李星恕,靳莉珍,張 博,熊秀芳,張海輝(. 西北農(nóng)林科技大學(xué)機械工程與電子學(xué)院,楊凌 7200; 2. 陜西省農(nóng)業(yè)裝備工程技術(shù)研究中心,楊凌 7200)
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基于電阻抗的蘋果干燥過程含水率實時檢測及動力學(xué)分析
李星恕1,2,靳莉珍1,張博1,熊秀芳1,張海輝1
(1. 西北農(nóng)林科技大學(xué)機械工程與電子學(xué)院,楊凌 712100;2. 陜西省農(nóng)業(yè)裝備工程技術(shù)研究中心,楊凌 712100)
摘要:為了找到一種經(jīng)濟便捷的蘋果片干燥過程含水率實時檢測方法,分析熱風(fēng)溫度和風(fēng)速對干燥過程的影響,該研究實時檢測了不同風(fēng)速和熱風(fēng)溫度下蘋果片的電阻抗和含水率并分析了其隨時間變化的規(guī)律。結(jié)果表明,干燥過程中蘋果片電阻抗隨干燥時間的增加而增大,含水率隨干燥時間而減小,兩者線性負相關(guān)(R2≥9.3),因此可以通過電阻抗的變化實時檢測蘋果干燥過程。蘋果片電阻抗和含水率隨干燥時間的變化均符合薄層干燥L(fēng)ogarithmic模型;基于電阻抗和含水率分別擬合得出不同條件下的干燥速率,并利用阿倫尼烏斯公式求出蘋果試樣干燥過程活化能,當(dāng)風(fēng)速為0.5和1.0 m/s時,依據(jù)電阻抗計算所得活化能分別為32.447和23.212 kJ/mol,含水率計算所得活化能為27.320和22.947 kJ/mol,依據(jù)電阻抗計算所得活化能與前人研究活化能值更一致。研究結(jié)果可為蘋果片干燥過程在線檢測和分析提供參考。
關(guān)鍵詞:電阻抗;干燥;動力學(xué);模型;活化能;蘋果片
李星恕,靳莉珍,張博,熊秀芳,張海輝. 基于電阻抗的蘋果干燥過程含水率實時檢測及動力學(xué)分析[J]. 農(nóng)業(yè)工程學(xué)報,2016,32(2):266-272.doi:10.11975/j.issn.1002-6819.2016.02.038http://www.tcsae.org
Li Xingshu, Jin Lizhen, Zhang Bo, Xiong Xiufang, Zhang Haihui. Real-time monitoring of moisture content and kinetics analysis of apple drying process by impedance measurement[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2016, 32(2): 266-272. (in Chinese with English abstract)doi:10.11975/j.issn.1002-6819.2016.02.038 http://www.tcsae.org
蘋果是中國第一大水果,占中國全部果品總量的30%左右,是最具競爭力的出口農(nóng)產(chǎn)品之一[1]。蘋果生產(chǎn)季節(jié)性強,上市量集中,而在營銷、儲運、保鮮等方面的發(fā)展還比較落后,出現(xiàn)了時間上和空間上的相對過剩,腐爛損耗率高,造成采后損失巨大。深加工是解決這一問題的較好方法。蘋果片是在保持蘋果原有品質(zhì)基礎(chǔ)上加工干燥而成的一種休閑食品[2-4]。在蘋果片的生產(chǎn)加工中,干燥工藝對其品質(zhì)影響很大[3]。
近年來提高產(chǎn)品質(zhì)量和降低能耗是果蔬干燥的研究熱點[5-6],實時監(jiān)測干燥過程中產(chǎn)品的含水率,對判斷干燥終點、提高產(chǎn)品質(zhì)量、降低能耗具有重要意義[7-9]。許多方法如CT[10-11]、核磁共振[8,12]、計算機視覺[13-14]和激光散射[15-16]等被用來檢測果蔬的干燥過程。上述方法均能檢測干燥過程中果蔬含水率的變化,但是由于操作繁瑣,成本較高等原因很難應(yīng)用于實際生產(chǎn)。另外,為了分析果蔬干燥過程,優(yōu)化干燥工藝,需要對果蔬的干燥過程進行動力學(xué)分析。很多研究均是基于含水率的變化建立果蔬干燥過程動力學(xué)模型,分析溫度、風(fēng)速等因素對干燥速率的影響[17-18],但是含水率檢測一般采用取樣并稱量的方法,不能實現(xiàn)實時檢測。因此找到一種經(jīng)濟、快捷的果蔬干燥過程在線檢測方法非常必要。
果蔬的阻抗特性與果蔬中含水率和活性密切相關(guān)[19]。有研究發(fā)現(xiàn)干燥過程中胡蘿卜、蔥、蘑菇和生菜的電容與其含水率的變化趨勢相同[20-21],不同含水率蘋果的電特性也不同[22-23]。有學(xué)者研究了干燥過程中蘋果的電特性隨含水率的變化[23-25],但并未通過電特性的變化建立具體的數(shù)學(xué)模型對干燥過程進行深入的分析及建模。
為了找到一種簡單、經(jīng)濟的果蔬干燥過程在線檢測方法,本文測量熱風(fēng)干燥過程中蘋果片含水率和阻抗特性的變化,建立基于電阻抗和含水率的蘋果干燥動力學(xué)模型,分析干燥溫度和風(fēng)速對蘋果片干燥過程的影響,為優(yōu)化干燥工藝,改進干燥設(shè)備提供理論依據(jù)。
1.1蘋果試樣制備
試驗材料為新鮮成熟、無損傷、大小相近的洛川富士蘋果,購于本地超市。清洗擦干后放置在實驗室平衡5 h使蘋果完全恢復(fù)至室溫(22℃),然后除去蘋果花萼、果柄、果皮和果核,將蘋果切成15 mm×15 mm×11 mm長方體試樣,并立即進行干燥試驗。試樣初始濕基含水率為84%。
1.2試驗系統(tǒng)與方法
將蘋果試樣置于洞道干燥裝置(DG100D,浙江中控科教儀器設(shè)備有限公司)內(nèi),調(diào)節(jié)干燥裝置分別設(shè)定干燥溫度為40、50、60和70℃,風(fēng)速為0.5和1.0 m/s,進行不同條件的熱風(fēng)干燥試驗[26-28],并在干燥過程中檢測蘋果試樣阻抗特性的變化??諝庀鄬穸葹?%。蘋果試樣熱風(fēng)干燥及阻抗檢測系統(tǒng)如圖1所示。首先在儀表盤上設(shè)定干燥溫度和熱風(fēng)速度,干燥系統(tǒng)啟動后,空氣從進口進入到電加熱裝置,變成熱風(fēng)后進入干燥室;待儀表控制盤上顯示實際溫度達到設(shè)定溫度后,打開干燥室,把6個相同的蘋果試樣均勻放置在干燥室中網(wǎng)狀托盤上進行單層干燥試驗。其中1個試樣用來檢測阻抗特性的變化,另5個試樣用來檢測含水率的變化。干燥過程中,每20 min檢測一次含水率和阻抗特性的變化。每個試驗重復(fù)3次。
檢測含水率的變化時,每隔20 min迅速取出其中的5塊樣品測其質(zhì)量并迅速放回干燥設(shè)備,直至前后兩次質(zhì)量差小于0.01 g終止試驗。
干燥過程中蘋果試樣濕基含水率按式(1)計算
式中MC為試樣濕基含水率,%;Mi為i時刻試樣質(zhì)量,g;M∞為干燥結(jié)束時試樣質(zhì)量,g。
利用LCR測試儀(LCR3532-50,日置公司)和自制不銹鋼針狀電極測量蘋果試樣的阻抗特性,兩電極間距為5 mm,插入蘋果樣品深度為8 mm。預(yù)試驗結(jié)果表明,采用針狀電極能夠有效避免干燥過程中體積收縮對測量結(jié)果的影響。每隔20 min,計算機控制LCR測試儀連續(xù)測量蘋果試樣在21個頻率點下的阻抗值和相位角,計算機自動記錄并保存數(shù)據(jù)。含水率測量結(jié)束時也停止檢測試樣阻抗特性。測量電壓為1 V,頻率范圍為42 Hz~5 MHz[27]。
溫度不同時干燥過程中阻抗值相差很大,因此在一張圖中比較分析阻抗變化趨勢很困難,需對電阻抗作歸一化處理。對測得電阻抗值按式(2)作歸一化處理,
式中Z為歸一化阻抗;Zi為干燥過程中i時刻阻抗,?;Z0為干燥開始時阻抗,?;Z∞為干燥結(jié)束時阻抗,?。
圖1 蘋果試樣電阻抗檢測系統(tǒng)Fig.1 Electrical impedance measurement system of apple slice
1.3數(shù)據(jù)處理方法
采用Spss軟件對蘋果片干燥過程數(shù)據(jù)進行處理和擬合。采用Origin軟件繪圖。
2.1干燥過程中蘋果試樣阻抗特性
干燥過程中蘋果試樣阻抗隨頻率變化如圖2所示。由圖2a可知,在干燥過程中同一時刻,阻抗隨著頻率的增加而減?。桓稍镞^程中,蘋果試樣的阻抗隨著時間的增加而增大;增加的幅度隨著頻率的增加而減小,高于1 MHz時每條時間曲線幾乎重合。
圖2 干燥過程中蘋果阻抗和相位角隨頻率的變化Fig.2 Change of impedance and phase angles with frequencyduring drying process of apple slice
蘋果組織是由細胞和細胞間隙組成的有序結(jié)合體,分為液泡、細胞質(zhì)、細胞膜、細胞壁和細胞間隙[24]。液泡膜和細胞膜具有電容的特性,低頻時具有較大的阻抗;高頻時,電流能穿過細胞膜,阻抗明顯減小[26,29],故阻抗隨頻率增大而減小。另外,在熱風(fēng)干燥過程中,含水率持續(xù)降低,組織細胞會收縮進而出現(xiàn)質(zhì)壁分離,細胞膜破裂組織內(nèi)部形成空洞[30-31],導(dǎo)致干燥過程中同一頻率下蘋果試樣阻抗增大[32]。
干燥過程中蘋果試樣相位角隨頻率的變化如圖2b所示。由圖2b可知,干燥過程中,相位角隨頻率的增大呈增大-減小-增大馬鞍形變化,隨著干燥進行,局部馬鞍形變化逐漸消失,干燥后期蘋果試樣相位角隨頻率的增加而一直減小。這是因為低頻時總阻抗中電阻成分占主導(dǎo)相位角較小,隨著頻率的增加,電流逐漸通過細胞膜流入細胞內(nèi)部,容抗在總阻抗中所占比例逐漸增加,相位角逐漸增大[33]。隨著干燥進行,蘋果組織含水率降低,細胞結(jié)構(gòu)遭到破壞,這種現(xiàn)象逐漸消失。
干燥過程蘋果試樣Cole-Cole圖如圖3所示。從右到左頻率依次增大,橫坐標(biāo)為阻抗實部(電阻成分),縱坐標(biāo)為虛部(容抗成分)。整個干燥過程中蘋果組織的Cole-Cole圖均為一段圓弧,與生物組織的電學(xué)特征相符。由于電極極化現(xiàn)象,低頻段曲線略有翹曲;隨著干燥的進行,蘋果試樣的Cole-Cole圖半徑逐漸增大。
圖3 干燥過程中蘋果試樣的Cole-Cole圖Fig.3 Cole-Cole plot of apple slice during drying process
2.2干燥過程中電阻抗與含水率的關(guān)系
由圖2a可知,頻率為1 kHz時,阻抗隨時間變化明顯且均勻,而且消除了電極極化對阻抗的影響[31],因此選擇1 kHz下的阻抗研究蘋果試樣干燥過程。熱風(fēng)速度為0.5 m/s時不同溫度條件下電阻抗隨時間的變化如圖4所示。熱風(fēng)速度1m/s時電阻抗隨溫度的變化趨勢相同,這里沒有給出。
圖4干燥過程阻抗隨時間的變化關(guān)系Fig.4 Impedance changes with time during drying process
由圖4可知,蘋果試樣阻抗隨干燥時間先緩慢增大后期急劇增加,這是因為隨著干燥過程的進行,蘋果組織含水率逐漸減小,組織細胞的面積和當(dāng)量直徑亦隨之減小,細胞逐漸收縮變小,電阻抗緩慢增大[30,34];干燥后期,蘋果組織細胞收縮進而出現(xiàn)質(zhì)壁分離,細胞會破裂進而形成空洞,電阻抗急劇增大[23,30];不同溫度干燥完成時間不同,主要原因是溫度與水分?jǐn)U散系數(shù)正相關(guān)[35],溫度越高,水分?jǐn)U散系數(shù)越大,水分?jǐn)U散越快,干燥時間越短。
干燥過程中蘋果試樣含水率和歸一化電阻抗隨時間的變化如圖5所示。由圖5可知,干燥初期含水率急劇減小,隨著干燥的進行,含水率減小緩慢,后期逐漸趨于穩(wěn)定;而干燥初期阻抗變化緩慢,后期急劇增大;干燥過程中含水率和阻抗的變化趨勢相反。為了找到一種能實時檢測蘋果試樣干燥過程的方法,本文擬合得出相同條件下歸一化阻抗和含水率之間的線性方程,如圖6a和圖6b所示。
圖5 歸一化阻抗和含水率隨時間變化Fig.5 Normalized Impedance and moisture content change with time
圖6 干燥過程含水率與歸一化阻抗關(guān)系Fig.6 Relationship of moisture content and normalized impedance during drying process
不同溫度時歸一化阻抗和含水率之間的線性方程擬合結(jié)果如表1所示。由表1可知,熱風(fēng)溫度為40、50、60℃時線性方程斜率較小,但70℃時直線斜率很大。主要原因是當(dāng)溫度大于65℃時,蘋果組織細胞壁和細胞膜完全破裂,細胞內(nèi)水分更易排出[27],故70℃熱風(fēng)干燥時,阻抗變化趨勢較其他溫度較大。
表1 歸一化阻抗與含水率的線性擬合Table 1 Fitting parameters between normalized impedance and moisture content
由表1可知,干燥過程中蘋果試樣歸一化阻抗和含水率線性負相關(guān)(R2≥0.93),通因此可以過檢測蘋果試樣阻抗特性來預(yù)測干燥過程中含水率的變化,從而實現(xiàn)在線監(jiān)控蘋果試樣干燥過程含水率的變化。實現(xiàn)加工終點判斷,干燥工藝優(yōu)化以及產(chǎn)品質(zhì)量控制[10,24]。
2.3蘋果試樣干燥過程動力學(xué)分析
為了分析溫度和時間對蘋果試樣干燥過程的影響,對蘋果試樣干燥過程進行動力學(xué)分析。干燥過程中歸一化阻抗和含水率隨時間的變化如圖7所示。干燥初期阻抗變化很小,而干燥后期,阻抗急劇上升,含水率剛開始變化急劇,后期變化緩慢。這種變化趨勢符合薄層干燥logarithmic模型[36],分別對干燥過程中蘋果試樣的歸一化阻抗和含水率進行l(wèi)ogarithmic動力學(xué)方程擬合,擬合曲線分別如圖7所示。logarithmic模型為
式中MR為歸一化阻抗或含水率;k為干燥速率常數(shù),h-1;ɑ,c為模型參數(shù)。
不同溫度和風(fēng)速條件下,分別根據(jù)歸一化阻抗和含水率計算擬合logarithmic模型參數(shù),結(jié)果如表2和表3所示。由表2和3中擬合結(jié)果可知,基于電阻抗和含水率的擬合效果均很好,蘋果干燥過程符合薄層干燥logarithmic模型(R2>0.99);隨著溫度升高干燥速率常數(shù)增加;同樣條件下基于電阻抗所得干燥速率明顯高于基于含水率所得干燥速率常數(shù),究其原因,主要是由于蘋果組織水分主要存在于細胞內(nèi),在熱風(fēng)干燥過程中,組織細胞結(jié)構(gòu)首先收縮破裂,然后細胞內(nèi)水分才能流出[27,30];而細胞結(jié)構(gòu)破壞和水分溢出均能導(dǎo)致阻抗的變化,阻抗的變化更快,因此阻抗計算擬合得到的干燥速率更大。
圖7 干燥過程中歸一化阻抗和含水率隨時間的變化Fig.7 Normalized impedance and moisture content change over drying time in different temperatures
表2 基于電阻抗的干燥動力學(xué)模型擬合結(jié)果Table 2 Drying kinetics parameters by normalized impedance
表3 基于含水率的干燥動力學(xué)擬合結(jié)果Table 3 Drying kinetics parameters by moisture content
為分析干燥溫度對蘋果試樣干燥過程的影響,利用阿倫尼烏斯公式分別描述了不同風(fēng)速下干燥速率常數(shù)與溫度之間的關(guān)系[37-38],阿倫尼烏斯公式為
式中k為干燥速率常數(shù),h-1;A為指前因子,h-1;Eɑ為活化能,kJ/mol;R為摩爾氣體常量,8.314 J(mol·K);T為絕對溫度,K。
根據(jù)logarithmic模型擬合可得不同干燥溫度下干燥速率常數(shù),然后以lnk和1/T作線性回歸,直線的斜率為?Eɑ/R,截距為lnA,可求出蘋果試樣干燥過程活化能Eɑ。當(dāng)風(fēng)速為0.5和1.0 m/s時,基于阻抗得到的活化能分別為32.447和23.212kJ/mol;基于含水率計算得出的活化能分別為27.320和22.947 kJ/mol。Mohsen的研究結(jié)果表明,蘋果干燥過程活化能范圍為26.72~35.83 kJ/mol[28],基于電阻抗得到的活化能更接近于該值,也間接驗證了通過電阻抗檢測分析蘋果干干燥過程的可行性。風(fēng)速增大,活化能減小,說明相同溫度條件下,風(fēng)速越大蘋果試樣越容易干燥。出現(xiàn)這種現(xiàn)象的原因,是由于不同的風(fēng)速對于蘋果試樣的組織狀態(tài)、結(jié)構(gòu)等的影響不同,從而影響到干燥活化能[39]。
1)熱風(fēng)干燥過程中蘋果試樣阻抗先是緩慢增大,后期急劇增大,含水率的變化趨勢相反;阻抗與含水率呈線性負相關(guān)(R2≥0.93)。故干燥過程中可以通過實時檢測蘋果試樣阻抗值來預(yù)測其含水率的變化,實現(xiàn)蘋果干燥過程的在線監(jiān)控。
2)蘋果片干燥過程符合薄層干燥L(fēng)ogarithmic模型(R2>0.99);基于電阻抗擬合所得干燥速率常數(shù)大于基于含水率所得干燥速率常數(shù)。
3)在風(fēng)速為0.5和1.0 m/s時由電阻抗擬合得到的干燥速率計算所得活化能分別為32.447和23.212 kJ/mol;由含水率計算得出的活化能分別為27.320和22.947 kJ/mol。
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Real-time monitoring of moisture content and kinetics analysis of apple drying process by impedance measurement
Li Xingshu1,2, Jin Lizhen1, Zhang Bo1, Xiong Xiufang1, Zhang Haihui1
(1. College of Mechɑnicɑl ɑnd Electronic Engineering, Northwest A&F University, Yɑngling 712100, Chinɑ; 2. Shɑnxi Engineering Reseɑrch Center for Agriculturɑl Equipment, Yɑngling 712100, Chinɑ)
Abstract:Apple is one of the most competitive agricultural exports in China, accounting for about 30% of the total production of fruits. Unsuitable preservation or storage methods can lead to the loss, which may amount for 30% of the total. To reduce the loss of apples after harvesting, deep processing is an appropriate method. As a kind of snack food, dried apple slice is produced by a drying process and the natural flavor of apple can be retained. During the drying process, the drying temperature and time have a great influence on the apple slice quality. To develop a new method to monitor and characterize the drying process of apple slices as well as evaluate the influence of the drying temperature and time on this process, the electrical impedance and moisture content of apple tissues were measured during the drying process under different conditions. The hot air temperature was set to 40, 50, 60 and 70℃, and the hot air speed was set to 0.5 and 1.0 m/s, respectively. The 6 same apple samples were placed in the drying chamber for drying test. One sample was to detect the change of the impedance characteristics, and the other samples were used to detect the change of the moisture content. The interval measurement time was 20 min. Until the mass difference of the 2 successive measurements for one sample was less than 0.01 g, the experiment was stopped. Results showed that the electrical impedance of apple slices increased with the increase in the drying time, and t he increase rate was slow in the early stages and fast in the latter part of the drying process. The moisture content decreased during the drying process, and the decrease was fast at the beginning and slow in the latter part. Thus, the variation tendency of moisture content was contrary to that of electrical impedance during the process. The electrical impedance of apple slices showed a negative linear correlation with the moisture content when the moisture content was more than 20%. So electrical impedance measurement could provide a simple and rapid approach for predicting the moisture content, and furthermore it was capable of monitoring and evaluating the drying process of apple slices. The curves of the normalized electrical impedance and moisture content with the drying time could be approximated with the logarithmic drying model which could describe the characteristics of the drying process. The rate constant of the model at various temperatures was estimated by the normalized impedance and moisture content. The rate constant increased with the increasing of drying temperature. Then to analyze the effect of temperature on the drying rate constant, the rate constants at different temperatures were formulated by the Arrhenius equation. Based on the obtained rate constants at different drying temperature, the activation energy was calculated. It was found that when the speed of drying hot air was 0.5 and 1.0 m/s, the activation energy calculated from the normalized impedance was 33.925 and 28.912 kJ/mol, respectively; the calculated activation energy from the moisture content was 27.320 and 22.947 kJ/mol, respectively. The activation energy decreased with the increasing of hot air speed, which indicated that the higher the hot air speed, the faster the drying of apple slices. The findings are useful to monitor the drying process of apple slices and have the potential applications in the control of fruit drying process.
Keywords:electrical impedance; drying; kinetics; models; activation energy; apple slices
作者簡介:李星恕,男,河南駐馬店人,博士,副教授,主要從事農(nóng)業(yè)裝備研發(fā)與農(nóng)產(chǎn)品無損檢測方面的研究。楊凌西北農(nóng)林科技大學(xué)機電學(xué)院,712100。Email:xingshu-li@nwsuaf.edu.cn
基金項目:陜西省科技統(tǒng)籌創(chuàng)新工程計劃項目(2014KTCL02-15)和陜西省自然科學(xué)基礎(chǔ)研究計劃(2015JM3113)
收稿日期:2015-09-03
修訂日期:2015-12-10
中圖分類號:TS255.4
文獻標(biāo)志碼:A
文章編號:1002-6819(2016)-02-0266-07
doi:10.11975/j.issn.1002-6819.2016.02.038