Advanced computational modelling for drying processes – A review

Low frequency ultrasound pretreatment of carrot slices: Effect on the moisture migration and quality attributes by intermediate-wave infrared radiation drying

Low-field bench top 1H nuclear magnetic resonance (LF-NMR) relaxometry instruments have been increasingly popular as analytical tools for engineering research. Magnetic resonance imaging, which is a more advanced approach to NMR technology, provides the researcher with images of the internal structure without any disruption to the sample and has been commonly used in medical applications in analysis of soft tissue. The non-invasive and non-destructive nature coupled with the high discriminative power of LF-NMR and MRI, makes them invaluable tools of analysis for a wide range of applications in food science. This review covers the basic concept behind NMR/MRI technology and discusses some of its most commonly used food applications. The review addresses the food scientist with no prior knowledge of NMR/MRI and aims to supply the reader with both the theory of the method and its fundamentals, as well as the practical uses in scientific research and industrial applications.

1H Nuclear Magnetic Resonance Relaxometry and Magnetic Resonance Imaging and Applications in Food Science and Processing

Changes in non-volatile taste components of button mushroom (Agaricus bisporus) during different stages of freeze drying and freeze drying combined with microwave vacuum drying.

The cacao pod husk (CPH) is the primary waste byproduct of the chocolate industry. One ton of cacao beans generate approximately ten times that amount of fresh CPH. The husk is rich in phenolic compounds. The aim of this study is to determine the effects of microwaves, hot air and freeze-drying on the phenolic compound content, antioxidant capacity, enzyme activity and microstructure of CPH. The results showed that fresh CPH contain 323.7 ± 26.5 mg gallic acid equivalents (GAE)/100 g dry mass (d.m.) of total phenolic compounds, and dehydration had a positive effect on the CPH phenolic content and antioxidant capacity. Catechin, quercetin, and (−)-epicatechin and gallic, coumaric and protocatechuic acids were identified in fresh and dried CPH. Drying the material using a microwave and freeze-drying preserved the husk microstructure. The results showed that microwave drying was a better drying method than hot air and as good as freeze-drying. Industrial relevance The food industry is currently searching for new sources of natural antioxidants. The cacao pod husk (CPH) is a good source of phenolic compounds that can function as antioxidants and could be used as ingredients in functional foods. However, due to their high water content, fresh CPH are perishable products, and for this reason, the first step would be dry the pod husk to improve its shelf life. The findings of this study showed that the use of microwaves to dry CPH may release polyphenols, thereby enhancing the antioxidant capacity and decreasing the polyphenol oxidase (PPO) activity while maintaining the microstructure. Thus, microwave drying could be of interest as a method for enhancing the extractability of phenolic compounds from CPH.

Effects of microwaves, hot air and freeze-drying on the phenolic compounds, antioxidant capacity, enzyme activity and microstructure of cacao pod husks (Theobroma cacao L.)

The impact of freeze-drying on microstructure and rehydration properties of carrot

Ice crystal interspacing in frozen foods

article i nfo Rehydration kinetics by two modes of imbibition is studied in pieces of freeze-dried winter carrot, after different thermal pre-treatments. Water ingress at room temperature is measured in real time by in situ MRI and NMR relaxometry. Blanched samples rehydrate substantially faster compared to non-blanched samples, independent of their porous microstructure. It is proposed that for non-blanched tissues immobilized sugars result in nearly complete swelling of the solid matrix, hindering the ingress of water through the porous network. Non- blanched carrot pieces frozen at �28 °C rehydrate faster compared to those frozen at �150 °C, due to blocking of smaller pores by swelling. In blanched tissues the mobilization of sugars results in a more homogeneous sugar distribution, leading to less swelling of the solid matrix and allowing fast ingress of water via capillary suction. Industrial relevance: The dried fruits and vegetables that arecurrently available on themarketare a poorcompro- mise betweenconvenience(rehydration kinetics) and sensorial quality. Thisis a major bottleneck for consumers to "Make the Healthy Choice the Easy Choice" and this also negatively impacts market growth. Currently, rational optimization of drying processes is impeded by lack of insight which structural features determine rehydration kinetics (convenience) and texture (sensorial quality) upon rehydration. We therefore started a program to quantitatively assess and model microstructural features and rehydration behavior of freeze-dried carrots as a model system.

Rehydration kinetics of freeze-dried carrots

Complementary imaging techniques (µCT, SEM, MRI), NMR and quantitative image analysis methods were used to obtain multi-length scale information of the porous structure of carrots in their freeze-dried, rehydrating and rehydrated form. This work established a predictive relation between freezing rate and freeze damage. Water imbibition rates could be explained from the porous structure induced during freeze-drying. In the rehydrated state, remaining structural integrity and anisotropy could be correlated with the textural quality.

Multi-length scale structural imaging of freeze-dried carrots and their rehydration behaviour.

Water ingress is studied in freeze-dried winter carrot pieces, prepared with different thermal pre-treatments before freeze-drying, as a model system for rehydration of dried vegetables in the food industry. Various drying protocols for carrot pieces result in different microstructures and its rehydration properties. Water ingress at room temperature is measured in real-time by in situ 3D magnetic resonance imaging (MRI) and NMR relaxometry. Two modes of imbibition were studied: water ingress into dried carrot pieces by full submersion and floating at the air-water interface. Differences in the time scale and degree of water ingress are observed by MRI, where floating samples show faster and more extensive rehydration. These alterations were not observed in NMR relaxometry at the same field strength. The contrasting observations are explained by susceptibility effects that strongly affect the MRI results, even at low field strength, due to the presence of air-water boundaries in samples rehydrated by full submersion. Clear effects on rehydration kinetics of non-blanched carrot tissue are observed, dependent on pre-treatment induced changes in the meso/microstructure. For blanched samples rehydration kinetics is strongly increased, which is explained by an extra, more dominant effect that enhances water ingress. Moreover, NMR relaxometry shows a slow decreases of T2 of the water surrounding the carrot pieces, due to leaching of sugars and other solutes. Since no effect on the leaching kinetics is observed for different pre-treatments and the process is much slower than rehydration, it is concluded that this leaching is a diffusion driven process following rehydration.

Arno Duijster

Papers

The impact of freeze-drying on microstructure and rehydration properties of carrot

Ice crystal interspacing in frozen foods

Rehydration kinetics of freeze-dried carrots

Multi-length scale structural imaging of freeze-dried carrots and their rehydration behaviour.

The Effect of Structure and Imbibition Mode on the Rehydration Kinetics of Freeze-dried Carrots