Analysis of Process of Microwave Puffing Blueberry Snacks

Introduction

Blueberry, which is rich in anthocyanins (Liu, 2013),has unique flavor and soft texture.There is only a small variety of blueberry products on the market,due to its intolerance in storage and transportation.The blueberry snacks made by pulp, starch and other materials, under microwave method are novel snack food with crisp taste.

Microwave technology, as a volume heating method(Yang et al., 2008), has puffing effect for the drying of agricultural products (Li et al., 2003) and is proved by Kparulis et al (2005).Fruit and vegetable crisp puffed by microwave technology retains high nutrients (Sun,1996).Therefore, microwave processing method was used in this experiment.Study of Ran et al.(2004)showed that the thickness in the range of 2-6 mm of the material dose not affect the drying rate, but different slice thickness has a significant effect on the temperature.Mu et al.(2010) studied on the effects of different processing parameters on the sensory evaluation and texture characteristics of black currant snacks using microwave vacuum technology.Zheng et al.(2012) indicated that the effects of microwave power, vacuum and initial moisture content on the dewatering and structural characteristics of raspberry crisps are significant.The study con firms the existence of the relationship between the volume expansion and the dehydration rate of raspberry snacks (Zheng et al.,2013a).As for the simulation of puffing, Ressing et al.(2007) used finite element method to simulate the temperature distribution of the dough depended on the depth of microwave penetration inside dough in microwave vacuum puffing considering the heat transfer and solid mechanical properties.Liu et al.(2009; 2010) established a mathematic model of microwave vacuum puffed black gallon slices to analyze the volume change of the crisp and the diffusion of internal moisture, the law of temperature transmission.Mu (2011) carried out the multi-physics coupling simulation of expansion of raspberry snacks and the internal pressure gradient and temperature distribution in the process of microwave vacuum puffing.The effects of microwave intensity and initial moisture content on the heat transfer and mechanical properties of puffed snacks are discussed based on above-mentioned studies.These simulations of the puffing process describe the distributions of the temperature and the volume change of materials.However, limited information on analysis of process of microwave puffing blueberry snacks was reported.The aims of this study were to determine the actual value of the internal pressure and temperature of the slices in the process of puffing and obtain the changes of volume based on the amount of water vapor generated inside the snacks.

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Materials and Methods

Experimental materials and experimental program

Blueberry pulp provided by Heilong jiang Academy of Agricultural Sciences of Harbin in China was stored at room temperature (25℃).Distilled water was added into the blueberry pulp to adjust its moisture content to setting level.Then mixed the pulp, modified starch (Pregel-A, Tianjin Dingfeng Starch Development Co., Ltd., Tianjin, China) and maltodextrin(Changchun Dihao Food Development Co., Ltd.,Changchun, China) evenly in proportion.The proportion based on the golden section method (Zheng et al., 2013b) at the ratio of starch dough was consisted of blueberry pulp of 32.6 g · 100 g-1, modified starch of 41.7 g · 100 g-1, maltodextrin of 25.7 g · 100 g-1.Finally, the starch dough was prepared to a sheet with a diameter of 18 mm, a thickness of 3 mm and a mass of (5±0.20) g.

以往进行的神经阻滞，多是麻醉医生凭经验盲探针刺或电流刺激患者神经以探寻异感或肌肉收缩以完成阻滞，对于一些存在神经变异者，阻滞效果较差，阿片类药物用量较大及更换麻醉方法现象频频发生。而在超声技术引导下进行神经阻滞，有助于强化神经及局麻定位的精确性，降低局麻药用量且有效提升了阻滞的成功率。若能联合应用超声与神经刺激仪技术，神经阻滞成功率将会进一步提升，而且神经阻滞相关并发症发生率明显降低[1]。本次研究选择接受周围神经阻滞的70例患者资料，对相关方法进行分析探讨，做出如下汇报。

The internal temperature and pressure of the snacks were measured by using two fiber optic sensors connected to the microwave workstation (MWS MAN-00074 R1 type, FISO, Quebec, Canada).The fiber optic sensors were immersed into the center of the snack to measure its temperature and pressure.The same conditions were repeated three times to report the average value.

The amount of water vapor in the snacks was directly related to the pressure in the snacks.The height of the snacks could be observed through the paper listed height scale in the puffing process with the rotation of the turntable.It could be learnt from the ideal gas state Eq.(2).

对于稳态运行的AHP，吸收器吸收从蒸发器而来的气态被吸收剂，并伴随着放热，故吸收器的温度Ta大于蒸发器的温度Te；由于发生器和冷凝器分别进行沸腾和冷凝过程，因此可知发生器的温度大于冷凝器的温度。同时结合前文所说溶液泵进出口侧压力的区别，可知AHP的温度分布为Tg＞（Tc，Ta）＞Te。同样分析可知，对于稳态运行的AHT，其温度分布为Ta＞（Tg，Te）＞Tc。正如图2中所示，AHP需要外界输入高温热源，AHT的热量输出端在吸收器。

The water content of blueberry pulp was determined by using direct drying (GB 5009.3-2010).A small amount of blueberry pulp was poured into three aluminum boxes, and then placed into a vacuum oven(DHG-9053A, Yiheng Experimental Equipment Co.,Ltd., Shanghai, China).The pulp was dried to constant weight at a temperature of 105℃.The moisture content of berry pulp was calculated according to the ratio of the mass of the removed water to the mass of the pulp.The water content of the dough was calculated by using Eq.(1).

Where, m1 was the quality of the blueberry pulp (g),m2 was the mass of modified starch (g), m3 was the mass of maltodextrin (g), M1 was the moisture content of the blueberry pulp (%), and M0 was the moisture content of the starch dough (%).

Determination of temperature and pressure

考虑到要求DE的长，所以应以DE为边构造直角三角形，即分别过点D、E作DE的垂线(DE为直角边)，或作BC、AB的垂线(DE为斜边)，但究竟作哪一条垂线更实用，还需结合其他条件进一步分析．

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Microwave puffing equipment

我痛感到自己这一辈子欠下白丽筠的感情债，决不是几滴眼泪所能报答的。她与我非亲非故，本无太多的干系，仅仅是小学同学而已，可是她却为了给我找一份工作受了那么大的委屈，她甚至为了我付出了她所能付出的全部，而我却一心想要逃开白丽筠。在她把她所有的人脉关系全都葬送了之后，我可耻地做了最后一名逃兵，我还是人吗？我还有人情味吗？像我这样的人还有良心吗？

Determination of amount of water vapor in snacks

The microwave workstation (MWS MAN-00074 R1 type, FISO, Quebec, Canada) was employed for puffing blueberry snacks.The equipment consisted of magnetrons installed on the right of the drying chamber and fiber optic sensors which were installed on the top of the drying chamber.Ten intensities of microwave output powers at 0-1 100 W with the interval of 100 W.

Where, P was the pressure in the snacks (Pa),ΔV was the increment of the snacks' volume (m3), n represented the amount of water vapor (mol), R was the ideal gas constant (8.31441 J · mol-1 K-1), and T was the temperature in the snacks (K).In a closed system,the product of pressure and volume was constant at constant temperature.To improve the accuracy and simplify the solution process, some assumptions were as the followings.The snacks before being puffed were treated as cylinders and ignore the interior air.The puffed snacks were considered as hemispherical,the height of snack was the ball radius r (mm).The volume increment ΔV of the snacks in MP was considered to be filled with saturated water vapor.The amount of water vapor in the snacks with puffing time was calculated by using Eq.(3).

The meanings of the parameters in the equation were the same as Eq.(2).

Fig.1 depicted the changes in internal pressure and temperature over time under different microwave intensities at the initial moisture content of 23% (w.b.)and puffing time of 150 s.In the early puffing stage,there was few changes in pressure and temperature inside the berry snacks.With the accumulation of microwave energy, the pressure and temperature in the slices increased rapidly for the puffing time of 1 min.The greater the microwave intensity resulted in the more evaporation of moisture in berry slabs.But in the end of the process, the pressure of the snacks tended to similar trends under different microwave intensities.

The hardness of puffed blueberry snacks, defined as the maximum force before the sample underwent crack (Liu, 2013), was measured by a texture analyzer(TA-XT-Plus, Stable Microsystems Ltd., Surrey,UK).The brittleness value referred to the number of crests that occurred, when the anchor was detected in the fragmentation of the crack.The running speed of cylindrical probe was set to 5, 2 and 10 mm · s-1 with 2.5 mm diameter, and the pressing distance was 35 mm,and the force threshold was 100 g.The software TEE32 was used to obtain the force-time curve.The hardness and the brittleness of the snacks could be calculated according to these curves.

The stress-strain curves of berry snack subjects stress were obtained according to the force-time curves.The elastic modulus was determined by fitting the linear portion of the curve, between 1/6 and 5/6 of the rupture stress, to a straight line and finding its slope (Ressing et al., 2007).

科研评价体系及科技人员奖励制度方面存在的问题科研评价体系和激励机制的改革与服务创新驱动要求不完全协调，推动科技成果转化力度和深度与服务地方经济社会发展不完全同步。目前，江苏师范大学还需加大应用性科技成果及转化在学校聘岗、职称晋升等工作中的权重。同时，对于那些从事推广的科研管理人员也要有相应的一系列激励措施，而且要不断加大对其激励的程度，这样才有利于整个科技成果转化更加顺利地进行。

Data processing

Sigma Plot (Ver12.5) was used to establish regression models of temperature and pressure over time.Design Expert (Ver8.0.6) was used to design the four-factor five-level center combination experiment.Factor level and coded level are shown in Table 1.

再其次，巴西内陆运输补贴有望增加，下半年进口将加快进度。因巴西内陆运输补贴问题，5月份起发生的物流系统罢工事件将在8月上旬有所定论，目前进口商已稳定心态，下半年将加快采购进度。

Results

Changes of internal pressure and temperature inside berry snacks

Effects of microwave intensity on pressure and temperature inside berry snacks

Moisture content measurement

Determination of hardness, brittleness and elastic modulus

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There was a regression equation of pressure and temperature over time.The precision of the model was determined by the determination coefficient R2 and the root mean square error (RMSE) (Chen et al., 2013).The model with higher R2 and lower RMSE was chosen as the most suitable regression model.

The trend of the internal pressure and temperature of the snacks were simulated under different microwave intensities at the initial moisture content of 23% (w.b.)and the expansion time of 150 s.Several models were selected to fit the pressure-time and temperature-time curves, and Eq.(4) was the most suitable model by comparing R2 and RMSE of different models.

In this model, the fitting of pressure and temperature with time was higher, and the related statistical parameters and values are shown in Table 2.

Changes of pressure and temperature under different initial water contents Fig.2 showed the changes of the pressure and temperature in the process of the snacks with initial moisture content at the microwave intensity of 11 W · g-1 and puffing time of 120 s.As shown in Fig.2(a), the pressure of the snacks was inversely proportional to the initial moisture content at the beginning.The pressure inside the snacks increased slowly at low initial moisture content.However, the pressure of the snacks increased rapidly at high initial moisture content.In the late stage of the puffing process, the internal pressure of the snacks increased with the increase of initial moisture content.Because the high dielectric constant of water, the higher initial moisture content of the snacks resulted in the higher dissipation of the microwave energy, which contributed to raise temperature of snacks as shown in Fig.2(b).

Table 1　Experimental values and coded level

Since microwave station doesn't have a 500 W stall, actual test replaces 500 W with 400 W or 600 W at the same microwave intensity.

Coded value (Zj)Factor Microwave power (W)　Initial moisture content (%) (w.b.)　Processing capacity (g)　Puffing time (s)+2　600　32　50.00　180 1 500　29　42.50　150 0 400　26　35.00　120–1　300　23　27.50　90–2　200　20　20.00　60

Fig.1　Changes of pressure and temperature in snacks of different microwave intensities [initial moisture content of 23%(w.b.), expansion time of 150 s]

Table 2　Statistical parameters and values of coefficients specific to Eq.(4) for different microwave intensities

Item　Microwave intensity (W · g-1)Coefficient　R2　RMSE a b x0　y0 7 0.1318　 9.1015　32.7253　0.0320　0.8944　0.0153 Pressure　11　 68.4557　117.7147　703.1333　　–0.1766　0.9702　0.0226 17　 0.4300　13.0246　66.6694　0.0847　0.9609　0.0348 7 68.4566　 9.8156　32.7683　31.1504　0.9910　2.1653 Temperature　11　81 247.1279　60.7592　522.5977　43.3998　0.9785　6.7680 17　 379.6649　49.5561　117.0104　17.8309　0.9772　10.2817

Fig.2　Changes of pressure and temperature in snacks of different initial water contents (microwave intensity of 11 W · g-1,puffing time of 120 s)

Changes of pressure and temperature of snack with puffing time

As shown in Fig.3, the changes of pressure and temperature with puffing time at microwave intensity of 11 W · g-1 and the initial moisture content of 29%(w.b.).The changes of pressure and temperature were non-significant with puffing time.

Changes of amount of water vapor and volume expansion of snacks

Amount of water vapor and volume of snacks with microwave intensity

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Change of the volume of water vapor in the snacks is shown in Fig.4, under different microwave intensities at the initial moisture content of 23% (w.b.) and the expansion time of 150 s.Points represented the actual experiment value, dotted line represented the trend.As the amount of water vapor in the snacks was directly related to the pressure within the snacks, so the change in the amount of water vapor was similar to the change in pressure.The amount of water vapor in snacks was closely related to the volume of snacks, the more water vapor in the snacks, the greater the volume of the berry snacks.Volume of water vapor and volume of snacks with different initial water contents

Fig.5 showed the change of the volume of water vapor under different initial water contents at the microwave intensity of 11 W · g-1 and the puf fing time of 120 s.The volume of water vapor in the snacks of high moisture content was little at the beginning of the puffing process.With the rotation of turntable in the microwave workstation, microwave penetration depth showed cyclical changes, so the amount of water vapor and volume of berry snacks.

Fig.3　Changes of pressure and temperature in snacks of different expansion time [microwave intensity of 11 W · g-1, initial moisture content of 29% (w.b.)]

Fig.4　Water vapor volume of substance in snacks of different microwave intensities [initial moisture content of 23% (w.b.) ,expansion time of 150 s]

Fig.5　Water vapor volume of substance in snacks of different initial water content (microwave intensity of 11 W · g-1, puffing time of 120 s)

Enough puffing time was helpful for volume expansion of snacks at proper microwave intensity and initial moisture content in microwave puffing.As shown in Fig.6, the puffing time had no significant impact on the amount of water vapor at the microwave intensity of 11 W · g-1 and the initial moisture content of 29% (w.b.).

Fig.6　Water vapor amount of substance in snacks of different expansion time (microwave intensity of 11 W · g-1, initial moisture content of 29% (w.b.))

Discussion

From the results, it could be seen that in the initial stage of the expansion, there were few changes in pressure, temperature, volume of water vapor and volume of snacks.The reasons could be that in the initial stage of volume expansion of berry snacks,temperature and vapor water raised, due to absorption of microwave energy within snacks.No significant effect of microwave intensities on the internal pressure and the temperature of the berry snacks was observed in the initial stage.A small amount of water vapor in the snacks caused the radial extension; therefore, few changes of the volume occurred in the drying stage(Han et al., 2006).The accumulation of microwave energy in berry snacks caused the evaporation of liquid water inside the snacks, which contributed to improving the internal pressure, expanding the volume and forming the porous structure.

The depth of the microwave penetration inside material had positive relation to the absorption.The more absorption of microwave energy inside berry snacks resulted in more amounts of water evaporation to form fine pores of the interior.During the experiment, it could be clearly observed when the snacks were close to the waveguide, the volume was significantly increased.The pressure, temperature, volume of water vapor and volume of snacks increased rapidly for the puffing time of 1 min.This could be explained that with the accumulation of microwave energy absorbed by the snacks, a large number of water had been vaporized, resulting in a sharp increase in the pressure inside the snacks.Large amounts of water vapor could also lead to an increase in the volume of berry snacks.

With the reduction of moisture inside berry snacks,the ability of the snacks to absorb microwave energy decreased, which resulted in the decrease of the amount of water vapor.After the water vapor was lost,its original volume left pores.As a result, the volume of the snacks expanded.

It was difficult to form a stable structure in the highly initial moisture content or low microwave intensity.When the initial moisture content was low or the microwave intensity was high, the moisture in the snacks was removed to form a stable structure in a short time.It was necessary to make appropriate choices of the initial moisture content and microwave intensity in order to obtain a good puffing effect.

Conclusions

In the initial stage of microwave puffing, the moisture inside the berry snacks absorbed microwave energy to increase the temperature and the moisture evaporation.Porous structure appeared and the volume of the berry snacks rose, due to the vaporization of the liquid water.In the end of the puffing process, the moisture content in the slices had been greatly reduced leading to a large number of pores in the snacks till constant.

(1) In the initial stage of microwave puffing, the internal pressure and temperature of the snacks was proportional to the microwave intensity and inversely proportional to the initial moisture content.In later stage, the pressure increased rapidly with the generation of the water vapor.Finally, the pressure inside the snacks achieved 0.3-0.6 bar under high microwave intensity and moderate initial moisture content.

依次将3.2和3.3节的结果与Lambert-Beer定律计算的结果进行比较.选择与3.2节中相同的条件，基于Lambert-Beer定律，计算三种辐射波在平流雾和辐射雾中的透过率TLB，并与Monte Carlo仿真的透过率TMC进行比较，结果如图7所示，纵坐标Difference value=TMC-TLB.

(2) The volume of the snacks was directly related to the amount of water vapor.In the beginning, less amount of water vapor in the snacks was diffucult to change the volume of the berry snacks.With the accumulation of microwave energy inside slices, a large amount of water vapor increased rapidly the volume of the berry snacks.

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