Effects of steam-microwave blanching and different drying processes on drying characteristics and quality attributes of Thunbergia laurifolia Linn. leaves

Research progress in fluid and semifluid microwave heating technology in food processing

Microwave is a form of electromagnetic radiation that has high penetration and heating efficiency in food processing. Uneven heating is the main problem of microwave processing, especially in solid foods. Fluid and semifluid media, which are good carriers in microwave processing, have uniform dielectric properties and good material fluidity. Herein, we review the development, application prospects, and limitations of microwave in fluid and semifluid food processing and the research progress in microwave heating with steam as carrier. The mixture of generated steam and tiny micro droplets from food material under the action of microwave can absorb microwave and transfer heat evenly, which effectively improves the uniformity of microwave heating. Due to the relatively uniform dielectric properties and consistent texture of fluid and semifluid food materials, uneven heating phenomenon during their microwave processing can be significantly inhibited. Based on the development of microwave heating technology and equipment design, the microbial inactivation and enzyme inhibition in fluid and semifluid food were improved and food product with better retention of nutrients and sensory profile were produced. Also, microwave radiation can be used to prepare the printing material or process the printed product for 3D food printing, which enhances the added value of 3D printed products and the personalization of food manufacturing. In future research, intelligent control technology can be applied in the microwave processing of fluid and semifluid food materials for various applications. Therefore, the processing conditions can be adjusted automatically.

New insights into moisture sorption characteristics, nutritional composition, and antioxidant and morphological properties of dried duckweed [Wolffia arrhiza (L.) Wimm]

BACKGROUND

Duckweed has been considered as an alternative future food material as a result of its high nutritional values, although it also has a high moisture content resulting in a short shelf life. Moisture sorption isotherms are used to design dehydration and storage conditions to prolong the shelf life food products. To date, information regarding the sorption isotherm of duckweed has not been reported. Scanning electron microscopy (SEM) is frequently used to study food microstructure. However, this technique has to be performed under high-vacuum conditions and takes a long time. In the present study, two-photon imaging microscopy was selected to investigate the microstructure of dried duckweed instead of SEM.

RESULTS

Among five sorption isotherm models, the Peleg model gave the highest goodness of fit. The monolayer moisture content (M₀ ) of duckweed was in the range 7.43-7.92% dry basis (d.b.) and 8.87-8.86% d.b. for the GAB and BET multilayer kinetic models, respectively. The moisture changing behavior at each relative humidity step could be described by two exponential and reaction order kinetics. A clear cell structure (hexagonal shape) and stomata, as well as structural images (both 2D and 3D), were obtained using the two-photon microscopy technique.

CONCLUSION

The Peleg model best described the moisture sorption behaviors of dried duckweed and the shape of sorption isotherms were classified as type III isotherm. The M₀ of dried duckweed ranged from 7.43 to 8.86% d.b. Two-photon microscopy was a potent tool for investigating the microstructure and composition of dried duckweed. © 2021 Society of Chemical Industry.

Recent Trends in Pretreatment of Food before Freeze-Drying

Drying is among the most important processes and the most energy-consuming techniques in the food industry. Dried food has many applications and extended shelf life. Unlike the majority of conventional drying methods, lyophilization, also known as freeze-drying (FD), involves freezing the food, usually under low pressure, and removing water by ice sublimation. Freeze-dried materials are especially recommended for the production of spices, coffee, dried snacks from fruits and vegetables and food for military or space shuttles, as well as for the preparation of food powders and microencapsulation of food ingredients. Although the FD process allows obtaining dried products of the highest quality, it is very energy- and time consuming. Thus, different methods of pretreatment are used for not only accelerating the drying process but also retaining the physical properties and bioactive compounds in the lyophilized food. This article reviews the influence of various pretreatment methods such as size reduction, blanching, osmotic dehydration and application of pulsed electric field, high hydrostatic pressure or ultrasound on the physicochemical properties of freeze-dried food and drying rate.

Effect of microwave freeze drying on quality and energy supply in drying of barley grass

BACKGROUND

Young barley grass leaves are well-known for containing the antioxidant substances flavonoid and chlorophyll. However, low product quality and energy efficiency exist with respect to the dehydration of barley grass leaves. To improve energy supply and the quality of barley grass, microwave heating instead of contact heat was applied for the freeze drying of barley grass at a pilot scale at 1, 1.5 and 2 W g^-1 , respectively; After drying, energy supply and quality parameters of color, moisture content, chlorophyll, flavonoids, odors of dried barley grass were determined to evaluate the feasibility of the study.

RESULTS

Microwave freeze drying (MFD) allowed a low energy supply and high contents of chlorophyll and flavonoids. A lightness value of 60.0, a green value of -11.5 and an energy supply of 0.61 kW h^-1  g^-1 were observed in 1.5 W g^-1 MFD; whereas drying time (7 h) decreased by 42% compared to contact heating. Maximum content of flavonoid and chlorophyll was 11.7 and 12.8 g kg^-1 barley grass. Microwave heating leads to an odor change larger than that for contact heating observed for the freeze drying of barley grass.

CONCLUSION

MFD retains chlorophyll and flavonoids, as well as colors and odors of samples, and also decreases energy consumption in the freeze drying of barley grass. © 2017 Society of Chemical Industry.

Effect of packaging types and storage conditions on quality aspects of dried Thunbergia laurifolia leaves and degradation kinetics of bioactive compounds

Thunbergia laurifolia leaves were dried by freeze drying (FD) and microwave heat pump dehumidified air drying (MHPD). The dried leaves were stored in polypropylene (PP) or aluminum laminated pouches (ALP) at 15, 25 or 35 °C and 60% RH. The samples were held for 180 days to observe changes in moisture content, color, total phenolics, antioxidant activity, catechin and caffeic acid. In general, samples in PP had a greater increase in moisture. Total phenolics content and ferric-reducing antioxidant power values increased for 120 days, then decreased thereafter. After 180 days, both FD and MHPD samples packaged in ALP and stored at 15 °C had the greatest total phenolics, antioxidant activity and bioactive compounds amongst the storage conditions. The first-order equation best described degradation behavior of catechin and caffeic acid for both drying technologies investigated.