of lactose powder on product quality

The effect of morphology and amorphous content, of three types of lactose, on the properties of ribbon produced using roller compaction was investigated. The three types of lactose powders were; anhydrous SuperTab21AN, α-lactose monohydrate 200 M, and spray dried lactose SuperTab11SD. The morphology of the primary particles was identified using scanning electron microscopy (SEM) and the powder amorphous content was quantified using NIR technique. SEM images showed that 21AN and SD are agglomerated type of lactose whereas the 200 M is a non-agglomerated type. During ribbon production, an online thermal imaging technique was used to monitor the surface temperature of the ribbon. It was found that the morphology and the amorphous content of lactose powders have significant effects on the roller compaction behaviour and on ribbon properties. The agglomerated types of lactose produced ribbon with higher surface temperature and tensile strength, larger fragment size, lower porosity and lesser fines percentages than the non-agglomerated type of lactose. The lactose powder with the highest amorphous content showed to result in a better binding ability between the primary particles. This type of lactose produced ribbons with the highest temperature and tensile strength, and the lowest porosity and amount of fines in the product. It also produced ribbon with more smooth surfaces in comparison to the other two types of lactose. It was noticed that there is a relationship between the surface temperature of the ribbon during production and the tensile strength of the ribbon; the higher the temperature of the ribbon during production the higher the tensile strength of the ribbon.

Tracking Structural Changes in Lipid-based Multicomponent Food Materials due to Oil Migration by Microfocus Small-Angle X-ray Scattering

One of the major problems in the confectionery industry is chocolate fat blooming, that is, the formation of white defects on the chocolate surface due to fat crystals. Nevertheless, the mechanism responsible for the formation of chocolate fat blooming is not fully understood yet. Chocolate blooming is often related to the migration of lipids to the surface followed by subsequent recrystallization. Here, the migration pathway of oil into a cocoa butter matrix with different dispersed particles was investigated by employing microfocus small-angle X-ray scattering and contact angle measurements. Our results showed that the chocolate powders get wet by the oil during the migration process and that the oil is migrating into the pores within seconds. Subsequently, cocoa butter is dissolved by the oil, and thus, its characteristic crystalline structure is lost. The chemical process provoked by the dissolution is also reflected by microscopical changes of the surface morphology of chocolate model samples after several hours from the addition of oil to the sample. Finally, the surface morphology was investigated before and after oil droplet exposure and compared to that of water exposure, whereby water seems to physically migrate through the particles, namely cocoa powder, sucrose, and milk powder, which dissolve in the presence of water.

Prediction of Powder Stickiness along Spray Drying Process in Relation to Agglomeration

The spray drying process consists of a fast convective drying of liquid droplets by hot air. Initially, the water activity (aw) of a drop is close to 1. During drying, the drop surface aw decreases while viscosity increases until reaching a sticky rubbery state before further drying. This can be observed for products such as carbohydrates, leading to particles sticking on walls (product losses) or to adhesion between particles leading to agglomeration. In this study, particle stickiness was investigated in a cocurrent pilot spray dryer by measuring drying air properties (temperature and relative humidity) at different positions. This allowed describing the evolution of temperature and mean water content of the drying drops. Two model products (maltodextrin DE12 and DE21) were spray dried varying process parameters liquid flow rate (1.8, 3.6, and 5.4 kg/h), air temperature (144°, 174°, and 200°C), airflow rate (80–110 kg/h), and rotary atomizer speed (22,500–30,000 rpm). The two products exhibit different drying behaviors in relation to their affinity towards water (sorption isotherms) and glass transition temperature evolution with aw (stickiness). Depending on drying conditions and product, the drop stickiness was observed very rapidly, close to the atomizer, or later, along the chamber. This approach can be used to identify conditions and positions corresponding to sticky particles.

Chapter 13 Agglomeration of dehydrated consumer foods

Publisher Summary Foods are dehydrated to reduce their transport weight and to increase their shelf life. Various dehydrated food products are currently offered to the consumer. Amongst them are dairy powders, infant formulas, bakery mixtures, beverage powders, dehydrated convenience foods, kitchen aids, confectionery, granulated sugar, tabletted sweeteners, and cereal-based products. Most of these products are agglomerated. Agglomeration by definition is a process during which primary particles are fixed together to form larger, porous secondary particles. Within these agglomerates individual primary particles are still visible. Freely dosable powders are agglomerated to provide superior instant properties. They should re-hydrate or dissolve quickly without forming lumps. In addition, the flowability of powders is improved by agglomeration. Good flowability is especially important for the precise dosing of powders as required in vending machines. Food products are also offered as pre-dosed quantities in the form of tablets. Tablets are agglomerates, which can have a distinctive, easily recognizable shape. Sometimes undesired agglomeration effects can be observed while processing dehydrated food powders. Caking of powders or post-hardening of agglomerates during storage significantly deteriorates the product quality. Stickiness and encrustation of equipment is observed during processing (which can be considered as an undesired agglomeration) and requires increased cleaning efforts. However, desired and undesired agglomeration of food products can be explained by the same basic principles and kinetics. These mechanisms are strongly linked to the material properties of the food particles.

A micromanipulation particle tester for agglomeration contact mechanism studies in a controlled environment

Pressure agglomeration of powders is widely applied in various industries and an increasing interest lies in the identification and description of contact mechanisms between particles, which are responsible for the compaction product properties. In this paper, the design and development of a novel micromanipulation particle tester (MPT) is presented. This device makes it possible to measure the deformation kinetics and resulting adhesion of two individual particles in contact under load, which are strongly influenced by the applied process conditions. The MPT set-up is, therefore, designed to offer a unique control over the process conditions most relevant to the compaction of powders: external stress, dwell or holding time at constant deformation, compression velocity as well as relative humidity and temperature determining the physical state and mechanical characteristics of hygrosensitive amorphous particles. The latter are often part of powder formulations, e.g. in the food industry, and have been used for force and contact-zone development studies with the MPT. The experimental results on the microscale level will deliver valuable quantitative information for an improved tailoring of pressure agglomeration process conditions of bulk solids.

Roller compaction: Effect of relative humidity of lactose powder

The effect of storage at different relative humidity conditions, for various types of lactose, on roller compaction behaviour was investigated. Three types of lactose were used in this study: anhydrous lactose (SuperTab21AN), spray dried lactose (SuperTab11SD) and α-lactose monohydrate 200M. These powders differ in their amorphous contents, due to different manufacturing processes. The powders were stored in a climatic chamber at different relative humidity values ranging from 10% to 80% RH. It was found that the roller compaction behaviour and ribbon properties were different for powders conditioned to different relative humidities. The amount of fines produced, which is undesirable in roller compaction, was found to be different at different relative humidity. The minimum amount of fines produced was found to be for powders conditioned at 20-40% RH. The maximum amount of fines was produced for powders conditioned at 80% RH. This was attributed to the decrease in powder flowability, as indicated by the flow function coefficient ffc and the angle of repose. Particle Image Velocimetry (PIV) was also applied to determine the velocity of primary particles during ribbon production, and it was found that the velocity of the powder during the roller compaction decreased with powders stored at high RH. This resulted in less powder being present in the compaction zone at the edges of the rollers, which resulted in ribbons with a smaller overall width. The relative humidity for the storage of powders has shown to have minimal effect on the ribbon tensile strength at low RH conditions (10-20%). The lowest tensile strength of ribbons produced from lactose 200M and SD was for powders conditioned at 80% RH, whereas, ribbons produced from lactose 21AN at the same condition of 80% RH showed the highest tensile strength. The storage RH range 20-40% was found to be an optimum condition for roll compacting three lactose powders, as it resulted in a minimum amount of fines in the product.