Performance of Drying Technologies to Ensure Microbial Safety of Dried Fruits and Vegetables

Dried fruits, vegetables, herbs, and spices are produced in and sourced from many countries worldwide, but they have been increasingly reported to be involved in outbreaks and alerts due to the presence of foodborne pathogens such as Salmonella. These dried products are mainly produced by solar drying and conventional air drying, but a wide range of drying technologies are available. From a technological point of view the general trend is to optimize and standardize the drying process to ensure high‐quality products to be offered. Drying technologies are mainly evaluated for their performance to reduce water activity at low energy cost while maintaining good sensorial quality of the dried product. However, as low water activity foods are increasingly recognized to support microbial survival and dried products are often consumed as they are, or are used as ingredients in many ready‐to‐eat foods, there is increasing attention to the microbiological quality and safety aspects of these products. This review presents traditional and emerging technologies to dry fruits, vegetables, herbs, and spices and discusses their potential to inactivate bacteria and viruses throughout the drying process. Overall, the microbial inactivation effect of the presented technologies has not yet been thoroughly assessed, even for traditional methods like solar drying, conventional air drying, or freeze‐drying. Emerging technologies such as dielectric (assisted) drying and low‐pressure superheated steam drying have been shown to reduce microbial populations; however, the number of studies is still low. Very few studies have focused on viral inactivation during drying processes.     

OPTIMIZATION OF MARINE FISH DRYING USING SOLAR TUNNEL DRYER

Studies were conducted to optimize fish-drying process in a Hohenheim-type solar tunnel dryer to produce safe and high quality dried fish products. Five commercially important tropical marine fish species in the Bay of Bengal such as silver jew fish, Bombay duck, big-eye tuna, Chinese pomfret and ribbon fish were used and drying was performed at 45 to 50 and 50 to 55C temperature ranges. Moisture content of the fish samples reached 16% after 36 and 32 h of drying at temperature ranges of 45 to 50 and 50 to 55C, respectively. Products produced at 45 to 50C were found to be excellent on the basis of flavor, color and texture. Their rehydration ability ranged from 65 to 80% with minimum in big-eye tuna and maximum in silver jew fish. Values of total volatile base, peroxide and aerobic plate count of all the final dried products were within the acceptable limit.

PRACTICAL APPLICATIONS

In this experiment, we optimized the drying process in a Hohenheim-type solar tunnel dryer, constructed using locally available materials, for several commercially important marine fish species in the Bay of Bengal over a wide range of temperature, humidity and moisture content. The data obtained in this study are now successfully used for large-scale production of dried fish of high quality, providing better market price for its producers as well as greater acceptability to the consumers. We also developed another low cost solar tunnel dryer using bamboo, hemp, canvas, rope and polythene sheets, which became very popular among small-scale dry fish producers in the coastal areas of Bangladesh.     

皮革低温除湿（热泵）干燥过程研究

皮革热泵除湿干燥设备是八十年代才开发的新技术设备，它利用热泵技术设定极为接近活动物生活环境的工况条件而保证获得极高的皮革质量，干燥快速、稳定，不受外部环境影响也不影响(污染)环境，且能耗显著降低和减小生产场地，因而得到普遍肯定和迅速推广。本文论述了皮革热泵干燥的原理，介绍了在摸拟样机上干燥皮革时进行的干燥时间，干燥室结构影响、能量消耗，以及干燥的皮革质量等实验，从而得出：热泵干燥的皮革质量在丰满、柔软、均一性及收缩率等方面与自然干燥基本一致而显著优于其它方法干燥的结果，在干爆速度、干燥时间、能量消耗等方面．也比各种传统方位优越。     

ACCELERATED STORAGE TESTING OF FREEZE-DRIED IMMOBILIZED LACTOBACILLUS ACIDOPHILUS-FERMENTED BANANA MEDIA

ABSTRACT

Freeze‐drying was applied as a drying method for the production of dehydrated immobilized Lactobacillus acidophilus‐fermented banana medium containing high levels of probiotics and appropriate prebiotic (mainly fructooligosaccharides) contents in an attempt to develop dried synbiotic products. Ca‐alginate and κ‐carrageenan were used for the immobilization of L. acidophilus, and the immobilized bacteria were employed directly in the banana media fermentation. The fermented products were then freeze‐dried. Results indicated that cell immobilization could provide effective protection to L. acidophilus, and reduce the damage caused by freezing and freeze‐drying. Meanwhile, accelerated storage testing using temperatures of 50, 60 and 70C was applied to the dehydrated products. Gel‐entrapped L. acidophilus appeared to have lower decimal reduction, and Ca‐alginate immobilized cells had a better survival than κ‐carrageenan. Results of accelerated storage test showed that immobilization could effectively increase the thermal resistance of entrapped microorganisms, and Ca‐alginate showed a better effect than κ‐carrageenan, and the beneficial effects increased with the decrease of the storage temperature. The freeze‐dried products exhibited little hygroscopicity because of the consumption of monosaccharides during fermentation. Results revealed that freeze‐drying could be used as a proper method for the development of dried product of immobilized L. acidophilus‐fermented banana media, and accelerated storage test could be used to evaluate the storage stability of the dried products.

PRACTICAL APPLICATIONS

This research demonstrates the feasibility of the application of accelerated storage testing to the prediction of shelf life and storage stability of freeze‐dried probiotic products manufactured by lactic acid bacteria fermentation. The Arrhenius equation was associated in this study. This will provide an efficient approach for the estimation of shelf life of probiotic products.

     

Food dehydration by solar energy

Solar driers that are currently being investigated for drying of agricultural products can be divided into two major divisions, depending upon how they transfer the incident solar energy to the product to be dried. These two divisions are direct and indirect drying, with some work also being done on combination drying procedures. In direct solar driers, the product to be dried is usually either inside a tent, greenhouse, or a glass‐topped box, where the product to be dried is heated by the direct rays from the sun and the moist air is removed by ambient wind movement. These dryers do accelerate moisture loss rate and the product is usually safe from inclement weather. These dryers usually do not require fans for forced air circulation. With indirect drying, the opposite is true, where most require powered fans for forced air circulation. With this type of dryer, both flat‐plate and inflated tube solar heat absorbers are used, with each offering certain advantages. Also, combination dryers have been built that utilize both direct and indirect principles. Product evaluation of solar dried foods indicate that in most cases the physical properties, flavor, and vitamin A and C retention were as good as, or better than, conventional dried foods. The economics of the solar systems indicate that most drying procedures are economically feasible for use in small‐scale operations only, with the exception of grain drying.     

Some quality characteristics of solar-dried cocoa beans in St Lucia

A field comparison of solar drying and open-air sun-drying of cocoa (Theobroma cocoa L) beans was carried out in St Lucia. Four methods of drying (indirect solar drier, direct solar drier, open air/perforated steel surface and open air/non-perforated wooden surface) were examined at three loading rates: 13·7, 26·9 and 40·4 kg m⁻². Beans from the open air had a higher incidence of external mould and poorer external appearance, though differences were minor. Beans dried at the lower rate of 13 kg m⁻² showed the best colour, but the highest titratable acidity. Conversely, beans dried at the higher loading rate of 40·4 kg m⁻² showed significantly lower titratable acidity, but poorer colour. Differences in cut-test score, colour, pH and titratable acidity between the open air and closed driers were small or not significant. While not significant, the indirect drier did show the highest cut-test score and the direct drier the poorest. Beans from the indirect drier were darker and more purple, while those from the direct drier were lighter coloured and less purple. The beans from the direct drier, dried to 6% moisture (WB) were, though not significantly so, more brittle and higher in titratable acidity than those from either the open air or indirect drier. Overall the beans from the indirect drier showed the highest quality and those from the direct drier the poorest. Whether the modest improvement of the indirect drier over the open air driers is sufficient to warrant investment in such a drier is unclear. © 1998 SCI.     

SELECTION OF CHEMICAL AND THERMAL PRETREATMENT COMBINATION TO REDUCE THE DEHYDRATION TIME OF SOUR CHERRY (Prunus cerasus L.)

Sour cherry (Prunus cerasus L.) is a good source of anthocyanins showing antioxidant and anti‐inflammatory activities. Contemporary drying processes of fruits and vegetables should be fast and at relatively low drying temperatures to minimize the energy consumption and the thermal degradation of their nutritional composition. Therefore, sour cherries placed into sample bags were dipped in one of eight different pretreatment combinations consisting of chemical solutions (2% ethyl oleate, 2% K₂CO₃, 1% citric acid or water) and dipping temperature (23 or 60C) for 1 min to accelerate the skin moisture diffusivity by breaking down the waxy cuticular surface of sour cherry. Pretreated sour cherry samples were dehydrated by three different methods (artificial drying, solar drying and open sun drying). Sour cherries treated with 2% ethyl oleate solution at room temperature (23C) were the fastest to reach the final weight loss percentage (75%). They dried in 24 h in the laboratory tray dryer providing 45.2C drying air. The untreated samples reached the same weight loss percentage of 75% at a drying time of 92 h. A high dipping temperature (60C) eliminated the positive effect of ethyl oleate. Solar drying and open sun drying of sour cherries treated with 2% ethyl oleate at room temperature finished approximately in 149 h and 372 h, respectively. The color of dried sour cherries was used as a quality indicator. The increase of dehydration time decreased redness values as seen in solar drying and open sun drying. The recommended pretreatment and drying processes for sour cherries are dipping them into 2% ethyl oleate solution at room temperature for 1 min and then drying them artificially at 45.2C air temperature to effectively retain their original color.     

Variation of Textural Attributes of S. Bartolomeu Pears at Maturation,Storage, and Drying

In the present work, the pears were dried in two different systems, both having the sun as the energy source: a solar stove with ventilation (ESAV) and a solar drier with natural convection (ESTV). The objectives of this work were to evaluate how instrumental textural properties changed during maturation, storing, and drying, as well as to compare the two different drying methods. The results obtained allowed to see that drying affected the textural properties of pears in a way that hardness decreased very much during drying irrespective of the drying system used.     

The analysis of phenolic compounds in daylily using UHPLC‐HRMSn and evaluation of drying processing method by fingerprinting and metabolomic approaches

The flowers of daylily (Hemerocallis fulva (L.) L.) are called “Jinzhen” which are commonly used as vegetables or seasoning in East Asia. In this study, we evaluated the chemical profile changes with different processing method after harvest. Five different drying methods including solar drying, vacuum freeze drying, steam drying, and hot air drying method were investigated regarding their impact on the chemical constituents in daylily. The major phenolic compounds were tentatively characterized with ultra‐high performance liquid chromatography tandem high‐resolution mass spectrometric with multistage fragmentation techniques (UHPLC‐HRMSⁿ). The UHPLC‐UV fingerprinting and UHPLC‐HRMSⁿ‐based metabolomic approaches were used for identifying the difference between different processing methods. The quercetin 3‐O‐rutinoside, 5‐O‐caffeoylquinic acids are obviously higher in freeze‐dried and steam‐dried samples. Colchicine was believed to be one of the toxic constituents found in daylily fresh flowers; however, no colchicine was found in any the daylily samples investigated.

Practical applications

The drying process is particularly important for handling and distribution of daylily flowers after harvest and it can also prolong the shelf life. However, the drying process may result in degradation of nutrients and quality loss in food. Steamed sunlight dried or hot air drying methods preserve most of the phenolic composition of fresh daylily samples. The purpose of this study was to analyze the phenolic compounds in daylily using ultra‐high performance liquid chromatography tandem high‐resolution mass spectrometric with multistage fragmentation techniques and to evaluate the drying processing method by fingerprinting and metabolomic approaches.     

EFFECT OF DEHYDRATION ON PHYSICOCHEMICAL PROPERTIES OF MUSTARD, MINT AND SPINACH

Mustard, mint and spinach were dehydrated in a sun and cabinet drier and were subsequently studied for rehydration characteristics. Dehydration ratio ranged from 12.37:1.0 to 23.37:1.0, while rehydration ratio varied from 1.0:2.84 to 1.0:3.76 for three green leafy vegetables. The bulk density of dried vegetables varied from 55.1 to 171.5 kg/m³, whereas the angle of repose was in the range of 33.2–52.4°. Color values L, a, b and ΔE were reduced with blanching and drying. Retention of chlorophyll, β‐carotene and ascorbic acid in dehydrated products varied from 41.87 to 61.35%, 22.26 to 55.16% and 13.36 to 37.53%, respectively. On rehydration, the ascorbic acid retention was further reduced to 4.17–11.55%.     

THIN-LAYER DRYING OF SPINACH LEAVES IN A CONVECTIVE DRYER

ABSTRACT

In the present work, spinach samples were dried using a pilot‐scale cabinet‐type convective dryer. Drying experiments were conducted using a constant air velocity of 1.2 m/s and four drying air temperatures of 50, 60, 70 and 80C. Drying rate increased with the increase in air temperature and thus reduced the drying time. The experimental drying data of spinach were applied to four moisture ratio models, namely, the Henderson and Pabis, Lewis, Page, and logarithmic models. Nonlinear regression analysis was performed to relate the parameters of the model with the drying conditions. The performance of these models is evaluated by comparing the coefficient of determination, R², and the reduced chi‐square,χ², between the observed and predicted moisture ratios. Among all the models, the logarithmic model was found to be the best for explaining the drying characteristics of spinach leaves. The effective moisture diffusivity varied from 6.590 × 10^?10to 1.927 × 10^?9 m²/s over the temperature range studied, with an activation energy of 34.35 kJ/mol for spinach leaves.

PRACTICAL APPLICATIONS

Drying, which is the oldest of food preservation practiced by humans, is the most important process to preserve agricultural products because it has a great effect on the quality of the dried products. The objective in drying foods is the reduction of the moisture content to a level that allows safe storage over an extended period. In this study, the drying kinetics of spinach leaves was studied in a convectional hot air dryer. The effect of air temperature on drying behaviors was determined. For the optimal products, the drying characteristics, namely, drying time and rehydration ratio, are considered. The dried spinach leaves could be used in the preparation of foods like puree, soups and baked products.

     

SUPERHEATED STEAM-DRYING OF MATE LEAVES AND EFFECT OF DRYING CONDITIONS ON THE PHENOL CONTENT

Four drying experiments of mate leaves (Ilex paraguariensis St. Hilaire) were carried out in a packed bed superheated steam dryer by varying outlet steam temperature from 120 to 140C and equivalent particle diameter from 4.7 × 10^?3 to 6.95 × 10^?3 m. The influence of these variables on the drying coefficient calculated by assuming a simplified drying kinetic model was investigated. A classical statistical approach revealed the significant effect of both factors on this parameter. Two additional drying runs were performed at identical conditions with conventional hot air and low pressure superheated steam, respectively. The influence of drying atmosphere on the total content of phenols was evaluated. A conventional process of extraction by using an aqueous methanol solution was adopted. Analyses for total phenols were performed by spectrophotometry at 715 nm by applying Folin‐Denis assay. The leaves dried with superheated steam had approximately 47% higher retention of these compounds.     

Influence of Antioxidant Addition and Processing Treatment on the Formation of Lipid Oxidation Compounds in Spray-dried Bovine Plasma Protein

The effects of antioxidant (tert‐butylhydroquinone, propyl gallate, butylated hydroxytoluene, and rosemary) addition, processing treatments, and storage time on the formation of lipid oxidation compounds in spray‐dried bovine plasma protein (BPP) were determined. Lipid oxidation is initiated by spray‐drying; formation of lipid oxidation compounds was highest in BPP processed using a gas‐fired spray‐drier. The most effective antioxidant in inhibiting the formation of hexanal and other lipid oxidation compounds was tert‐butylhydroquinone. Increased spray‐drier residence time and extended storage contributed to increases in lipid oxidation products. Use of effective antioxidants and selection of processing parameters to minimize lipid oxidation reactions can improve the flavor quality of spray‐dried BPP.     

EFFECT OF DIFFERENT DRYERS AND DRYING CONDITIONS ON ACCEPTABILITY AND PHYSICO-CHEMICAL CHARACTERISTICS OF DURIAN LEATHER

Optimization study on drying condition ofdurian leather was carried out using Response Surface Methodology (RSM). The optimization was conducted for two different dryers, oven dryer and forced-air cabinet dryer, with temperature and time of drying as independent variables. It was shown that both variables significantly affected most of the sensory properties. For both dryers, the optimum points for 5 sensory attributes evaluated including taste, aroma, texture, appearance and overall acceptability, were at temperatures ranged from 47–55 C for 10–14 h. Based on the overall acceptability, the optimum condition for oven-dried leather was achieved at temperature 50C for 12.6 h, while for cabinet-dried leather at 52.5C for 10 h. Results showed that panelist preferred product dried with low temperature-long time compared with high temperature-short time. Except for Hunter color b value of cabinet-dried product, results also indicated that drying condition was significantly influencing the physico-chemical properties such as moisture, A_w, nonenzymatic browning, texture, vitamin C and color of durian leather.     

不同干燥方式对苹果蟠桃纸的感官品质和理化特性的影响

为了研究不同干燥方式对苹果蟠桃纸的感官品质和理化特性的影响,本文采用五种干燥方式（烘箱热风干燥、烤箱干燥、微波炉干燥、烤箱-烘箱联合干燥、烤箱-微波炉联合干燥）制备苹果蟠桃纸,考察不同干燥方式制备的苹果蟠桃纸在感官评定、质构、色泽、褐变度、总酚和维生素C含量、抗氧化活性等方面的差异。结果表明,烘箱热风干燥的产品的形态、口感、色泽、风味更能吸引感官评定人员,感官评定分数为最高（86.7）,总酚类含量最高（5.66 mg GAE/g dw）、抗氧化活性较强（DPPH自由基清除率66.00%,FRAP法测定的总还原能力0.32 mmol Trolox/g dw）,但维生素C含量较低（4.56 mg/100 g）、褐变程度较高。微波炉干燥的苹果蟠桃纸维生素C含量最高（9.78 mg/100 g）,但感官评定分数最低为66.7,总酚含量（4.11 mg GAE/g dw）和抗氧化活性最低（DPPH自由基清除率58.00%,FRAP法测定的总还原能力 0.18 mmol Trolox/g dw）。烤箱-烘箱联合干燥和烤箱-微波炉联合干燥的产品则没有突出优势。综上,建议在苹果蟠桃纸生产中不采用微波炉干燥,采用烘箱热风干燥。     

DRYING METHODS AND QUALITY OF SHRIMP DRIED IN A JET-SPOUTED BED DRYER

The objective of this study was to experimentally investigate the effects of various parameters, i.e., size of shrimp, level and pattern of inlet drying air temperature, on the drying kinetics and various quality attributes of dried shrimp viz. shrinkage, rehydration ability, texture and color during drying in a jet‐spouted bed dryer. It was found that the use of a constant inlet air temperature of 100C yielded dried shrimp of the best quality in terms of low percentage of shrinkage, high percentage of rehydration, low maximum shear force and high value of redness compared to shrimp dried using other conditions. A simple mathematical model that enables prediction of the drying behavior of shrimp in a jet‐spouted bed dryer has also been developed.     

Retention of Ascorbic Acid and Total Carotene in Solar Dried Vegetables

Four vegetables (African spinach, cow pea, sweet potato, and cassava leaves) were dried in the open sunshine and in enclosed convectional solar driers with and without shade provision. Total carotene and ascorbic acid content in the fresh leaves and in leaves dried under the three drying conditions were assayed. The study showed that maximum retention of the two vitamins is obtained by drying vegetables in enclosed solar drier with shade. Direct sun exposure of vegetables as often practiced in the tropics resulted in marginal retention of the two vitamins.     

CHEMICAL CHARACTERIZATION AND PREPARATION OF SALTED MINCES FROM BIGEYE GRUNT AND LONGFIN BONEFISH

The proximate chemical composition, amino add and fatty add profile of bigeye grunt (Brachydeuterus auritus) and longfin bonefish (Pterothrissus belloci) minces were determined. Both minces contained a low fat content. Polyunsatu‐rated fatty adds represented more than 50% of the fatty adds in bigeye grunt but in the case of longfin bonefish the percentages of saturated, mono‐, and polyunsaturated were similar. The amino add profile of minces from these species was very similar. The preparation of salted minces using different drying methods is described. The chemical and physical characteristics of raw and dried salted minces as well as cooked products were measured. Dried salted minces from these species had high water uptake and cooked products presented a low salt content (1 %). Cooked rehydrated salted minces from longfin bonefish were lighter in color than those from bigeye grunt and color values were not significantly influenced by the salt level used in the preparation.     

Development of quick-cooking dehydrated pulses by high temperature short time pneumatic drying

Quick-cooking dehydrated pulses were made from husked split grains of red gram, black gram and Bengal gram using high temperature short time (HTST) pneumatic drying technique wherein porosity was brought about in the dried pulses by exposure of the cooked grains initially to air at a high temperature (170–200°C) for a short duration (4–6 min) in a laboratory model HTST pneumatic drier followed by finish drying at a lower temperature (60–70°C) in a conventional tray or fluidized bed drier. Optimum temperature and time for the HTST drying of the grains were worked out and the HTST dried products were evaluated with respect to their drying behaviour, rehydration characteristics and shelf stability vis-a-vis the conventional hot air dried products.
Considerable reduction in drying and rehydration times could be achieved by the HTST process. The increase in porosity achieved and the HTST treatment did not adversely affect the shelf stability of the grains to any measurable extent.
The technique involves simple equipment and less capital investment as compared to other methods of drying, such as explosive puffing, achieving the same objective. It is amenable to continuous processing as the time of high temperature drying is very short.     

Microwave drying of mango ginger (Curcuma amada Roxb): prediction of drying kinetics by mathematical modelling and artificial neural network

Curcuma amada (Mango ginger) was dried at four different power levels ranging 315–800 W to determine the effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective diffusivity. Among the fifteen thin layer drying models considered for evaluating the drying behaviour, the semi‐empirical Midilli et al., model described the drying kinetics very well with R² > 0.999. Drying rate and effective diffusivity increased as the microwave power output increased. Activation energy was estimated by a modified Arrhenius type equation and found to be 21.6 kW kg^?1. A feed‐forward artificial neural network using back‐propagation algorithm was also employed to predict the moisture content during MW drying and found adequate to predict the drying kinetics with R² of 0.985.