Ultrasound as pretreatment for drying garlic slices in microwave and convective dryer

Integrating Image Analysis and Machine Learning for Moisture Prediction and Appearance Quality Evaluation: A Case Study of Kiwifruit Drying Pretreatment

The appearance of dried fruit clearly influences the consumer's perception of the quality of the product but is a subtle and nuanced characteristic that is difficult to quantitatively measure, especially online. This paper describes a method that combines several simple strategies to assess a suitable surrogate for the elusive quality using imaging, combined with multivariate statistics and machine learning. With such a convenient tool, this study also shows how one can vary the pretreatments and drying conditions to optimize the resultant product quality. Specifically, an image batch processing method was developed to extract color (hue, saturation, and value) and morphological (area, perimeter, and compactness) features. The accuracy of this method was verified using data from a case study experiment on the pretreatment of hot-air-dried kiwifruit slices. Based on the extracted image features, partial least squares and random forest models were developed to satisfactorily predict the moisture ratio (MR) during drying process. The MR of kiwifruit slices during drying could be accurately predicted from changes in appearance without using any weighing device. This study also explored determining the optimal drying strategy based on appearance quality using principal component analysis. Optimal drying was achieved at 60 °C with 4 mm thick slices under ultrasonic pretreatment. For the 70 °C, 6 mm sample groups, citric acid showed decent performance.

Application of novel pretreatment technologies for intensification of drying performance and quality attributes of food commodities: a review

Drying is an energy-intensive process that can be reduced by the application of pretreatment prior to drying to enhance mass transfer and minimize energy consumption. This review summarizes the mechanistic aspects and applications of emerging pretreatment approaches, namely ohmic heating (OH), ultrasound (US), high pressure processing (HPP), and pulsed electric field (PEF), with emphasis on the enhancement of mass transfer and quality attributes of foods. Novel pretreatments significantly improved the drying efficiency by increasing mass transfer, cavitation, and microchannel formation within the cell structure. Various processing parameters have great influence on the drying performance and quality attributes of foods. Several studies have shown that novel pretreatments (individual and combined) can significantly save energy while improving the overall drying performance and retaining the quality attributes. This work would be useful for understanding the mechanisms of novel pretreatment technologies and their applications for future commercial research and development activities.

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.

Advances in drying techniques for retention of antioxidants in agro produces

Antioxidants are compounds that are essential for the human body which prevents cell from disease causing free radicals. Antioxidants are present in a wide range of fruits, vegetables, and spices. However, a considerable amount of antioxidants is lost during the post-harvest drying operation of agro produces for their shelf-life enhancement. Hence, retention of antioxidants becomes utmost importance in preserving the nutritional aspects of fruits and vegetables. Compared to conventional hot air drying, methods like freeze drying, vacuum drying, and dehumidified drying helps in the retention of antioxidants. However, the drawbacks prevalent in current drying practices, such as high-power consumption and high capital cost, could be eliminated by adopting novel drying mechanisms. This review focuses on various pretreatment methods like ultra-sonication, high pressure processing, pulsed electric field and ethanol treatment prior to drying operation helps in enhancing the drying efficiency with maximum retention of antioxidants. In addition, hybrid drying technologies such as microwave assisted drying, IR-radiated drying and electro-magnetic assisted drying methods also could significantly improve the retention of antioxidants.HIGHLIGHTSDrying is the most commonly adopted unit operation for enhancing the shelf life of perishable agro produces.However, drying is accompanied by loss of bioactive, color, texture, and sensory attributes.Compared to conventional drying techniques like hot air drying, methods like freeze drying, vacuum drying and dehumidified drying helps in the retention of antioxidants present in agro/food produces.Pretreatment methods like Ozonation, ultra-sonication, and UV radiation prior to drying are also found to improve the drying performance with good retention of antioxidants.Recent developments like microwave-assisted and IR-assisted drying methods perform well in the retention of antioxidants with less energy consumption.

Effect of intermittent microwave convective drying on physicochemical properties of dragon fruit

The study was carried out to investigate the effect of Intermittent microwave convective drying (IMCD) on the overall quality of dried dragon fruit in terms of total phenolic content, color change, and rehydration ratio. Three levels of microwave power (200-600 W) and a temperature of 60 °C for hot air were applied alternately throughout the process with three levels of pulse ratio such as 1:10, 1:20, and 1:40, respectively. The total phenolic content of the dragon fruit slice obtained by IMCD was ranged between 5.750 and 6.575 mg GAE/g dry weight. Within the experimental range of process variables under IMCD conditions, the drying efficiency, color change, and rehydration ratio of the dried dragon fruit slices were 15.287-51.930%, 18.643-24.847, and 1.908-3.239, respectively. The Weibull model scale (α) parameter was found to vary between 27.512 - 498.174 , while the shape (β) parameter was found to vary between 0.769 - 0.851 . The Weibull model parameters were shown to decrease with increasing microwave power at constant pulse ratio. The IMCD method produced a dried dragon fruit slices with reduced color changes and higher total phenolic content and rehydration ratio values. This investigation would contribute to the development of effective drying techniques for increased food quality and product consistency in the drying of diverse fruits and vegetables.

Enhancing the Efficacy of Microwave Blanching-cum-black Mould Inactivation of Whole Garlic (Allium sativum L.) Bulbs Using Ultrasound: Higher Inactivation of Peroxidase, Polyphenol Oxidase, and Aspergillus niger at Lower Processing Temperatures

The freshly harvested whole garlic bulbs require the inactivation of peroxidase (POD), polyphenol oxidase (PPO), and Aspergillus niger. However, the conventional hot water blanching and modern pretreatment like ultrasound (US) and microwave (MW) cannot individually inactivate both the enzymes and Aspergillus niger to the desired levels without compromising the quality of the garlic due to either of the higher process temperatures (> 85 °C) or prolonged treatment times. Therefore, a two-stage sequential US followed by MW pretreatment for garlic bulbs was developed for simultaneous inactivation of POD, PPO, and Aspergillus niger to the desired levels and overcome the individual pretreatment drawbacks. The separate experiments were conducted for US and MW pretreatment using central composite design, and optimization was carried out using response surface methodology. When temperature constraint was considered during optimization, the US was able to reduce POD, PPO, and Aspergillus niger by 80.87%, 93.80%, and 2.60 logs, respectively, whereas MW reduced POD, PPO, and Aspergillus niger by 77.84%, 77.04%, and 1.90 logs, respectively. The US treatment (58.43 WL⁻¹ ultrasound power density for 40 min with an initial bath temperature of 60 °C) followed by MW treatment (3 Wg⁻¹ MW power density for 120 s) resulted in 90.37% POD and 92.38% PPO inactivation with 2.62 log reduction in Aspergillus niger. The maximum temperature reached in US + MW process was 83 °C which ensured no severe thermal damage to the garlic bulbs. The scanning electron microscopic images indicated that ultrasonication induced the porous structure in garlic and helped microwaves increase the product temperature rapidly and achieve the higher inactivation of enzymes and Aspergillus niger. Thus, the US was found to be enhancing the efficacy of the MW heating process.

Osmotic, osmovacuum, sonication, and osmosonication pretreatment on the infrared drying of Ginkgo seed slices: Mass transfer, mathematical modeling, drying, and rehydration kinetics and energy consumption

This study evaluated the mass transfer, drying, and rehydration kinetics (drying and rehydration curve, moisture diffusivity [D_eff ]), energy consumption (specific energy consumption [SEC], moisture extraction rate (MER), and specific moisture extraction rate [SMER]), and mathematical modeling of infrared dried Ginkgo biloba seed (GBS) using the various nonthermal pretreatments namely: osmotic (OS), osmovacuum (V + OS), ultrasound (US, ginkgo seed immersed in a distilled water with US), and osmosonication (US + OS, ginkgo seeds immersed in an OS solution with US). Results showed that various pretreatments affected mass transfer, drying, and rehydration characteristics, and energy consumption, which was confirmed by principal component analysis. In terms of mass transfer, US pretreatment recorded the highest weight loss while the osmosonication pretreatment registered the highest solid gain. The entire drying process occurred in the falling-rate period. The D_eff values were within the normal range of agroproducts (10^-11 to 10^-8 m² /s). The modified Page-I and Weibull model best fitted the drying and rehydration kinetics, respectively, with the coefficient of determination (R² ) > 0.991, root mean square error, residual sum of squares, and reduced chi-square closer to zero, compared with the other models. The untreated GBS (control) had the lowest energy efficiency (lowest SMER and MER) and the highest SEC than the pretreated GBS. Among the various pretreatments, the US pretreatment of GBS was superior, with the highest D_eff , MER, SMER, and drying rate, and lowest drying time and SEC. Based on the findings, sequential US pretreatment and infrared drying is a feasible drying technique for GBS that could be used commercially. PRACTICAL APPLICATION: Ginkgo tree cultivation in China has exceeded market needs with 60,000 tons per annum of GBS produced. Hence, there is a compelling need to explore new chances to use GBS availability irrespective of the seasonality and address the problem where GBS utilization is limited to the early phases of home-cooked dishes. Although drying increases the shelf life of ginkgo seeds, there is a higher operation cost. Thus, pretreatment can reduce energy consumption and augment the product quality is ideal. This research reported the impact of nonthermal pretreatments on ginkgo seeds' mass transfer, drying, and rehydration characteristics. The present results will provide a comprehensive understanding of the engineering application of ginkgo seed pretreatment, allowing for the best technique to be selected.

Optimization of foaming process: drying behaviour, physicochemical, and powder properties of hot air-assisted foam-mat dried nectarine

Nectarine powder is widely used in the industries of baking and confectionery. The production of nectarine powder can be made by several drying techniques such as spray, tray, drum, freeze, and foam mat. This study was aimed to optimize the parameters of the nectarine foaming process. Besides, hot air-assisted foam-mat drying of nectarine was carried out to evaluate the effect of different temperatures (50, 60, and 70 °C) on drying kinetics, physicochemical and powder properties of nectarine powder. Factors studied were egg albumin concentration, carboxymethyl cellulose concentration, and whipping time that varied between 10 and 30% (by weight), 0.2–0.8% (by weight), and 3–5 min, respectively. Optimum conditions were determined as 30% of egg albumin, 0.8% carboxymethyl cellulose, and a whipping time of 5 min to get maximum foam expansion, high foam stability, and minimum foam density. The drying rate and effective moisture diffusivity of nectarine foam powder increased with increasing drying temperature. Carr Index and Hauser Ratio values were in the range of 32.31–47.00 and 1.48–2.00, respectively. Foamed nectarine powder dried at 70 °C had the lowest hygroscopicity value and the highest wettability value. No significant difference was found between the powders’ porosity (p > 0.05). The powders produced at 50 °C resulted in higher total phenolic, vitamin C, and carotenoid content.

Influence of Ultrasound and the Conditions of Convective Drying with Dehumidified Air on the Course of the Process and Selected Properties of Apple Tissue

The conditions of convective drying induce a negative effect on the quality of the dried products, and thus, innovative solutions, such as the utilization of ultrasound and dehumidified air are gaining importance. Therefore, the aim of this study was to evaluate the impact of ultrasound pretreatment and variable air temperature on the kinetics of convective drying with dehumidified air and the quality of dried apples. Apples were dried with dehumidified and non-dehumidified air at 55, 70 and 85 °C. Preliminary ultrasound treatment was conducted by immersion for 60 min. The effect of both ultrasound and dehumidified air utilization was more prominent in the terms of drying time reduction, when lower drying temperatures were used. Drying of apples with dehumidified air, preceded by ultrasound pretreatment, resulted in a greater rehydration capacity of the products, and limited the browning process. Dehumidified air increased the lightness of the dried products, while sonication darkened them. The use of ultrasound before drying coupled with a drying with dehumidified medium at a temperature of 70 °C reduced the exposure of the product to a high temperature and oxygen. Products treated before dehumidified air drying with ultrasounds were characterized by high total phenolic content and the greatest antioxidant activity. This was because such technological treatment reduced exposure of the product to a high temperature and oxygen.

Enhancing carrot convective drying by combining ethanol and ultrasound as pre-treatments: Effect on product structure, quality, energy consumption, drying and rehydration kinetics

Ultrasound was combined with ethanol to improve different aspects of carrot convective drying, evaluating both processing and product quality. The ultrasound in water treatment resulted in cellular swelling and small impact on texture. Differently, the ultrasound in ethanol and ethanol treatments modified both carrot microstructure (cell wall modifications of parenchymatic tissue) and macrostructure (shrinkage and resistance to perforation). Pre-treatments with ultrasound in ethanol and ethanol improved the drying kinetics, reducing the processing time (~50%) and the energy consumption (42-62%). These pre-treatments also enhanced rehydration, whose initial rate and water retention were higher than the control. In addition, the carotenoid content was preserved after drying, for all the treatments. Any impact on shrinkage was observed. A mechanistic discussion, based on structural modification (microstructure and macrostructure) and physical properties of water and ethanol, was provided. As conclusion, this work not only described positive aspects of combining the technologies of ultrasound and ethanol as pre-treatments to convective drying, but also proposed mechanisms to explain the phenomena.

Sonication, osmosonication and vacuum-assisted osmosonication pretreatment of Ghanaian garlic slices: Effect on physicochemical properties and quality characteristics

This work aimed to assess the impact of sonication (US), osmosonication (US + OD) and vacuum-assisted osmosonication (V + US + OD) pretreatments on the quality of Ghanaian garlic prior to relative humidity convective drying. Hence, slices of fresh garlic subjected to US, US + OD and V + US + OD pretreatments were dried and the following assessed: antioxidant activities, total phenolic content (TPC), total flavonoid content (TFC), polyphenol oxidase (PPO) enzyme inactivation, rehydration ratio (RR), drying kinetics, energy consumption, chromatographic fingerprinting, allicin content and Fourier-transform infrared (FT-IR) spectroscopy. V + US + OD pretreatment gave the best results for antioxidant activities, TPC, TFC, RR and PPO enzyme inactivation. It also recorded the shortest drying time and was more energy efficient. Finally V + US + OD pretreatment maintained the chemical integrity of the finished product and recorded the highest content of allicin. A general trend was however observed for all quality parameters assessed for the various pretreatment methods as: V + US + OD > US + OD > US.

Effects of physical and chemical pretreatments on drying and quality properties of blackberry (Rubus spp.) in hot air dryer

This research examines the impact of various pretreatments on effective moisture diffusivity coefficient (Deff ), activation energy (Ea ), specific energy consumption (SEC), color, and shrinkage of blackberry (Rubus spp.). Hot air drying experiments were conducted under three different temperatures (50, 60, and 70°C) and four pretreatments, including thermal pretreatment by hot water blanching at 70, 80, and 90°C, pulse pretreatment with microwave having power of 90, 180, and 360 W, chemical pretreatment using ascorbic acid (1% in distilled water), and mechanical pretreatment using ultrasonic vibration with working frequency of 28 ± 5% kHz for 15, 30, and 45 min. The results show that the highest Deff value, which was 1.00 × 10-8 m2/s, could be achieved by using a microwave pretreatment with power and drying temperature of 360 W and 70°C͘, respectively. Moreover, the lowest Deff value obtained from this similar pretreatment condition was 3.10 × 10-9 m2/s at a drying temperature of 50°C, while Ea ranged from 13.61 to 26.02 kJ/mol. The highest and lowest SECs were 269.91 kW hr/kg for the control sample and 75.63 kW hr/kg for the microwave pretreatment, respectively. Furthermore, the largest color change and shrinkage were detected in ascorbic acid pretreatment and control sample, respectively.

Application of non-thermal pretreatment techniques on agricultural products prior to drying: a review

BACKGROUND

Most agricultural crops contain high moisture content (80-95% wet basis (wb)) which makes them very susceptible to microbial damage leading to shorter shelf-life and high postharvest losses. The high perishability of these agricultural products requires preservation techniques to prolong their shelf-lives. Drying remains an important component of processing in this regard. Therefore, any pretreatment methods for drying agricultural product that decreases the moisture content and minimizes drying time by conserving the quality of the crop product is of prime significance. This article is a comprehensive review of recent developments of non-thermal pretreatment (NTP) methods. A summary of their significance, emerging and innovative methods of this technology together with its applications and limitations are discussed. This article further examines the environmental impact of NTP techniques.

RESULTS

NTP techniques, such as high pressure, ultrasound, pulsed electric field and osmotic dehydration methods are essential operations for pre-dehydration of agricultural products prior to drying. These techniques can avoid the deleterious effects of heat on nutritive value, colour and flavour of agricultural products compared to thermal pretreatments. They also enhance the inactivation of the enzymes, improve energy efficiency and mass transfer, reduce processing time, preserve bioactive compounds, improve drying kinetics and drying rate, minimize enzymatic browning, and enhance product quality.

CONCLUSION

These findings will provide a better understanding of different NTP methods and also make available more information for selecting pretreatment techniques for drying of agricultural products. © 2020 Society of Chemical Industry.