Analysis on Status quo and Future Development of Fruit and Vegetable Protreatment

Xuhai YANG，Qian ZHANG，Hongjuan Ll，Xinjian ZHANG

Mechanical and Electrical Engineering College of Shihezi University，Shihezi 832000，China

Analysis on Status quo and Future Development of Fruit and Vegetable Protreatment

Xuhai YANG，Qian ZHANG*，Hongjuan Ll，Xinjian ZHANG

Mechanical and Electrical Engineering College of Shihezi University，Shihezi 832000，China

Pretreatment on vegetable and fruit is a key chain of drying process.The research analyzed mechanisms and applications of different pretreatments such as blanching，freezing，liquid glucose pretreatment，NaCl dipping，and high-voltage electric field，and proposed the development prospect.

Fruit and vegetable；Pretreatment；Drying；Mechanism；Application；Development trend

D omestic fruit and vegetable yields always rank the first in the world and dehydrated vegetable and fruit occupy about 2/3 worldwide［1］.It is known that fruit and vegetable are the sources of vitamin，mineral substances，organic acid，dietary fiber and bioactivator essential for human beings.However，most are inclined to rotting and drying treatment becomes necessary to prolong shelf life.As living standard improves，people have increasingly high demands on food nutrients and safety，so that drying industry is expected to be highefficient，good-quality，and environment friendliness.

What’smore，withS&T advancement，much progress has been made on the drying technology and hot-air drying technology is much popular currently in drying fruit and vegetable because of its convenience and low price.Nevertheless，fruit and vegetable cells are prone to spoilage destruction and rehydration property tends to be poor，resulting in changes of color，tissue，taste and nutrients.In order to prevent or reduce the negative effects，it is necessary to conduct pretreatment on materials before drying，such as blanching with water，freezing at low temperature，and dipping in NaCl solution and sugar liquor.

The pretreatment before drying generally includes mechanical pretreatment，physical and chemical pretreatments.Specifically，vegetable and fruits are mainly influenced by physical and chemical pretreatments，but the mechanism seems much more complicated.Comparatively speaking，the mechanical pretreatment has less effects on dehydration rate and product quality［2］.

Pretreatment on Fruit and Vegetable

Blanching

Blanching refers to heating fresh raw materials，cut or complete，in hot water，boiling water or atmospheric steam.In general，hot ironing improves deformation，passivation enzymes，and crispness，accelerates water loss，reduces bad flavor and hardness while soften product surface.For example，air-impingement jet is a new technology directly spraying high-temperature and high-humidity gases on surface of reserve products through round or narrow-slit shaped nozzles，with a higher heat transfer coefficient，which performs significantly in terms of drying rate and color protection.At present，the technology is widely used for some products such as carrot，apricot and pepper［4-6］.The materialswould go through pectin substance solubilization，tissue structure change，decreasing of cell adhesion，drying rate growth，reduction of drying period after undergoing air-impingement jet blanching，as well as avoiding fruit and vegetable cell softening，dehulling and loss of soluble solid and nutrients［5］. Currently，many new blanching methods are also available，including steam blanching，microwave blanching，acid blanching at room temperature，ohmic blanching，ultrasonic wave heating blanching and manothermosonication. With drying industry development，the new blanching technologies are increasingly applied in drying industries of vegetables and fruits.

Many researches have been conducted on blanching technology in vegetable and fruit drying.For example，Zhao et al.discovered that potato would lose enzyme activity，accelerating drying rate of potato and improving rehydration rate without brown stain and concluded drying after blanching is best［3］.Ma et al.researched that humidity and temperature of blanching are key factors influencing drying time of Lycium barbarum.For instance，the drying time reduced by 4.75 h in the treatments with temperature at 115℃，humidity of 40%and time of 30 s b［7］.

However，excessive blanching is not suitable for color maintenance and should be decided upon material characters.In general，proper blanching is conductive to reducing moisture content，puffing，and color quality of puffed products.What’s more，vegetables and fruits blanching is increasingly fast，energy-saving and convenience with drying quality guaranteed，as drying technology advances.

Frozen pretreatment

Frozen pretreatment refers to cool down vegetable or fruit temperature below its freezing point before further processing or storage.It is much effective and the best method in drying heat sensitive materials and maintaining bioactivator［8］.In freezing environment，metabolism of vegetable and fruit would be inhibited，so that nutrient consumption would be lowered，and the lower the temperature，the lower the consumption.Therefore，frozen pretreatment is conductive to fixing fiber structure of materials，so that material size can be guaranteed.

Frozen pretreatment would improve texture structure and increase puffing degree and crispness to accelerate vegetable and fruitdrying. Specifically，after the treatment，liquid in black edamame tissues would ice up，capillary expand，and cytoderm thaw a little.Compared with the product without freezing treatment，water content and hardness both declined，and crispness grew remarkably［9］.According to treatments on sweet potato，rehydration capability increases and performance density reduces，so that sweet potato looks better［10］.

Liquid glucose pretreatment

Liquid glucose pretreatment refers to the practice immersing materials in liquid glucose，which accelerates water loss and maintains materials in color and shape well.What’s more，it performs better in heat and quality transferring as well as rehydration capacity of the materials.Besides，the pretreatment lowers activation energy of dry sweet potato and the higher activation energy，the lower drying rate［1］.With increasing of sugar concentration，materials would absorb sugar crystal and sugar liquid would be dissolved by inner part of the materials，preventing heat transferring.With decreasing of water vapor pressure within，water movement rates would go down at drying.In addition，sugar also lowers water absorption rate by dehydrated materials and rehydration property declines.Therefore，itis necessary to well control sugar concentration according to material characters at pretreatment.For example，the pretreated carrot would reduce drying period and energy consumption provided proper humidity of drying medium［1-2］.

The pretreatments with sugar liquid have significant effects on product color and hardness.For instance，the color is better of pretreated puffing products with high fructose corn syrup and malt syrup，and the hardness is significantly higher in the treatment with high fructose corn syrup than that treated with maltitol.It is recommended to choose malt syrup or high fructose corn syrup as dipping solution in the pretreatment，which would accelerate drying rate.

NaCl-based pretreatment

The pretreatment refers to the practice immersing materials in NaCl solution，functioning permeation and dehydration and product moisture is decreasing and drying rate is growign upon NaCl concentration［10］.For instance，Sun et al.researched the effects of over-saturated or boiled NaCl solution on drying period and rehydration quality of trepang and the results showed that drying period and rate tended to be volatile.For example，the drying process of trepang was dominated by water expansion and initial moisture rate was different，as well as drying period and rehydration quality［13］.

It is notable that NaCl solution at high concentrations performs excellently in color maintenance.For instance，Hami melon usually looks better in shape and color if pretreated with NaCl［14］and moisture content of puffing Hami melon is decreasing as NaCl concentration grows.These indicated that NaCl solution accelerates water loss of materials.In addition to that，the lightness index of puffing products keeps increasing upon NaCl concentration，which demonstrated that NaCl solution has considerable effects on color of the treated materials.

Pretreatment technology by pulsed electric field

The pretreatment is a dehydration technology before drying by pulsed electric field［16］.Specifically，external electric field would induce electric charge clustering in the sides of cell membranes，formulating transmemberane potential differences.If the differences are higher than membrane potential difference，the cell membrane is prone to fracture and local structures prone to change，rapidly improving membrane permeability and accelerating water loss rate.The technology functions well in reducing energy consumption and lowering cost.What’s more，it is featured by small temperature increase，rapid transmission，and short treatment term and can be sued for increasing drying rate，reducing energy consumption for dehydration and improving quality of dried vegetable or fruit with few damages on food.For example，Bajgai et al.conducted research on the effects of high-voltage electric field on drying spinach leaf and concluded that material temperature did not grow，which is conductive to maintaining nutrient elements［17］.Liu et al.conducted the effects of high-voltage electric field on drying apple and concluded that averaged dehydration rate of apple sample was higher than that without treatments，which indicated that high-voltage electric field pretreatment would effectively improve dehydration rate of vegetable and fruit to reduce dry period and enhance drying rate［18］.Therefore，it enjoys broader development prospect.

It is notable that treatment effects tend to be volatile upon materials by high-voltage electric field，because mechanical property would be changed by adipocyte parameter and structure of different materials.The higher pretreatment，the higher stress on cell membrane.

Other pretreatments

Pretreatment methods available are diversified and it is recommended to choose one or more methods according to specific species and characters of the materials，which would accelerate drying rate， improving product quality and maintain nutrients. Still，it should recognize that enzyme oxidative system would be destroyed and enzyme activity passivated by blanching，and pretreatment with sulfur dioxide，thiourea and ascorbic acid. With living standard improvement，people have increasingly high demands on food quality，as well as dried products and it is necessary to use rational ways in food pretreatment such as integration of different methods in order to speed up drying rate，improve product quality and lower cost and energy consumption.Zhang et al.researched the effects of three pretreatment methods on chili pepper in terms of drying period，rate and moisture by gas jet impingement and the results showed coefficient of moisture expansion in the treatment with threesection cutting method was the highest，reducing haematochromeand brown stain［6］.Hence，the integrated treatment actually integrates advantages of different pretreatment methods，effectively improving drying rate and quality.

Although every pretreatment would undergo rate decreasing phase，the characters are diversified，as well as initial moisture content.The choice ofpretreatmentmethod basedon vegetable or fruit species would accelerate drying rate，improve product quality and meet market demand to get maximized economic benefits.

Development Trends

Domestically，fruit and vegetable drying has increasingly drawn attention and global technology exchange becomes frequent.Currently，it is necessary to analyze problems existing in pretreatment technology with references of foreign achievements，which would advance sustainable development of fruit and vegetable dehydration industry.It is safe，nutrient and convenient food that consumers are always pursuing，which can be accomplished by scientific pretreatment on vegetable and fruit would accelerate water loss，improve drying rate and guaranteeing productquality. Still，product quality characters should be taken into consideration before drying to select optimal method.

Some keys are proposed to guarantee quality of pretreated vegetable and fruit.It is recommended to research quality elements of different materials to seek the rules for pretreatment and maximize development of drying industry.At present，pretreatment in many small-scale industries is by manual operation，affecting treatment period and quality.In contrast，pretreatment equipment makes mechanization possible.Finally，it is of significance to integrate two or more pretreatment methods to optimize pretreatment results according to specific characters of materials from singlefactor to multi-factor control.

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Responsible editor：Xiaoxue WANG

Responsible proofreader：Xiaoyan WU

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果蔬预处理现状分析及未来发展趋势

杨旭海，张 茜*，李红娟，张新建 （石河子大学机械电气工程学院，新疆石河子832000）

果蔬预处理是果蔬制干的重要环节之一。主要介绍了烫漂、冷冻、糖渍、NaCl浸渍和高压脉冲电场预处理等当前果蔬预处理方法的机理及应用现状，并对果蔬预处理的未来发展趋势作了展望。

果蔬；预处理；干燥；机理；应用；发展趋势

国家自然科学基金 （编号：31360399）。

杨旭海（1980-），男，新疆人，硕士生，研究方向：农业装备工程，E-mail：1554608278@qq. com。*通讯作者，E-mail：zqq80@163．com。

2015-10-04

Supported by National Natural Science Foundation of China（31360399）.

*Corresponding author.E-mail:zqq80@163.com

Received:October 4，2015 Accepted:November 16，2015

修回日期 2015-11-16