Recent processing of fruits and vegetables using emerging thermal and non-thermal technologies. A critical review of their potentialities and limitations on bioactives, structure, and drying performance

Fruits and vegetables have rich bioactive compounds and antioxidants that are vital for the human body and prevent the cell from disease-causing free radicals. Therefore, there is a growing demand for high-quality fruits and vegetables. Nevertheless, fruits and vegetables deteriorate due to their high moisture content, resulting in a 40-50% loss. Drying is a common food preservation technique in the food industry to increase fruits and vegetables' shelf-life. However, drying causes chemical modifications, changes in microstructure, and bioactives, thus, lowering the final product's quality as a considerable amount of bioactives compounds and antioxidants are lost. Conventional pretreatments such as hot water blanching, and osmotic pretreatment have improved fruit and vegetable drying performance. However, these conventional pretreatments affect fruits' bioactive compounds retention and microstructure. Hence, emerging thermal (infrared blanching, microwave blanching, and high-humidity hot-air impingement blanching) and non-thermal pretreatments (cold plasma, ultrasound, pulsed electric field, and edible films and coatings) have been researched. So the question is; (1) what are the mechanisms behind emerging non-thermal and thermal technologies' ability to improve fruits and vegetables' microstructure, texture, and drying performance? (2) how do emerging thermal and non-thermal technologies affect fruits and vegetables' bioactive compounds and antioxidant activity? and (3) what are preventing the large-scale commercialization of these emerging thermal and non-thermal technologies' for fruits and vegetables, and what are the future recommendations? Hence, this article reviewed emerging thermal blanching and non-thermal pretreatment technologies, emphasizing their efficacy in improving dried fruits and vegetables' bioactive compounds, structural properties, and drying performance. The fundamental mechanisms in emerging thermal and non-thermal blanching pretreatment methods on the fruits and vegetables' microstructure and drying performance were delved in, as well as what are preventing the large-scale commercialization of these emerging thermal and non-thermal blanching for fruits and vegetables, and the future recommendations. Emerging pretreatment approaches not only improve the drying performance but further significantly improve the retention of bioactive compounds and antioxidants and enhance the microstructure of the dried fruits and vegetables.

Effects of carboxymethyl cellulose/pectin coating combined with ultrasound pretreatment before drying on quality of turmeric (Curcuma longa L.)

Turmeric is an herb with multiple bioactive substances and health benefits. Drying is one of the most important steps of its processing and sales. In order to obtain high-quality turmeric products, we used five different pretreatment methods to treat turmeric prior to pulse-spouted microwave vacuum drying (PSMVD), including carboxymethyl cellulose coating (CMC), pectin coating (P), ultrasound (US) and their combination (CMCUS or PUS). The effect of different pretreatments on the drying kinetics, quality attributes and microstructure of turmeric were evaluated. Results showed that the US pretreatment had the shortest drying time (60 min), while coating treatment did not significantly affect drying rate. Dried turmeric with coating pretreatment had lower rehydration ratio and water adsorption capacity compared with individual ultrasound treatment. Carboxymethyl cellulose coating protected bioactive substances better than pectin coating. Moreover, CMCUS pretreatment showed significantly lower total color change, higher curcumin content, total phenols and flavonoid content as well as antioxidant capacity in all dried samples. Microstructure observation showed that the polysaccharide coating covering the surface of turmeric might reduce the degradation of bioactive compounds. Therefore, the CMCUS pretreatment before PSMVD of turmeric was recommended due to the efficiency and quality protections.

Edible coatings as osmotic dehydration pretreatment in nutrient-enhanced fruit or vegetable snacks development: A review

Edible coatings (ECs) are thin layers applied on food to protect it and improve quality. They are made from bio-based materials such as polysaccharides, proteins, lipids, or their composites. The incorporation of functional agents, such as bioactive compounds, vitamins, or antimicrobials into the EC, has been investigated to control the shelf life of many food products from horticulture ones to processed food. Osmotic dehydration (OD) as a mild technology may also positively impact the availability of innovative fruit snacks and consequently influence consumer health. Combination of the EC with the OD aims to remove water through the semipermeable membrane while limiting the transfer of solutes from the dehydrated tissue and in the opposite direction from the osmotic solution to the food. The development trend of the snack market is expanding, especially with health-promoting properties. Consumers pay increasing attention to quality of food and its beneficial effects on health. This review attempts to provide the advancement of recent studies on the application of the EC before the OD of different fresh or fresh-cut fruit and vegetables. A fundamental theory related to the methodology of creating the EC, their composition, and the influence on the physicochemical properties of products that are osmo-dehydrated to a medium water content or additionally dried to a low water content have been described. Efforts have been exerted to introduce hydrocolloids used in the production of the EC, including new sources of biopolymers such as agricultural waste and by-products. The perspectives of using ECs in the technology of producing pro-healthy snacks are emphasized.

Effects of sodium alginate-based coating pretreatment on drying characteristics and quality of heat pump dried scallop adductors

BACKGROUND

Drying efficiency and quality maintenance are the major concerns of both manufactures and consumers. Heat pump drying (HPD) is suitable for heat sensitive foodstuffs due to its ability to independently control the drying operation parameters. However, lower drying rate and energy efficiency in the later period of HPD are the bottlenecks that restrain its application. A novel approach using hydrocolloids as pretreatment coatings prior to drying was designed to solve these problems. The effects of sodium alginate (SA) coating, drying temperatures and air velocities on the drying characteristics and quality attributes of scallop adductors were evaluated.

RESULTS

Drying took place in the falling rate period. Drying time decreased with increasing temperature, air velocity and SA coating. The Two Term model and the Wang and Singh model gained the best fit for thin-layer drying of scallop adductors and SA film, respectively. Effective moisture diffusivity increased with temperature, velocity and SA coating and were in the range 7.352-14.620 × 10^-11 , 9.890-17.100 × 10^-11 and 2.348-4.604 × 10^-10  m²  s^-1 for uncoated scallop adductors, SA coated scallop adductors and SA films, respectively. The activation energies for SA films, coated and uncoated scallop adductors were 17.07, 20.78 and 26.17 kJ mol^-1 , respectively. Dried scallop adductors with SA coating pretreatment exhibited a significant lower value of shrinkage rate and hardness, and higher value of toughness than uncoated ones at 30 °C and 2.0 m s^-1 (P < 0.05).

CONCLUSION

Hydrocolloid coating is a promising pretreatment in improving HPD efficiency and enhancing quality attributes of dried scallop adductors. © 2019 Society of Chemical Industry.

Ultrasound-assisted intensification of a hybrid intermittent microwave - hot air drying process of potato: Quality aspects and energy consumption

The objective of this study was to intensify combined intermittent microwave - low temperature (40 °C) hot air drying with ultrasound. The process variables were ultrasound time (40 kHz for 0, 10, 20 and 30 min), microwave power (360, 600 and 900 W), and microwave pulse ratio (1, 2, 3 and 4). Results showed that the highest reduction in moisture content was observed in the samples pretreated with ultrasound for 10 min and then dried at 900 W microwave with the pulse ratio of 4. As ultrasound time, microwave power and pulse ratio increased, significant increases were noticed in D_eff by 4.89, 16.44 and 20.7%, respectively. Moreover, shrinkage was lower in the samples pretreated with lower ultrasound time and microwave pulse ratio. Besides, bulk density reduced when the microwave power was increased due to lower volume reduction. In addition, the highest increase in rehydration (32.23%) was observed in the samples dried using the high-power intermittent microwave. Finally, the highest significant reduction in specific energy consumption as a result of increased microwave power was 23.32%. In general, results of this study demonstrated that the ultrasound-intensified combined intermittent microwave - low temperature hot air drying may be a suitable alternative for industrial applications.

Influence of a three stage hybrid ultrasound-osmotic-frying process on production of low-fat fried potato strips

BACKGROUND

Fried potato is one of the most consumed products in the world. Due to consumers' growing tendency to use healthy and low-fat foods, reducing oil content in fried foods has become a necessity. Several studies have shown that higher initial water content results in increased oil uptake during frying. Therefore, pretreatments that reduce water content of the product could lower oil uptake in the final product. The aim of this study was to evaluate the influence of a three-stage hybrid ultrasound-osmotic-frying process on production of low-fat fried potato strips.

RESULTS

Results showed that, compared to control samples, osmotic pretreated samples using saline solutions at concentrations of 2% and 4% decreased oil uptake by 29.5% and 32.7%, respectively. Ultrasound (28 and 40 kHz) also showed a significant synergistic effect on reducing oil uptake in the samples pretreated with both ultrasound and osmotic dehydration so that different samples pretreated with both ultrasound and osmotic dehydration decreased oil uptake from approximately 40% to more than 50%, compared to untreated control samples.

CONCLUSION

Owing to production of low-fat fried potato strips, utilising osmotic dehydration pretreatment was desirable before the frying process. Regarding low mass transfer rate during osmotic treatment, ultrasound was applied to enhance mass transfer rate. The use of ultrasound pretreatment in the frying process can yield promising results in reducing oil uptake. © 2017 Society of Chemical Industry.

Modification of Cell Wall Polysaccharides during Drying Process Affects Texture Properties of Apple Chips

The influences of hot air drying (AD), medium- and short-wave infrared drying (IR), instant controlled pressure drop drying (DIC), and vacuum freeze drying (FD) on cell wall polysaccharide modification were studied, and the relationship between the modifications and texture properties was analyzed. The results showed that the DIC treated apple chips exhibited the highest crispness (92) and excellent honeycomb-like structure among all the dried samples, whereas the FD dried apple chips had low crispness (10), the minimum hardness (17.4 N), and the highest volume ratio (0.76) and rehydration ratio (7.55). Remarkable decreases in the contents of total galacturonic acid and the amounts of water extractable pectin (WEP) were found in all the dried apple chips as compared with the fresh materials. The highest retention of WEP fraction (102.7 mg/g AIR) was observed in the FD dried apple chips, which may lead to a low structural rigidity and may be partially responsible for the lower hardness of the FD apple chips. In addition, the crispness of the apple chips obtained by DIC treatment, as well as AD and IR at 90°C, was higher than that of the samples obtained from the other drying processes, which might be due to the severe degradation of pectic polysaccharides, considering the results of the amounts of pectic fractions, the molar mass distribution, and concentrations of the WEP fractions. Overall, the data suggested that the modifications of pectic polysaccharides of apple chips, including the amount of the pectic fractions and their structural characteristics and the extent of degradation, significantly affect the texture of apple chips.

Natural Pectin Polysaccharides as Edible Coatings

The most fashionable trends in food packaging research are targeted towards improvements in food quality and safety by increasing the use of environmentally-friendly materials, ideally those able to be obtained from bio-based resources and presenting biodegradable characteristics. Edible films represent a key area of development in new multifunctional materials by their character and properties to effectively protect food with no waste production. The use of edible films should be considered as a clean and elegant solution to problems related with waste disposal in packaging materials. In particular, pectin has been reported as one of the main raw materials to obtain edible films by its natural abundance, low cost and renewable character. The latest innovations in food packaging by the use of pectin-based edible films are reviewed in this paper, with special focus on the use of pectin as base material for edible coatings. The structure, properties related to the intended use in food packaging and main applications of pectins are herein reported.