Yunhong Liu

Technical aspects in freeze-drying of foods

ABSTRACT Drying is a widespread concept in the food industry, typically used to convert a surplus crop into a shelf-stable commodity. With advancement of technology, however, there is interest in moving forward from the traditional convective air drying that is most widely used today for foods, to maintain at a very high level the nutritional and organoleptical properties of the initial fresh product. Freeze-drying (FD) produces the highest quality food product obtainable by any drying method, but it is considered the most expensive operation for manufacturing a dehydrated product owing to high energy consumption and high costs of both operation and maintenance. Microwave freeze-drying (MFD) and atmospheric freeze-drying (AFD) have been developed to reduce the FD energy consumption. The product quality of these two drying methods is similar to FD, due to removal of water content in materials by sublimation in both MFD and AFD. Although a significant amount of scientific research has been carried out in the field of sublimation drying, there are only a few comprehensive summarizations about the various sublimation drying methods. As a result, this review aims to highlight some of the latest and most notable advancements in sublimation-drying of foods, with main emphasis given to recent developments of reducing energy consumption of FD process and suggests future research areas on sublimation-related drying.

Hot air drying of purple-fleshed sweet potato with contact ultrasound assistance

ABSTRACT A drying technique using a combination of a contact ultrasound apparatus and a hot air dryer is developed to investigate the strengthening effect of contact ultrasound on hot air drying. The effects of drying parameters such as ultrasound power and drying temperature on drying characteristics, effective moisture diffusivity (Deff), microstructure, glass transition temperature (Tg), rehydration ratio, and color difference are discussed. The results show that the application of contact ultrasound causes a significant acceleration of internal mass transfer, and higher ultrasound power applied leads to faster drying rate. The effect of ultrasound power on drying rate decreases along with the reduction of moisture content during drying process. The increase in drying temperature significantly reduces drying time but has a little negative influence on the strengthening effect of ultrasound. Deff values range from 1.0578 × 10−10 to 5.4713 × 10−10 m2/s in contact ultrasound-assisted hot air drying of purple-fleshed sweet potato and increase significantly with an increase in drying temperature as well as ultrasound power. The microstructure of purple-fleshed sweet potato is greatly different at different ultrasound powers during contact ultrasound-assisted hot air drying and shows more microchannels and dilated intercellular spaces in the cross-section of purple-fleshed sweet potato micrographs at higher ultrasound power. Contact ultrasound application during hot air drying could improve the mobility of water and consequently reduce glass transition temperature. Lower color difference and higher rehydration ratio could be achieved as drying temperature decreases and ultrasound power increases. The increase in contact ultrasound power could reduce energy consumption of drying process up to 34.60%. Therefore, contact ultrasound assistance is a promising method to enhance hot air drying process.

A Mathematical Model for Vacuum Far-Infrared Drying of Potato Slices

A mathematical model of mass and heat transfer for vacuum far-infrared drying of potato slices is introduced in this study on the basis of energy and diffusion equations. The finite difference method is used to mathematically simulate the sample temperature and moisture content in different drying conditions. Calculated results are compared with the experimental findings at varying conditions of heater temperature (120°C, 140°C, and 160°C), chamber pressure (1500, 8000, and 15000 Pa), sample thickness (0.004, 0.006, and 0.008 m), and radiation distance (0.08, 0.12, and 0.16 m). Comparison results show that the model fits well the changes in sample temperature and moisture content at different times of drying, with the values of the coefficient of determination close to 1.0 and the relative error values less than 10%.

The Influences of Drying Process on Crude Protein Content of Naked Oat Cut Herbage (Avena nuda L.)

The effects of thin-layer drying naked oat cut herbage process (hot air temperature, air velocity, and drying time) on crude protein content were investigated. It was observed that an increase in drying temperature could lead to a significant drop in the crude protein content of naked oat cut herbage. The crude protein content also decreased with an increase in drying time and air velocity, and air velocity had a minimal impact during processing. A thin-layer drying mathematical model of crude protein content for naked oat cut herbage was established. The optimal drying parameters were determined as hot air temperature of 88.6–94.2 ∘ C, air velocity of 0.28–0.32 m/s, and drying time of 16.4–20.6 min.

Detection Potential of Multi-Features Representation of E-Nose Data in Classification of Moldy Maize Samples

In order to assess rapidly and timely the moldy degree of maize samples using electronic nose (E-nose) and improve the correct classification rate of E-nose, the different feature representation modes (DFRM) for E-nose data were explored in depth. A determining method for multi-features vector of E-nose based on Wilks Λ statistic was introduced so as to obtain the best multi-features vector for characterizing E-nose data. And then a selection method of representation features of each sensor signals based on elimination transform with pivoting of the Λ statistic was also introduced for the different excitation characteristic of each gas sensor. The research results show that the classification effect of multi-features representation mode (MFRM) is better than that of single feature representation mode (SFRM), and the MFRM is not a regular pattern, but the best multi-features vector of E-nose in MFRM can be obtained by the determining method. Moreover, it is necessary to select the representation features of each sensor signals in the MFRM using the selection method. The visual inspection results based on SFRM and MFRM were examined by Fisher discriminant analysis (FDA) and proved that the introduced methods were very effective, the highest correct discrimination rate based on SFRM is 80%, while the correct discrimination rate of the five features combination is 97%. As an outlook, we believe that the research findings may be universally applied for the classification of other food and agriculture products using E-nose.

Potential of hyperspectral imaging for rapid identification of true and false honeysuckle tea leaves

Honeysuckle (Lonicerae japonicae Flos, LJF) tea is a favorite cool tea in China and Southeast Asia. However, some unscrupulous traders usually use Lonicera Flos (LF) as LJF to sell for earning high profit. In order to identify true and false honeysuckle tea leaves rapidly and precisely, hyperspectral imaging technology was applied to develop a nondestructive identification model for LJF and LF. Firstly, the original spectral data were analyzed by three pretreatment methods including Savitzky–Golay (SG) convolution smoothing, multiple scatter correct and standard normal variate transformation (SNV). Then, a full-band analysis model was established by using the partial least squares-discriminant analysis method. And after the selection of characteristic wavelengths by regression coefficients algorithm, the identification analysis models based on the back-propagation neural network and extreme learning machine (ELM) discriminant were established. The results showed that the BP neural network and ELM discriminant analysis model based on SNV denoising at 9 characteristic wavelengths could achieve the best identification results. The recognition rates of both modeling sets and forecasting sets could reach 100%. Therefore, the application of hyperspectral imaging technology can identify LJF and LF effectively and nondestructively, and has potential in the identification of true and false honeysuckle tea leaves.

Contact ultrasound strengthened far-infrared radiation drying on pear slices: Effects on drying characteristics, microstructure, and quality attributes

Abstract In order to investigate the feasibility and the enhancing effect of contact ultrasound application during far-infrared radiation (FIR) drying, a contact ultrasound strengthened FIR (CUFIR) drying equipment was fabricated and used, and CUFIR drying experiments on pear slices were carried out to explore the synergetic effects of ultrasound power and FIR heating on drying characteristics, microstructure, and quality of dried pear products. The results show that the application of contact ultrasound could be obviously helpful to accelerate internal mass transfer in pear slices and improve the drying rate of FIR drying, and higher ultrasound power could lead to stronger strengthening effect. The enforcing effect of ultrasound increased at higher FIR power, and weakened with the reduction of moisture content during CUFIR drying. The Deff values ranged from 4.76 × 10−10 m2/s to 13.94 × 10−10 m2/s in this study and the increase of both FIR power and ultrasound power had significant and positive influence on the increasing of Deff values. With scanning electrical microscope (SEM), it was observed that the improvement of ultrasound power could enlarge the size of microcapillaries and even generate new micropores and microchannels on the ultrasound-treated surface and in the organism structure of pear slices. The increase of ultrasound power could improve total phenolic content (TPC) and total flavonoids content (TFC) of pear slices at FIR powers of 100 and 220 W. Yet, the application of contact ultrasound with ultrasound power of 60 W had negative influence on TPC and TFC at FIR power of 340 W. Although the ascorbic acid content (AAC) reduced as FIR powers increased during FIR drying without contact ultrasound assistance, the increase of ultrasound power could improve AAC at all FIR powers. The CUFIR drying at ultrasound power of 60 W and FIR power of 220 W achieved the lowest energy consumption. Therefore, the application of FIR drying combined with contact ultrasound is a promising method to improve drying rate as well as protect product quality.