Freeze-drying Stage Research Articles

A Circular Economy Approach in the Development of Superabsorbent Polymeric Matrices: Evaluation of the Mineral Retention

This manuscript focuses on the production of polymeric matrices enriched in minerals and antioxidant compounds. The biopolymers employed are obtained from different by-products of the agro-food industry (porcine plasma protein, pea protein concentrate and soy protein isolate), which helps to revalorize them. Two different manufacturing techniques are employed to produce these matrices: 3D-printing and injection molding. Bioactivity was enhanced through immersion of the samples in magnesium glutamate and iron lactate solutions. To incorporate these minerals and bioactive compounds into the matrices, two additional stages are required: (1) an immersion stage in a mineral/bioactive containing solution, which allows simultaneous removal of the glycerol employed as plasticizer and entrapment of the minerals and bioactive compounds; and (2) a subsequent freeze-drying stage. Matrices produced through these manufacturing processes were assessed through water uptake capacity, mineral analysis, bioactivity and color measurements. The studied matrices have great potential in the food industry, as the threshold for claiming a significant mineral content was reached after the immersion stage. The presence of bioactive compounds could avoid the degradation of these matrices when food processing includes stages at relatively high temperatures.

Fruit and vegetable purees as cryoprotectants for vacuum freeze-dried fermented milk products

Fresh fermented milk products have a limited shelf life that can be extended by vacuum freeze-drying. Cryoprotectants are used to increase the survival of lactic acid microorganisms during freeze-drying. The most effective cryoprotectants are those of natural origin. Literature offers little information on the cryoprotective effects of fruit and vegetable purees. Therefore, we aimed to evaluate the effectiveness of fruit and vegetable purees in increasing the survival of lactic acid microorganisms during the freeze-drying and storage of fermented milk products. 
 We studied bioyogurt samples containing pumpkin, fig, and banana purees. Rational modes of freezing and freeze-drying were established on the basis of thermal analysis. The cryoscopic temperature was determined by differential scanning calorimetry. The proportion of frozen moisture was calculated using the Nagaoka formula. Standard methods were employed to evaluate the sensory characteristics of bioyogurts and determine their protein, fat, and non-fat milk solids contents, as well as titratable acidity and microbiological indicators. 
 The addition of pumpkin puree increased the cryoscopic temperature and reduced the freeze-drying stage and the total drying time by 13 h, depending on the amount of puree. However, the addition of sweet fig and banana purees decreased the cryoscopic temperature and increased the freeze-drying stage and the total drying time by 0.5–1.5 and 1.5–3 h, respectively. Based on the sensory evaluation of the freeze-dried bioyogurts, we selected the formulations with 15% of pumpkin and fig purees and 10% of banana puree. We found that the freeze-dried bioyogurts with puree had higher counts of lactic acid bacteria compared to the control. In the freeze-dried samples, the counts were higher at a storage temperature of 4 ± 2°C than at 20 ± 2°C. 
 Pumpkin puree provided the best survival of lactic acid microorganisms during freeze-drying and storage.

Comparison of vial heat transfer coefficients during the primary and secondary drying stages of freeze-drying

This study uses a heat flux sensor and temperature probes to directly measure vial heat transfer coefficients (Kv) during primary and secondary drying stages of lyophilization. It is observed that Kv is 40–80% smaller during secondary drying than primary drying, and this value exhibits a weaker dependence on chamber pressure than in primary drying. These observations arise because water vapor in the chamber significantly decreases between primary and secondary drying, which alters the gas conductivity between the shelf and vial. This study tabulates Kv values for secondary drying for different vials and different chamber pressures, and demarcates the contributions from gas conduction. Lastly, the study performs an energy budget analysis on two different vials (10R glass vial and 10 mL plastic vial) to determine the major factors that contribute to vial energy consumption. During primary drying, the majority of energy supplied goes towards sublimation, while for secondary drying, the majority of energy goes towards heating the vial wall rather than desorbing bound water. We discuss the consequences of this behavior for heat transfer modeling. During secondary drying, the heat of desorption can be neglected in thermal modeling for some materials (e.g., glass) but not others (e.g., plastic vials).

Freeze-drying precursor to construct carbon nitride photocatalyst with low addition and high adsorption performance

BackgroundIn photocatalytic reactions, how to maintain the high efficiency of the photocatalysts with a small number of catalysts is a key challenge. MethodsIn this paper, the alkali metal ions (Na+ or K+) doped carbon nitride (Na-CN/K-CN) were prepared by the combination of freeze-drying and molten salt assisted calcination to solve this problem. Significant findingsAlkali metal salts and dicyandiamide were rapidly nucleated and interconnected during the freeze-drying stage. After calcination, Na-CN and K-CN with strong interaction between Na+/K+ and carbon nitride were obtained. This strong interaction reduced the electron cloud density near the N atom and further caused strong adsorption capacity and high oxidation capacity, thus improving the photocatalytic performance of Na-CN and K-CN. Compared with K-CN, Na-CN had stronger electron-hole separation ability, which made it exhibit higher photocatalytic activity. After 60 min of visible light irradiation, only 5 mg of Na-CN could degrade 63.79% of MB with a concentration of 30 mg/L.

Experimental Study of the Impact of Pore Structure on Drying Kinetics and Sublimation Front Patterns

Freeze-drying frozen maltodextrin solutions with solid contents of 5% and 30% (w/w) was experimentally investigated using neutron imaging at PSI Villigen/Switzerland. Different solid contents, as well as annealing at −5 °C for 11 h, were used to modify the porous structure of the samples, which was quantified using X-ray computed tomography. Annealing of the 5% (w/w) sample, with a pore size distribution (PSD) of 23.7 ± 11.1 µm, yielded a very open pore space with high porosity (ε = 0.96) and a PSD of 33.0 ± 27.0 µm. In contrast, the higher solid content resulted in small, lamellar, narrow pores with high anisotropy and a porosity of ε = 0.65, as well as a PSD of 13.5 ± 4 µm. In operando neutron imaging was used to show the impact of the structure of frozen maltodextrin on the overall drying kinetics and shape of the sublimation front during freeze-drying. For this purpose, a freeze-drying stage was employed, which allowed a novel approach to time- and space-resolved monitoring of the ice phase. The sublimation front propagation was quantitatively analyzed based on ice saturation profiles and sublimation rates. The dependence of drying velocity on structure is nicely demonstrated by the data. In addition, it is shown that the sublimation front widened during freeze-drying, resulting in either rather concave or convex shape depending on morphological parameters.

Assessing the viability of three Lactobacillus bacterial species protected in the cryoprotectants containing whey and maltodextrin during freeze-drying process.

Freeze-drying of bacteria associates with different stresses such as osmotic pressure, temperature and oxidation, and decreases bacterial viability, which seem to reduce by applying cryoprotectants. The present study evaluated the effect of four cryoprotectants on decreasing the stress caused by freeze-drying process among three Lactobacillus species. Additionally, it highlighted the use of whey and maltodextrin as a substitute for peptone and sucrose in cryoprotectants respectively. The viability of lactobacilli was measured after freeze-drying, 1 month of storage at 25 and 4°C. Based on the results, the viability rate of bacteria in protectants during freeze-drying stage was dependent on their strains. The best viability of Lacticaseibacillus rhamnosus GG and Ligilactobacillus salivarius 20687 was, respectively, observed in the protectants containing sucrose and whey, while Lactiplantibacillus plantarum NRRL B-14768 viability was equal in all protectants. The number of live bacteria reduced significantly by storing bacteria for 1 month at 25°C compared to the 4°C storage. During the storage period, the viability of L. salivarius improved by adding sucrose in protectant. Due to the positive effect of whey and sucrose in the drying and storage stage, on bacterial viability, the protectant consisting of whey and sucrose is suggested for all of the species under study.

In situ micro-computed tomography to study microstructure and sublimation front during freeze-drying

• New method to visualize freeze-drying kinetics in situ using high resolution X-Ray imaging. • High resolution X-Ray imaging enables to visualize pore structure in situ during freeze-drying. • Link between pore structure and structure of sublimation front offers new details. In this study, a new method is presented to examine the freeze-drying process of frozen solutions in situ. By using a newly developed freeze-drying stage for high resolution micro-computed X-Ray tomography, the freeze-drying process can be investigated in more detail. For the proof of concept maltodextrin (c =0.05 w/w, c =0.2 w/w and c =0.3 w/w) and sucrose (c =0.2 w/w) solutions were used as model substances. The use of X-Ray radiography enables to analyze the drying kinetics, while X-Ray tomography describes the pore structure in more detail. The results are qualitatively compared and the proof of the concept will serve as a basis for further studies in the future.

Strengthening of Porcine Plasma Protein Superabsorbent Materials through a Solubilization-Freeze-Drying Process.

The replacement of common acrylic derivatives by biodegradable materials in the formulation of superabsorbent materials would lessen the associated environmental impact. Moreover, the use of by-products or biowastes from the food industry that are usually discarded would promote a desired circular economy. The present study deals with the development of superabsorbent materials based on a by-product from the meat industry, namely plasma protein, focusing on the effects of a freeze-drying stage before blending with glycerol and eventual injection molding. More specifically, this freeze-drying stage is carried out either directly on the protein flour or after its solubilization in deionized water (10% w/w). Superabsorbent materials obtained after this solubilization-freeze-drying process display higher Young’s modulus and tensile strength values, without affecting their water uptake capacity. As greater water uptake is commonly related to poorer mechanical properties, the proposed solubilization-freeze-drying process is a useful strategy for producing strengthened hydrophilic materials.

The Quality of Red Bell Pepper Subjected to Freeze-Drying Preceded by Traditional and Novel Pretreatment.

It has been demonstrated previously in the literature that utilization of PEF or a combination of a pulsed electric field (PEF) and ultrasounds (US) can facilitate dehydration processes and improve the quality of dried products even better than the application of thermal methods such as blanching. The aim of the study was to evaluate the quality of red bell pepper subjected to freeze-drying preceded by blanching or PEF or US treatment applied in a single and combined mode. Furthermore, the freeze-drying was preceded by shock freezing or vacuum freezing performed inside the freeze-dryer as a result of pressure drop during the first stage of freeze-drying. All of the analyzed technological variants enhanced the drying kinetics when compared to the intact material. Freeze-dried bell pepper subjected to non-thermal pretreatment exhibited higher vitamin C, total phenolic and carotenoids content than blanched material despite the fact that blanching reduced drying time the most compared to all other analyzed methods.

Development of a Freeze-Drying Stage for In-Situ µ-CT Measurements

This paper shows the development of a freeze-drying stage for in-situ μ-CT measurements. The stage can operate in a temperature range of −40 °C up to 70 °C, and a pressure range from atmospheric pressure to 7 Pa at the sample holder. To get the best visualization of the probe, it is fundamental that the materials around the sample holder are not absorbing most of the radiation. For this reason, we built an axial symmetrical stage built out of polyetheretherketon (PEEK). A test of the stage by different freeze-drying experiments with maltodextrin and sucrose particles and solutions demonstrated its suitability to visualize the freeze-drying processes in-situ. It was possible to track the drying front during the process by radiographic and tomographic measurements, as well as to visually resolve the ice crystals and porous structure in tomographic measurements. Using different samples and process parameters, we showed that the freeze-drying stage is not only suitable for in-situ µ-CT measurements, but also allows us to use the stage for other imaging methods such as neutron imaging, and for any sample where a controlled environment is needed.

Assessment of the Processing Method for the Development of Hybrid Biopolymer-Based Scaffolds

The present study deals with the development of scaffolds based on gelatin (G) and/or chitosan (CH) through modifications in the central processing method. This process consists of the fabrication of a hydrogel which is, then submitted to a freeze-drying stage. To compare the effects of the different modifications, the mechanical properties of the various systems were characterized, employing both dynamic compressive strain and frequency sweep tests. In addition, their porosity as well as the structure and fiber distribution, using an analytic model and scanning electron microscopy (SEM), were also evaluated. The obtained results demonstrated a strong dependence on the properties by the scaffolds with both the modifications introduced in the processing method as well as the proportion of materials used (G and CH). Furthermore, the properties were found to improve for systems with a high chitosan content after being submitted to a heat treatment at 50 °C with agitation.

Observation of Microstructure Formation During Freeze-Drying of Dextrin Solution by in-situ X-ray Computed Tomography

In-situ X-ray computed tomography (CT) was used to observe microstructure formations during freeze-drying of a dextrin solution. A specially designed freeze-drying stage was equipped at the X-ray CT stage. Frozen and dried microstructures were successfully observed. The CT images of the frozen solution clarified the ice crystal size increase and obvious boundary formation between the ice and freeze-concentrated phases upon performing post-freezing annealing at −5°C. These structural modifications emerged owing to Ostwald ripening and glassy phase relaxation. During the freeze-drying, pore microstructures formed as a consequence of water removal. The pores were replicas of the original ice microstructures; some pore microstructures newly formed by the removal of water. The latter mechanism was more obvious in the non-annealed sample than in the annealed sample. The glassy phase in the non-annealed solution was not perfectly freeze-concentrated; water was rapidly removed from this phase, losing its original microstructure. At this moment, the freeze-concentrated region piled up to new pore walls, which consequently thickened the pore walls. An image analysis estimated that the mean pore wall thicknesses for the non-annealed and annealed samples were 13.5 and 8.6 μm, respectively. It was suggested that the advantages of annealing are not only to reduce drying time owing to the modification of ice crystal morphologies but also to avoid quality loss related to the structural deformation of the glassy matters.

Use of microreactors and freeze-drying in the manufacturing process of chitosan coated PCL nanoparticles

This paper is focused on the synthesis of chitosan-coated polycaprolactone nanoparticles in microreactors and on the freeze-drying of the nanosuspension, to separate the particles from the liquid phase. Nanoparticles were produced in the confined impinging jets mixer (CIJM) and in the multi-inlet vortex mixer (MIVM), using the solvent displacement method, with acetone or tert-butanol (TBA) as polymer solvent. The study was initially carried out considering a feed flow rate of 80 ml min−1: using acetone, the mean particle size was lower (163 ± 7 nm) and the Zeta potential was higher (31.4 ± 37 mV) with the MIVM, with respect to the CIJM (265 ± 31 nm and 9.8 ± 2.4 mV, respectively). Slightly larger particles were obtained using TBA in the MIVM (mean diameter: 221 ± 44 nm): in this case it is no longer required removing the solvent before the freeze-drying stage. The effect of the liquid flow rate was then investigated, confirming that the best results were obtained at 80 ml min−1. With respect to the freeze-drying process, the effect of lyoprotectants and of steric stabilizers on particle stability was investigated. Best results were obtained with 5% sucrose and 2.5% Poloxamer 388 (mean diameter: 306 ± 8 nm); in all cases Zeta potential remained positive and larger than +30 mV. Preliminary results about the encapsulation of a test drug, ciprofloxacin, are also shown and discussed.

Microstructural Characteristics and Mechanical Behavior of Spark Plasma-Sintered Cu–Cr–rGO Copper Matrix Composites

Copper matrix composites were prepared through spark plasma sintering (SPS) process, mixing fixed amount of reduced graphene oxide (rGO) with the different amounts of Cr. In the sintered bulk composites, the layered rGO network and uniform Cr particles distributed in the Cu matrix. Both of mechanical blending and freeze-drying stages of the wet-mixing process obtained the Cu/Cr/rGO mixture powders, and then SPS solid-phase sintering realized the faster densification of these mixture powders. The hardness and compressive yield strength of the Cu–Cr–rGO composites depicted the higher values than those of pure Cu and single rGO-added composite, and they were gradually increased with increasing Cr. The rGO/Cr hybrid second-phases are believed to be beneficial to strengthening Cu matrix. The relevant formation and strengthening mechanisms involved in Cu–Cr–rGO composites were discussed.

Internal Nano Voids in Yttria-Stabilised Zirconia (YSZ) Powder.

Porous yttria-stabilised zirconia ceramics have been gaining popularity throughout the years in various fields, such as energy, environment, medicine, etc. Although yttria-stabilised zirconia is a well-studied material, voided yttria-stabilised zirconia powder particles have not been demonstrated yet, and might play an important role in future technology developments. A sol-gel synthesis accompanied by a freeze-drying process is currently being proposed as a method of obtaining sponge-like nano morphology of embedded faceted voids inside yttria-stabilised zirconia particles. The results rely on a freeze-drying stage as an effective and simple method for generating nano-voided yttria-stabilised zirconia particles without the use of template-assisted additives.

Properties of as-prepared and freeze-dried hydrogels made from poly(vinyl alcohol) and cellulose nanocrystals using freeze-thaw technique

Poly(vinyl alcohol), PVA hydrogels are potential materials for biomedical and biotechnogical applications. However, their low mechanical properties restrict their use. In this study, the effect of PVA concentration, addition of nanocrystalline cellulose, CNC, number of freeze-thaw cycles and freeze-drying stage on properties of resulting hydrogels were investigated. The results showed that increase in PVA concentration and the addition of CNC improved the compressive properties of the hydrogels. Overall, increase in number of freeze-thaw cycles from 3 to 5 did not show any improvements in properties of hydrogels. Concentration of PVA had great effect on morphology of freeze-dried hydrogels. The CNC reduced crystallinity of PVA/CNC hydrogels as compared to PVA hydrogels. Rehydrated PVA and PVA/CNC hydrogels had higher compressive characteristics than their as-prepared analogues. In general, an improvement of compressive properties of hydrogels was achieved via reduction of their water content. In case of 5% PVA hydrogel, an addition of CNC was found to be beneficial because it increased degree of swelling and water content on rehydration.

A mathematical model of the process of freeze-drying

The study explores the process of freeze-drying of biological materials and the basic parameters that affect the quality of the original sample. We have considered the existing methods of controlling freeze-drying stages and their main limitations. The research findings will be used for modeling and optimizing the lyophilisation process. The paper focuses on the problem of monitoring the temperature of biological materials in vials during the freeze-drying. It is necessary to control the temperature of the product to keep it below the triple point (transition point) in order to avoid spoilage. It is also important to determine the position of the moving boundary of sublimation, which informs about completing the stage of primary drying. There are many models of developing temperature sensors, but none fully satisfies the needs of the process. Therefore, we have calculated a mathematical model that will be used further to develop a temperature sensor to control the process of sublimation. Our experimental study was processed in COMSOL Multiphysics 5.0 Final and tested at the State Scientific-Control Institute of Biotechnology and Strains of Microorganisms. The study has revealed a significant impact of the zone of acontact between the heater and the object of drying on the duration of dehydration (about 20%). The devised mathematical model will furtherallow conducting multivariate calculations aimed at minimizing energy consumption and choosing optimal treatment regimes in obtaining and maintaining a certain level of product quality. The suggested calculation method can help estimate the real duration of the vacuum freeze-drying,taking into account the conditions of the contact.

On the Use of tert-Butanol/Water Cosolvent Systems in Production and Freeze-Drying of Poly-ε-Caprolactone Nanoparticles

This work deals with the use of a water/tert-butyl alcohol (TBA) system in the manufacturing process of poly-ε-caprolactone (PCL) nanoparticles, namely in the synthesis stage, using the solvent displacement method in a confined impinging jet mixer (CIJM), and in the following freeze-drying stage. The experimental investigation evidenced that the nanoparticles size is significantly reduced with respect to the case where acetone is the solvent. Besides, the solvent evaporation step is not required before freeze-drying as TBA is fully compatible with the freeze-drying process. The effect of initial polymer concentration, flow rate, water to TBA flow rate ratio, and quench volumetric ratio on the mean nanoparticles size was investigated, and a simple equation was proposed to relate the mean nanoparticles size to these operating parameters. Then, freeze-drying of the nanoparticles suspensions was studied. Lyoprotectants (sucrose and mannitol) and steric stabilizers (Cremophor EL and Poloxamer 388) have to be used to avoid nanoparticles aggregation, thus preserving particle size distribution and mean nanoparticles size. Their effect, as well as that of the heating shelf temperature, has been investigated by means of statistical techniques, with the goal to identify which of these factors, or combination of factors, plays the key role in the nanoparticles size preservation at the end of the freeze-drying process.

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

Button mushroom slices were dehydrated using freeze drying (FD) or FD combined with microwave vacuum drying (FMVD), and the non-volatile component profiles were studied. The results showed that the level of non-volatile components in button mushroom firstly increased during sublimation of FD/FMVD process and then fell during desorption in FD process and MVD in FMVD process. Compared to FD products, the contents of soluble sugars and polyols in FMVD products were relatively low, whereas the contents of total free amino acids were significantly higher, close to the level of fresh mushroom. However, there was no significant difference in the contents of 5′-nucleotides and organic acids between FD and FMVD products. The equivalent umami concentration (EUC) values for FD and FMVD products did not differ from fresh, indicating that both drying methods could effectively preserve MSG (monosodium glutamate)-like components in button mushroom.

Microscale Freeze-Drying with Raman Spectroscopy as a Tool for Process Development

Until recently, the freeze-drying process and formulation development have suffered from a lack of microscale analytical tools. Using such an analytical tool should decrease the required sample volume and also shorten the duration of the experiment compared to a laboratory scale setup. This study evaluated the applicability of Raman spectroscopy for in-line monitoring of a microscale freeze-drying process. The effect of cooling rate and annealing step on the solid-state formation of mannitol was studied. Raman spectra were subjected to principal component analysis to gain a qualitative understanding of the process behavior. In addition, mannitol solid-state form ratios were semiquantitatively analyzed during the process with a classical least-squares regression. A standard cooling rate of 1 °C/min with or without an annealing step at -10 °C resulted in a mixture of α, β, δ, and amorphous forms of mannitol. However, a standard cooling rate induced the formation of mannitol hemihydrate, and a secondary drying temperature of +60 °C was required to transform the hemihydrate form to the more stable anhydrous polymorphs. A fast cooling rate of 10 °C/min mainly produced δ and amorphous forms of mannitol, regardless of annealing. These results are consistent with those from larger scale equipment. In-line monitoring the solid-state form of a sample is feasible with a Raman spectrometer coupled microscale freeze-drying stage. These results demonstrate the utility of a rapid, in-line, low sample volume method for the semiquantitative analysis of the process and formulation development of freeze-dried products on the microscale.