楊瀟瀟，叢堃林，張衍國，李清海

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木薯高/低溫二段式干燥工藝參數(shù)優(yōu)化試驗(yàn)

楊瀟瀟，叢堃林，張衍國，李清?！?/p>

（熱科學(xué)與動(dòng)力工程教育部重點(diǎn)實(shí)驗(yàn)室，清華大學(xué)-滑鐵盧大學(xué)微納米能源環(huán)境聯(lián)合研究中心，清華大學(xué)能源與動(dòng)力工程系，北京 100084）

木薯收獲時(shí)含水率約為65%～70%，為了便于儲(chǔ)存和運(yùn)輸，必須在短時(shí)間內(nèi)對(duì)木薯進(jìn)行干燥處理。為開發(fā)和優(yōu)化木薯干燥工藝，該文在干燥箱中試驗(yàn)研究了木薯切片厚度、干燥溫度對(duì)干燥過程的影響。試驗(yàn)結(jié)果表明，干燥溫度過高，木薯經(jīng)過高溫后易糊化變質(zhì)，溫度太低，則干燥不充分；切片太薄，干燥過程中木薯片會(huì)斷裂，切片太厚，達(dá)到合格含水率所需時(shí)間將延長。用Wang and Singh模型關(guān)聯(lián)木薯干燥過程，根據(jù)干基含水率曲線確定了木薯干燥過程中的臨界含水率為105%。對(duì)于一段式木薯干燥工藝，木薯在干燥過程中達(dá)到臨界含水率時(shí)后段干燥速率呈下降趨勢(shì)，為了避免這一趨勢(shì)出現(xiàn)而導(dǎo)致干燥效率下降，提出了先高溫、后低溫的二段式優(yōu)化干燥方案，確定了各段工藝的參數(shù)，為木薯干燥設(shè)備的設(shè)計(jì)提供了必要的依據(jù)。

干燥；水分；質(zhì)量控制；木薯

0 引 言

木薯是熱帶和亞熱帶根莖植物，是世界三大薯（馬鈴薯、木薯、紅薯）之一。原產(chǎn)于亞馬遜流域，在我國及東南亞等地被廣泛種植，主要應(yīng)用于食品、醫(yī)藥、工業(yè)等領(lǐng)域，有“淀粉之王”之稱[1-4]。

木薯成熟時(shí)含水率在65%～70%之間，收獲3～7 d內(nèi)若不進(jìn)行干燥則很容易變質(zhì)腐爛。為便于存儲(chǔ)和運(yùn)輸，收獲后應(yīng)盡快將其含水率降低到安全含水率以下，需利用工業(yè)化干燥設(shè)備對(duì)木薯進(jìn)行快速干燥[5-10]。

截至目前，國內(nèi)外已有許多學(xué)者對(duì)木薯干燥進(jìn)行了大量的研究，但有關(guān)木薯干燥過程、干燥工藝及干燥設(shè)備的研究還不多見。國外研究人員大多針對(duì)太陽能干燥對(duì)木薯干燥過程進(jìn)行研究，主要研究了環(huán)境溫度、相對(duì)濕度、光線強(qiáng)度、氣體流速、裝載密度等因素對(duì)木薯干燥過程的影響，結(jié)果表明溫度、光線強(qiáng)度和氣體流速與干燥速率成正相關(guān)，裝載密度與干燥速率呈負(fù)相關(guān)[11-14]。由于薯類作物大多含淀粉和糖分，其干燥過程具有相似性，因而其他薯類的研究可以參考。李業(yè)波等[15]對(duì)馬鈴薯進(jìn)行了內(nèi)部的傳熱傳質(zhì)研究，建立了馬鈴薯內(nèi)部水分?jǐn)U散系數(shù)的數(shù)學(xué)模型。孟岳成等[16]研究了溫度、風(fēng)速和厚度對(duì)紅薯煩躁過程的影響，比較了12種干燥模型在紅薯熱風(fēng)干燥中的適用性，結(jié)果表明與Wang and Singh 模型的擬合程度最高。龍銘等[17]設(shè)計(jì)了用負(fù)壓氣流對(duì)木薯進(jìn)行干燥的設(shè)備。劉琨等[18-21]分別對(duì)木薯淀粉、木薯酒糟的干燥特性，結(jié)果表明干燥曲線為指數(shù)曲線。張鵬等[22-23]研究了紅薯、馬鈴薯、芋頭和山藥在干燥過程中溫度、厚度和切法對(duì)干燥特性的影響，對(duì)4種物料分別進(jìn)行了干燥特性與動(dòng)力學(xué)模型的擬合，結(jié)果表明，它們?cè)跓犸L(fēng)干燥過程中各類指標(biāo)變化基本一致，具有共性。上述研究分別是針對(duì)馬鈴薯、紅薯、木薯淀粉、木薯酒糟、芋頭、山藥等進(jìn)行的研究，但均未與木薯的干燥工藝相結(jié)合。

為了設(shè)計(jì)木薯干燥設(shè)備，本文利用干燥箱[24-25]研究了干燥過程中溫度和木薯切片厚度對(duì)干燥過程的影響，并提出相應(yīng)的木薯干燥工藝。

1 材料與方法

1.1 試驗(yàn)材料

試驗(yàn)采用的新鮮木薯購于廣西省容縣，木薯洗凈晾干后密封放置在2～5℃的冰箱中冷藏備用。

1.2 試驗(yàn)設(shè)備及方法

用切片模具將新鮮木薯分別切成厚度分別為5、6、7、8、9 mm的圓片，再將切好的木薯片放入提前設(shè)定好溫度為60、70、80、90、100 ℃的鼓風(fēng)干燥箱（DGF25012C）中，利用鼓風(fēng)干燥箱模擬熱風(fēng)干燥過程，每隔5 min從干燥箱中取出1個(gè)試樣，將取出來的木薯片放入到裝有變色硅膠的干燥器中冷卻降溫，冷卻至室溫后用電子天平（德國Sartorius BS210S，稱量范圍為0～210 g，精度為0.000 1 g）稱其質(zhì)量。根據(jù)實(shí)際稱量的質(zhì)量計(jì)算木薯的水分。

1.3 木薯水分的測(cè)量和表示方法

木薯的原始水分用GB28733-2012[26]的方法進(jìn)行測(cè)量。

木薯的干基含水率是表示木薯在干燥過程中的某一時(shí)刻水分與干物料的比，每個(gè)樣品的干基含水率按照式（1）進(jìn)行計(jì)算。

用水分比來表示一定干燥條件下的木薯的含水率，具體計(jì)算方法如下

式中MR為水分比；M為干燥過程中某一時(shí)刻樣品的含水率，%；M，木薯的終點(diǎn)含水率，%；0為木薯的原始含水率，%。

木薯干燥過程中的干燥速率用式（3）表示。

式中為干燥速率，%/min；為干燥時(shí)間，min。

2 結(jié)果與分析

木薯的原始水分為62%，根據(jù)預(yù)試驗(yàn)結(jié)果，若木薯切片太薄，干燥過程中會(huì)出現(xiàn)斷裂的現(xiàn)象，木薯片太厚干燥不完全，因此選取切片厚度為8 mm的試驗(yàn)數(shù)據(jù)研究溫度對(duì)木薯干燥特性的影響。

木薯主要成分為蛋白質(zhì)和淀粉，由于蛋白質(zhì)和淀粉在高溫下易變性，因此選取干燥溫度為80 ℃的試驗(yàn)數(shù)據(jù)分析厚度對(duì)木薯干燥特性的影響。

2.1 溫度對(duì)木薯干燥特性的影響

圖1給出了厚度為8 mm時(shí)干基含水率隨時(shí)間變化的干燥曲線。木薯與熱風(fēng)之間的傳熱傳質(zhì)機(jī)理主要受溫差的影響，干燥過程中的溫度越高，木薯與熱風(fēng)的溫度差越大，木薯內(nèi)部水分的蒸發(fā)速率加快，圖1中曲線的斜率增大，即木薯的干燥速率增大。試驗(yàn)結(jié)果表明，提高干燥溫度對(duì)木薯干燥過程有利，但考慮木薯的化學(xué)性質(zhì)會(huì)隨干燥溫度的升高而糊化、變質(zhì)，根據(jù)試驗(yàn)結(jié)果擬定干燥設(shè)備的干燥溫度為80～100 ℃，不僅能夠避免低溫造成能耗損失，同時(shí)也不會(huì)出現(xiàn)因高溫引發(fā)的木薯變質(zhì)、糊化的現(xiàn)象。

2.2 厚度對(duì)木薯干燥特性的影響

圖2給出了80℃下不同切片厚度的干燥曲線，試驗(yàn)結(jié)果表明，切片厚度越薄，達(dá)到特定含水率需要的時(shí)間越短。但在實(shí)際干燥過程中當(dāng)切片厚度為5 mm時(shí)，由于厚度太薄，干燥一段時(shí)間后木薯片表面易產(chǎn)生收縮裂口最終導(dǎo)致木薯片斷裂；當(dāng)厚度為9 mm時(shí)由于木薯內(nèi)部水分蒸發(fā)速率較低導(dǎo)致木薯干燥時(shí)間延長。木薯干燥時(shí)，薯片厚度6～8 mm為宜。

圖1 厚度為8 mm時(shí)不同干燥溫度下的干燥曲線

圖2 溫度為80 ℃時(shí)不同厚度的干燥曲線

2.3 干燥模型擬合

圖3 溫度為80℃水分比的擬合曲線

圖4 厚度為8 mm比水分的擬合曲線

相同溫度下5種不同厚度、相同厚度下5種不同溫度的木薯水分比MR干燥模型、干基含水率模型與試驗(yàn)數(shù)據(jù)擬合結(jié)果的相關(guān)系數(shù)22分別見表1、表2，其中22的值均在0.98～0.99之間。根據(jù)擬合得到的水分比干燥模型方程可以對(duì)木薯實(shí)際干燥過程進(jìn)行預(yù)測(cè)，便于判斷某一時(shí)刻的木薯水分比MR的值；根據(jù)不同條件下木薯片干基含水率的擬合方程式可以推出木薯片在不同溫度、不同厚度下的臨界含水率的值，及木薯干燥過程中恒速干燥與降速干燥的臨界點(diǎn)。確定相應(yīng)的臨界含水率后可得到與之對(duì)應(yīng)的達(dá)到木薯片臨界含水率的時(shí)間。

表1 80℃下不同厚度木薯片水分比MR和干基含水率M的擬合方程式

表2 8 mm切片厚度下不同溫度木薯片水分比MR和干基含水率M的擬合方程式

2.4 臨界含水率

圖5 厚度8 mm木薯片在溫度80 ℃時(shí)的干燥速率曲線

2.5 干燥工藝的設(shè)計(jì)

干燥過程中為了避免干燥速率下降的情況，木薯干燥設(shè)備的設(shè)計(jì)過程中一般木薯的干燥過程可分為恒溫一段式干燥和變溫分段式干燥[30]。試驗(yàn)結(jié)果表明，采用恒定溫度的一段式干燥方案在70 min內(nèi)只能將木薯的含水量降到35%左右，并不能滿足在60 min內(nèi)將木薯含水率降到安全含水率的設(shè)計(jì)要求。因此在實(shí)際干燥工藝的設(shè)計(jì)過程中考慮采用二段式干燥方案，即干燥過程中前后兩段選取不同的干燥溫度，利用前半段時(shí)間將含水率降低到原始水分的一半，后半段時(shí)間將含水率降到安全含水率。

二段式干燥試驗(yàn)分別在不同的溫度和不同的時(shí)間內(nèi)完成，將木薯切片后分別放入鼓風(fēng)干燥箱，在1溫度下干燥1時(shí)間后，將鼓風(fēng)干燥箱的溫度調(diào)至2溫度繼續(xù)干燥2時(shí)間，最終將木薯取出，分別測(cè)得在不同試驗(yàn)工況下干燥后木薯的含水率，以判斷經(jīng)過木薯是否達(dá)到安全含水率。其中設(shè)計(jì)的2種干燥試驗(yàn)方案見表3，其中方案1設(shè)定先低溫后高溫的干燥方案，即前半段干燥溫度為80 ℃，干燥時(shí)間為30 min，后半段干燥溫度為100 ℃，干燥時(shí)間為30 min；方案2設(shè)定先高溫后低溫的干燥方案，即前半段干燥溫度為100 ℃，干燥時(shí)間為30 min，后半段干燥溫度為80 ℃，干燥時(shí)間為30 min。

表3 二段式干燥試驗(yàn)方案

圖6a是先低溫后高溫的二段式干燥方案結(jié)果，圖6b是先高溫后低溫的二段式干燥方案結(jié)果。對(duì)比以上二段式干燥試驗(yàn)結(jié)果，發(fā)現(xiàn)木薯干燥過程中方案2的最終含水率均低于方案1的最終含水率。根據(jù)試驗(yàn)結(jié)果，擬定木薯干燥工藝中采用第一段溫度為100 ℃、第二段溫度為80 ℃的二段式干燥方案。

a. 第1干燥段80 ℃，第2干燥段100℃

a. 80℃at first stage and 100℃ at second stage

b. 第1干燥段100℃，第2干燥段80℃

b. 100℃at first stage and 80℃ at second stage

注：為分段干燥溫度。

Note:is stage drying temperature.

圖6 不同溫度的二段式木薯干燥工藝對(duì)比圖

Fig.6 Comparison chart of two-stage cassava drying process at different temperatures

圖7給出了木薯干燥工藝方案示意，該方案的設(shè)備主要包括風(fēng)機(jī)、給料機(jī)、第一干燥段、第二干燥段、熱交換器等。實(shí)際干燥過程中將具有一定原始含水率的木薯切成6～8 mm的片狀，用給料機(jī)將切好的木薯片送入第一干燥段內(nèi)，其中第一干燥段內(nèi)的熱風(fēng)由風(fēng)機(jī)1提供，經(jīng)熱交換器1加熱到1=100 ℃，第一干燥段的干燥時(shí)間1=30 min。經(jīng)第1干燥段干燥后的中間產(chǎn)品被送入第二干燥段，第二干燥段的干燥時(shí)間2=30 min，經(jīng)第二干燥段處理后的木薯達(dá)到13%的安全含水率，能夠進(jìn)行大量的儲(chǔ)存和運(yùn)輸。

其中干燥工藝的第二干燥段的熱風(fēng)由風(fēng)機(jī)2提供，并由熱交換器2加熱，其熱風(fēng)溫度為2=80 ℃。由于第1干燥段反應(yīng)后的氣體濕度較大不適合被循環(huán)利用，因此直接經(jīng)干燥氣流出口排放到大氣中。第二干燥段出口流出的氣體濕度小，因此設(shè)計(jì)將第二干燥段反應(yīng)后的干燥氣體流出后返回到熱交換器2中繼續(xù)循環(huán)利用，這樣在增加了第二干燥段循環(huán)風(fēng)流量的同時(shí)，提高了木薯的傳熱傳質(zhì)效率，加快木薯實(shí)際的干燥速率、節(jié)約了大量的能源。

圖7 木薯二段式干燥工藝流程圖

3 結(jié) 論

在干燥箱內(nèi)模擬木薯干燥設(shè)備，對(duì)不同溫度、不同厚度的木薯片進(jìn)行了干燥試驗(yàn)。試驗(yàn)研究發(fā)現(xiàn)，干燥溫度過低則干燥時(shí)間延長，溫度過高木薯則會(huì)變質(zhì)、糊化；木薯片厚度太薄，則在干燥過程中易斷裂；木薯片厚度太厚，達(dá)到安全含水率所需時(shí)間過長。用Wang and Singh模型對(duì)木薯干燥過程的水分比進(jìn)行了擬合，吻合較好。利用干基含水率曲線確定了木薯干燥的臨界含水率為105%。本文結(jié)果表明，干燥溫度取80～100 ℃、厚度選6～8 mm為宜，采用先高溫后低溫的二段式木薯干燥方案。

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Optimization experiment on two-stage drying process of high and low temperatures for cassavas

Yang Xiaoxiao, Cong Kunlin, Zhang Yanguo, Li Qinghai※

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Cassava is widely distributed in tropical and subtropical regions of the world. Cassava, potato and sweet potato are known as the world’s 3 major potatoes. Cassava is native to the Amazon basin, which are now widely grown in China and Southeast Asia. Cassava is mainly used in food, medicine, industry and other fields, and known as the king of starch. Cassava has to be dried in a period of short time prior to storage or transportation, since the water content of the fresh cassava is as much as 65%-70% at harvest. Up to now, many scholars have done a lot of research on cassava. However, there are few studies on the drying process, drying technology and drying equipment for cassava. In order to develop and optimize drying process of cassava, the effects of the thickness of cassava slices and the temperature of drying process were experimentally investigated. Fresh cassavas were obtained from Rongxian County, Guangxi Zhuang Autonomous Region. They were washed and dried before being stored in a fridge at 2-5 ℃. Cassavas were cut into 5-9 mm pieces, and then were placed in a drying oven with a preset temperature that could be adjusted from 60 to 100 ℃ with a 10 ℃ interval for different cases. The drying process by hot air was simulated with an electric drying oven with forced convective air flow. The samples were taken from the oven every 5 min during the experiment. The cassava slices taken from the drying oven were placed in a silica gel dryer. The cassava slices were cooled to room temperature and weighed with an electronic balance. The moisture of cassava was calculated according to the actual weight change. The effects of different temperatures and different thicknesses on the drying process of cassava were compared. The results showed that cassavas were denatured due to the high temperature, however, the low temperature made water evaporation incomplete. The slice with thinner thickness was prone to be broken up, and on the contrary, the thicker thickness prolonged the drying time to reach the targeted water content. The experimental results show that the drying temperature of 80-100 ℃ and the thickness of 6-8 mm are better. It was found that the cassava drying process was fitted well with the Wang and Singh model. Based on the water content curves on dry basis, the critical water contents of cassava with different thicknesses were obtained. The experimental results show that the one-stage scheme with constant temperature can only reduce the water content of cassava to about 35% when being dried for 70 min, and fails to reduce moisture content of cassava to safe moisture in the limited short time. In order to avoid the undesired sharp decrease in the drying rate as the water content reached the critical value during single-stage drying process, an optimal two-stage drying scheme of first low temperature and then high temperature was proposed. According to the two-stage drying scheme, the cassava drying process was also proposed. The equipment of the proposed drying process mainly includes the fan, the feeder, the first drying section, the second drying section, the heat exchanger, and so on. The cassava slices of 6-8 mm in thickness were fed into the first drying section by feeder. After the moisture content was reduced to a certain value, the cassavas were fed into the second drying section. Finally, the cassava slices eventually reached the safe moisture content. This cassava drying process provides a good solution for cassava drying and production.

drying; moisture; quality control; cassava

10.11975/j.issn.1002-6819.2018.02.037

S375

A

1002-6819(2018)-02-0272-06

2017-09-22

2017-12-29

國家重點(diǎn)研發(fā)計(jì)劃資助（No. 2017YFB0603901）

楊瀟瀟，工程師，主要從事熱能轉(zhuǎn)換與利用方向的研究。Email：yangxiaoxiao2589@126.com。

李清海，副研究員，博士，博士生導(dǎo)師，主要從事熱能轉(zhuǎn)換與利用的研究。Email：liqh@tsinghua.edu.cn

楊瀟瀟，叢堃林，張衍國，李清海. 木薯高/低溫二段式干燥工藝參數(shù)優(yōu)化試驗(yàn)[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2018，34(2)：272－277. doi：10.11975/j.issn.1002-6819.2018.02.037 http://www.tcsae.org

Yang Xiaoxiao, Cong Kunlin, Zhang Yanguo, Li Qinghai. Optimization experiment on two-stage drying process of high and low temperatures for cassavas[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(2): 272－277. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2018.02.037 http://www.tcsae.org