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Both nonthermal pretreatment and nondestructive analysis are effective technologies in improving drying processes. This study evaluated the effects of different pretreatment methods on the quality of beef dehydrated by microwave vacuum drying (MVD) and compared the MVD process performance comprising real-time moisture content (MC), MC loss, colour content, and shrinkage rate using different optical sensing methods including terahertz time-domain spectroscopy (THz-TDS) and near-infrared hyperspectral imaging (NIR-HSI). Results indicated that osmotic pretreatment improved the drying rate of MVD beef with lower changes in colour and shrinkage rate. Both THz-TDS-based and NIR-HSI-based on-site direct scanning and in-situ in-direct sensing showed accurate prediction results, with best R2p of 0.9646 and 0.9463 for MC and R2p of 0.9817 and 0.9563 for MC loss prediction, respectively. NIR-HSI visualisation of MC results showed that ultrasound pretreatment curbed but osmotic pretreatment promoted nonuniform distribution during MVD. This research should guide improving the industrial MVD drying process.

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Moisture content (MC) is a critical quality indicator for food drying processing, however achieving in-situ non-destructive analyses of dynamic MC of products during processing is still a challenge. This study developed an in-situ indirect measurement method using Terahertz time-domain spectroscopy (THz-TDS) for real-time MC prediction of foods during microwave vacuum drying (MVD). During MVD, THz-TDS continuously sense the dynamic moisture vapour from the desiccator through a polyethene air hose. The obtained THz spectra were processed to calibrate MC loss prediction models using support vector regression, Gaussian process regression and ensemble regression. Then the MC was calculated using moisture loss prediction results. The best real-time MC prediction results for beef and carrot slices achieved R2 of 0.995, RMSE of 0.0162, and RDP of 22. The developed system provides a novel method for drying kinetics research during MVD and expands the applicability of the THz-TDS technique in the food industry.

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The molten salt chlorination method is more suitable to produce TiCl4 using titanium-containing materials with high contents of CaO and MgO in China. However, there is a large amount of molten salt chloride slags generated from the molten salt chlorination process, which contains a variety of chlorides and is difficult to treat, often causing serious environmental problems such as direct piling or landfilling. A novel process was proposed to deal with molten salt chloride slags, and calcium chloride could be effectively removed by this process (as in our previous study). However, magnesium chloride is another impurity which can deteriorate the density and viscosity of the molten salt; it is often found in higher contents, and should be also removed from molten salt chloride slags to efficiently recycle NaCl in novel processes. Na2SiO3 is added to the molten salt chloride slags in the molten state to produce the Mg-containing solid phase, which could be separated with the molten NaCl in novel processes. Thus, the transition of Mg-containing phases and the recovery of NaCl in molten salt chloride slags at high temperature in a novel process were systematically investigated in this work, including thermodynamic analysis, the phase transition behavior of Mg-containing phases, NaCl recovery, etc. The removal rate of Mg was 99.56% when the molar ratio of MgCl2:Na2SiO3 was 1:1.5 at 1273 K and in a N2 atmosphere. The recovery rate of NaCl from the molten salt chlorination slag was 97.62% and the purity of NaCl obtained was 99.35 wt%, which could be used in the molten salt chlorination process.

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Effects of food microstructure and processing methods on moisture migration in food during processing are important for improving product quality. This study evaluated the effects of fibre directions and dehydration methods on beef dehydration and rehydration behaviours using near-infrared hyperspectral imaging. Beef slices (transversal cut) and splits (longitudinal cut) were dehydrated by hot air drying (HAD) and microwave vacuum drying (MVD). Results indicated the moisture in the slices evaporated faster than that in the splits during HAD and MVD processed samples exhibited higher rehydration rates than HAD ones. Principal component analysis showed great performance in clustering samples based on fibre directions, processing methods and time. The established simplified RC-SG-PLSR and RC-2nd Der-PLSR models possessed good prediction accuracies for moisture (R2p = 0.994) and rehydration rate (R2p = 0.889), respectively. Chemical distribution maps also confirmed the influence of fibre directions and dehydration methods on moisture migration in samples.

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Dehydration is one of the most widely used food processing techniques, which is sophisticated in nature. Rapid and accurate prediction of dehydration performance and its effects on product quality is still a difficult task. Traditional analytical methods for evaluating food dehydration processes are laborious, time-consuming and destructive, and they are not suitable for online applications. On the other hand, vibrational spectral techniques coupled with chemometrics have emerged as a rapid and noninvasive tool with excellent potential for online evaluation and control of the dehydration process to improve final dried food quality. In the current review, the fundamental of food dehydration and five types of vibrational spectral techniques, and spectral data processing methods are introduced. Critical overtones bands related to dehydration attributes in the near-infrared (NIR) region and the state-of-the-art applications of vibrational spectral analyses in evaluating food quality attributes as affected by dehydration processes are summarized. Research investigations since 2010 on using vibrational spectral technologies combined with chemometrics to continuously monitor food quality attributes during dehydration processes are also covered in this review.

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