孔丹丹,方 鵬,王紅英,陳 嘯,岳 巖,呂 芳,金 楠(中國農(nóng)業(yè)大學(xué)工學(xué)院,國家農(nóng)產(chǎn)品加工技術(shù)裝備研發(fā)分中心,北京 100083)
高含量乳清粉的仔豬配合飼料熱特性及調(diào)質(zhì)溫度控制
孔丹丹,方 鵬,王紅英※,陳 嘯,岳 巖,呂 芳,金 楠
(中國農(nóng)業(yè)大學(xué)工學(xué)院,國家農(nóng)產(chǎn)品加工技術(shù)裝備研發(fā)分中心,北京 100083)
為探究熱敏性飼料原料乳清粉及不同含量乳清粉的仔豬配合飼料的熱物理特性,該文以仔豬料配方中的4種主要飼料原料玉米、豆粕、乳清粉和魚粉為研究對(duì)象,采用混料設(shè)計(jì)的方法得到33種不同含量(0~30%)乳清粉的仔豬配合飼料,并利用差示掃描量熱法(differential scanning calorimetry,DSC)測(cè)定了4種單一原料在25~120 ℃范圍內(nèi)以及33種仔豬配合飼料在25~110 ℃范圍內(nèi)的比熱,分析了乳清粉及高含量乳清粉(質(zhì)量分?jǐn)?shù)≥14.548%)的仔豬配合飼料的熱變性過程。結(jié)果顯示:玉米、豆粕和魚粉的比熱分別與溫度(25~120 ℃)呈線性、對(duì)數(shù)和二次關(guān)系,而乳清粉的比熱與溫度(25~110 ℃)遵循三次多項(xiàng)式的關(guān)系;當(dāng)配合飼料中含有≥6.25%的乳清粉時(shí),其比熱與溫度遵循三次多項(xiàng)式的關(guān)系;配合飼料的比熱顯著受溫度、原料配比以及二者交互作用的影響(P<0.001),其中,溫度的影響最為顯著,而乳清粉含量的影響次之。DSC熱焓曲線上,乳清粉在109.79 ℃會(huì)出現(xiàn)吸熱峰,為乳清蛋白的熱變性導(dǎo)致;而隨著溫度由20 ℃升高到110 ℃,乳清粉顆粒由存在許多凸起與微孔的粗糙表面結(jié)構(gòu)逐漸過渡為光滑、粘結(jié)的狀態(tài)。與乳清粉相似,高含量乳清粉的配合飼料也會(huì)在77.95~87.69 ℃出現(xiàn)吸熱峰。在仔豬配合顆粒飼料的加工過程中,為降低乳清蛋白的變性程度、減少環(huán)模制粒機(jī)的堵機(jī)現(xiàn)象,應(yīng)將調(diào)質(zhì)溫度降低至70 ℃以下為宜。研究結(jié)果為高含量乳清粉的仔豬配合飼料的調(diào)質(zhì)、制粒等熱處理過程的工藝優(yōu)化提供理論指導(dǎo)。
比熱;溫度;物理特性;乳清粉;配合飼料;熱變性;差示掃描量熱法
飼料級(jí)乳清粉一般含有質(zhì)量分?jǐn)?shù)61%~70%的乳糖和2%~12%的粗蛋白質(zhì),適口性好,是早期斷奶仔豬日糧中必不可少的能量來源。許多研究表明:在基礎(chǔ)的玉米-豆粕型日糧中加入20%或25%的乳清粉,能夠顯著提高3~4周齡的斷奶仔豬在斷奶后3~5周內(nèi)的日增重和日采食量[1-5],并降低料肉比[1,5]。Graham等[6]的研究顯示在超早期斷奶仔豬(2周齡)日糧中添加25%的乳清粉可以顯著提高仔豬胰腺、腸道中總的淀粉酶、蛋白酶以及乳糖酶的活性,改善其在斷奶后28 d內(nèi)的生長性能。
乳清粉屬于熱敏性飼料原料,在仔豬顆粒飼料調(diào)質(zhì)、制粒的熱加工過程中容易產(chǎn)生乳糖焦化、乳清蛋白變性等問題。目前,國內(nèi)斷奶仔豬顆粒料配方中乳清粉的添加量一般為6%~12%[7],當(dāng)其添加量增至15%~20%時(shí),則會(huì)造成顆粒料硬度高、制粒機(jī)堵機(jī)[8]、生產(chǎn)困難、生產(chǎn)效率嚴(yán)重降低的現(xiàn)象。因此,國內(nèi)飼料生產(chǎn)企業(yè)通常將高含量乳清粉的配方加工成粉狀配合料。相較于顆粒料而言,粉料飼喂損失較大、保存期較短、易產(chǎn)生成分的自動(dòng)分級(jí)[9]。有研究顯示[1,10],國外可將含20%或25%乳清粉的仔豬料配方加工成顆粒料。而高含量乳清粉的仔豬配合料顆?;a(chǎn)一直是中國飼料行業(yè)的瓶頸問題。基于以上分析,研究乳清粉添加量對(duì)仔豬配合料比熱的影響、掌握高含量乳清粉的仔豬配合料的熱變性規(guī)律,從而合理設(shè)計(jì)其顆粒化熱加工過程中調(diào)質(zhì)、制粒的工藝參數(shù)勢(shì)在必行。
差示掃描量熱法(differential scanning calorimetry,DSC)已成為一種研究物質(zhì)的比熱[11-17]、熱變性[18-21]和相變[22-24]等熱物理特性的主要技術(shù)。許多學(xué)者采用DSC探究了配合飼料[11]、谷物面粉[14]、王不留行籽[15]、雙低油菜籽[16]等農(nóng)業(yè)物料的比熱與溫度、含水率的關(guān)系。王紅英等[17]采用DSC測(cè)定了乳清粉在25~150 ℃范圍內(nèi)的比熱,發(fā)現(xiàn)其在58.8 ℃時(shí)會(huì)出現(xiàn)峰值。還有許多學(xué)者采用DSC研究了熱處理[18]、乳清蛋白濃度和pH值[19-21]對(duì)溶液中乳清蛋白變性的影響。
本文以斷奶仔豬料配方中的4種主要原料玉米、豆粕、乳清粉和魚粉為研究對(duì)象,采用混料設(shè)計(jì)的方法得到33
種不同含量(0~30%)乳清粉的仔豬配合飼料,在此基礎(chǔ)上,利用DSC研究4種單一原料以及33種配合飼料在25~120 ℃范圍內(nèi)的比熱,建立其比熱關(guān)于溫度的可靠預(yù)測(cè)模型;分析乳清粉及高含量乳清粉的仔豬配合飼料的熱變性條件,并進(jìn)一步探究乳清粉在加熱過程中微觀結(jié)構(gòu)的變化,以期為高含量乳清粉的仔豬配合顆粒飼料的調(diào)質(zhì)、制粒等熱加工過程的工藝優(yōu)化提供理論指導(dǎo)。
1.1 材料與儀器
1.1.1 試驗(yàn)材料
玉米:鄭單958,源自中國農(nóng)業(yè)大學(xué)涿州試驗(yàn)基地;豆粕:益海(泰州)糧油工業(yè)有限公司;乳清粉:美國帝王(Empire Cheese Inc)奶酪公司;魚粉:秘魯Tecnológica de Alimentos S.A.(TASA)公司。將采集到的玉米和豆粕分別自然晾干到12%和11%的安全水分,然后用配有Φ1.5 mm篩片的粉碎機(jī)粉碎備用。將4種粉狀原料用聚乙烯自封袋密封,置于4 ℃的冷藏柜中保存。
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1.1.2 儀器設(shè)備
AL204分析天平:梅特勒-托利多儀器有限公司;DHG-9240A電熱恒溫鼓風(fēng)干燥箱:上海精宏實(shí)驗(yàn)設(shè)備有限公司;15B萬能粉碎機(jī):江陰市宏達(dá)粉體設(shè)備有限公司;ISO3310不銹鋼標(biāo)準(zhǔn)篩:英國endecotts(恩德)公司;Kjeltec 2300凱氏定氮儀:丹麥FOSS公司;Soxtec TM 2050粗脂肪分析儀:丹麥FOSS公司;Fibertec TM 2010纖維含量測(cè)定系統(tǒng):丹麥FOSS公司;CWF通用馬弗爐:英國Carbolite公司;DK-8D恒溫水浴鍋:上海精宏實(shí)驗(yàn)設(shè)備有限公司;DSC-60差示掃描量熱儀:日本島津公司;SU8010掃描電鏡:日本日立公司。
1.2 試驗(yàn)方法
1.2.1 飼料原料常規(guī)成分測(cè)定
玉米、豆粕和魚粉3種原料中粗蛋白、粗脂肪、粗灰分含量分別按照GB/T 6432-1994、GB/T 6433-2006、GB/T 6438-2007進(jìn)行測(cè)定;玉米、豆粕的粗纖維、中性洗滌纖維和酸性洗滌纖維含量分別按照GB/T 6434-2006、GB/T 20806-2006、NY/T 1459-2007進(jìn)行測(cè)定;玉米水分和淀粉含量分別按照GB/T 10362-2008和GB/T 5514-2008進(jìn)行測(cè)定;豆粕和魚粉的水分依照GB/T 6435-2014進(jìn)行測(cè)定;乳清粉中水分、蛋白質(zhì)、脂肪、灰分、乳糖分別依照GB 5009.3-2010、GB 5009.5-2010、GB 5413.3-2010、GB 5009.4-2010、GB 5413.5-2010進(jìn)行測(cè)定。試驗(yàn)均重復(fù)測(cè)定3次,取3次的平均值為最終結(jié)果。4種原料的常規(guī)成分分析結(jié)果見表1。
玉米中的主要成分為淀粉(63.98%,濕基),是斷奶仔豬日糧中首要的能量來源。對(duì)比中國飼料成分及營養(yǎng)價(jià)值表[25],本試驗(yàn)選用玉米的淀粉(72.71%,干基)、粗蛋白(9.18%,干基)、粗脂肪(3.95%,干基)等質(zhì)量分?jǐn)?shù)均與2級(jí)玉米相應(yīng)指標(biāo)極為接近。
表1 4種飼料原料的常規(guī)成分Table 1 Proximate composition of four feed ingredients %(濕基)
豆粕是最佳的植物性蛋白源飼料,在斷奶仔豬日糧中的配比僅次于玉米。對(duì)比中國飼料成分及營養(yǎng)價(jià)值表[25],本試驗(yàn)選用的豆粕粗蛋白質(zhì)量分?jǐn)?shù)(53.08%,干基)與1級(jí)去皮豆粕(53.82%,干基)相接近,而粗灰分和粗纖維質(zhì)量分?jǐn)?shù)(6.55%和6.45%,干基)與普通1級(jí)豆粕(6.85%和6.63%,干基)相近。
乳清粉中的主要成分為乳糖,質(zhì)量分?jǐn)?shù)高達(dá)70.94%(濕基),其次為粗蛋白質(zhì),為11.24%(濕基),屬于高蛋白乳清粉。
魚粉通常含有60%~72%的蛋白質(zhì)、10%~20%的灰分和5%~12%的脂肪[26],是極好的蛋白質(zhì)、氨基酸來源。本試驗(yàn)選用的魚粉粗蛋白質(zhì)質(zhì)量分?jǐn)?shù)高達(dá)65.73%(濕基),而粗灰分、粗脂肪質(zhì)量分?jǐn)?shù)和含水率分別為16.26%、8.90%和7.28%(濕基),這與中國飼料成分及營養(yǎng)價(jià)值表[25]中進(jìn)口魚粉的營養(yǎng)成分含量相近。
1.2.2 DSC熱特性分析
本試驗(yàn)采用DSC掃描玉米、豆粕、乳清粉、魚粉4種單一飼料原料在25~120 ℃范圍內(nèi)以及由4種原料不同配比組成的配合飼料在25~110 ℃范圍內(nèi)的熱譜曲線,并計(jì)算其在不同溫度下的比熱值,分析物料在加熱過程中的熱變性。
先采用標(biāo)準(zhǔn)物質(zhì)銦對(duì)DSC儀器進(jìn)行溫度校正和熱量校正,儀器校準(zhǔn)后開始進(jìn)行測(cè)定。所有測(cè)定過程均通入氮?dú)猓?0 mL/min)對(duì)樣品進(jìn)行保護(hù),裝樣品的坩堝均加蓋密封,以避免加熱過程中水分蒸發(fā)散失而影響比熱的測(cè)定結(jié)果。具體測(cè)定方法與孔丹丹等[11]描述的一致。每個(gè)樣品進(jìn)行3次重復(fù)試驗(yàn),取3次的平均值作為最終結(jié)果。
1.2.3 掃描電鏡圖像分析
取適量乳清粉樣品置于玻璃試管內(nèi),用橡膠塞封口,放入特定溫度的恒溫油浴鍋內(nèi),保溫5 min后取出,并置于冰水里迅速冷卻。取少量經(jīng)熱處理后的乳清粉樣品固定噴金后,使用SU8010掃描電鏡獲取乳清粉顆粒放大1 000倍后的微觀圖像。
1.3 試驗(yàn)設(shè)計(jì)
首先進(jìn)行單因素試驗(yàn),用DSC分別掃描玉米、豆粕、乳清粉和魚粉4種單一飼料原料在25~120 ℃范圍內(nèi)的熱譜曲線,并計(jì)算其在不同溫度下的比熱值。
其次,以文獻(xiàn)記載的仔豬料配方中4種主要原料玉米(22.6%~70%)、豆粕(13.7%~46%)、乳清粉(0~30%)和魚粉(0~10%)的質(zhì)量分?jǐn)?shù)范圍為因素水平的上下限,采用Design-Expert 軟件進(jìn)行D-最優(yōu)混料設(shè)計(jì),得到33個(gè)配比組合,編號(hào)為No.1~No.33(見表2)。其中,No.11和No.15、No.16和No.20、No.18和No.25、No.21和No.23、No.27和No.31是完全相同的配比組合,這是Design-Expert軟件為了考慮來自重復(fù)的純誤差、測(cè)試試驗(yàn)的重復(fù)性而在重要的頂點(diǎn)、邊緣等位置增加的5個(gè)重復(fù)設(shè)計(jì)。Design-Expert軟件的評(píng)估工具(Evaluation)顯示該試驗(yàn)設(shè)計(jì)的FDS為0.97,遠(yuǎn)大于0.8,說明試驗(yàn)設(shè)計(jì)足夠好。按表2中所顯示的4種原料的質(zhì)量分?jǐn)?shù)分別進(jìn)行配料混合,得到33種仔豬配合飼料,其中,20種配合飼料中乳清粉的質(zhì)量分?jǐn)?shù)≥14.548%,6種配合飼料中乳清粉的質(zhì)量分?jǐn)?shù)在6.25%~9.487%之間,其余7種配合飼料中(幾乎)不含乳清粉。用DSC分別掃描33種配合飼料隨溫度變化(25~110 ℃)的熱譜曲線,并計(jì)算其在不同溫度下的比熱值,分析乳清粉的含量對(duì)配合飼料比熱及DSC熱特性的影響。
表2 配合飼料的D-最優(yōu)混料設(shè)計(jì)Table 2 D-optimal mixture design for formula feeds %
1.4 數(shù)據(jù)處理
使用SPSS 22.0軟件對(duì)試驗(yàn)結(jié)果進(jìn)行主效應(yīng)方差分析,用OriginPro 9.1軟件作圖。比熱與溫度的最佳關(guān)系采用MATLAB R2014a 軟件的線性和非線性逐步回歸來確定,用決定系數(shù)(R2)、均方根誤差(RMSE, root mean square error)和平均相對(duì)百分誤差(e, mean relative percent error)來評(píng)估模型,選R2最高,RMSE 和e最小的為最佳模型。
2.1 單一飼料原料的比熱分析
玉米、豆粕、乳清粉和魚粉4種飼料原料的比熱隨溫度的變化如圖1所示。整體看來,4種原料的比熱均隨溫度的升高而呈逐漸增大的趨勢(shì),且在25~120 ℃范圍內(nèi),玉米、豆粕、乳清粉和魚粉的比熱分別在1.614~2.705、1.844~2.529、1.355~2.911和1.592~2.464 kJ/(kg·K)的范圍內(nèi)變化。溫度平均每升高1 ℃,4種原料的比熱值分別升高0.011、0.007、0.016和0.009 kJ/(kg·K)??梢姡榍宸圩鳛橐环N熱敏性的飼料原料,其比熱隨溫度升高的速率最快,而豆粕作為最常用的植物蛋白源飼料,其比熱隨溫度升高的速率最慢,表明其熱穩(wěn)定性較好。
表3顯示了4種飼料原料的比熱關(guān)于溫度的回歸預(yù)測(cè)模型,模型的R2均在0.988以上,P<0.000 1,且RMSE≤0.042 kJ/(kg·K),e≤1.927%,說明模型預(yù)測(cè)準(zhǔn)確度極高,預(yù)測(cè)穩(wěn)定性好。預(yù)測(cè)模型顯示:在25~120 ℃范圍內(nèi),玉米的比熱與溫度之間呈線性正相關(guān)關(guān)系,而豆粕的比熱與溫度之間的關(guān)系可以很好地用對(duì)數(shù)來描述,魚粉的比熱與溫度的關(guān)系則可以用二次多項(xiàng)式來表示;乳清粉在25~110 ℃范圍內(nèi)的比熱與溫度遵循三次多項(xiàng)式的關(guān)系。
圖1 4種飼料原料比熱隨溫度的變化曲線Fig.1 Specific heat curve of four feed ingredients with temperature
許多研究已報(bào)道了農(nóng)業(yè)物料的比熱與溫度之間的線性正相關(guān)關(guān)系。Kaletun?[14]的研究表明玉米、小麥、大米3種谷物面粉的比熱與溫度(20~110 ℃)之間存在線性關(guān)系。Jian等[27]的研究表明,0 ℃以上時(shí),高油含量的加拿大雙低油菜籽比熱隨溫度的升高而線性增大。還有研究報(bào)道了麻風(fēng)樹種仁[12]、阿月渾子[28]的比熱與一定范圍內(nèi)溫度的線性關(guān)系。這些均與本文中玉米比熱和溫度關(guān)系的研究結(jié)果相一致。
表3 4種飼料原料比熱的回歸模型及統(tǒng)計(jì)信息Table 3 Regression equations and statistical information of specific heat of four feed ingredients
此外,還有少數(shù)研究了農(nóng)業(yè)物料比熱關(guān)于溫度的非線性關(guān)系。Shrestha等[15]建立了王不留行籽比熱在25~80 ℃范圍內(nèi)溫度的二次模型。Yu 等[16]報(bào)道了貯藏的加拿大雙低油菜籽的比熱與溫度(40~90 ℃)的二次關(guān)系。此外,還有研究報(bào)道了琉璃苣種子比熱與溫度(5~80 ℃)的二次多項(xiàng)式關(guān)系[29-30]。這些均與本文中魚粉比熱和溫度關(guān)系的研究結(jié)果相一致??椎ささ萚11]報(bào)道了不同含水率的仔豬配合粉料比熱與溫度的對(duì)數(shù)關(guān)系,這與本文中豆粕的研究結(jié)果相一致。
由此可見,乳清粉作為一種熱敏性的物料,其比熱隨溫度的變化規(guī)律明顯區(qū)別于其他農(nóng)業(yè)物料。乳清粉的比熱在25~80 ℃范圍內(nèi)升高較為緩慢,80~110 ℃范圍內(nèi)升高較為迅速,且在109.79 ℃時(shí)出現(xiàn)比熱的峰值,這與王紅英等[17]報(bào)道的有關(guān)乳清粉比熱的研究結(jié)果存在一定差異。后者顯示,乳清粉的比熱在58.8 ℃時(shí)出現(xiàn)峰值,且65~110 ℃溫度段比熱升高速率較25~55 ℃段的低。這可能是由于兩次研究中所采用的乳清粉的組分差異較大導(dǎo)致。本試驗(yàn)所采用的乳清粉為高蛋白乳清粉,粗蛋白質(zhì)含量高達(dá)11.24%,且含水率較低(3.84%),后者所采用的為中蛋白乳清粉(粗蛋白質(zhì)含量僅為3.3%),且含水率較高(6.2%)。
2.2 配合飼料的比熱分析
2.2.1 溫度對(duì)配合飼料比熱的影響
33種配合飼料中,14種典型配合飼料的比熱隨溫度的變化如圖2所示。整體看來,在25~110℃的范圍內(nèi),所有的配合飼料比熱均呈現(xiàn)隨溫度的升高而逐漸增大的趨勢(shì),這與大多數(shù)單一農(nóng)業(yè)物料比熱的性質(zhì)相似。高含量乳清粉的20種配合飼料(乳清粉質(zhì)量分?jǐn)?shù)≥14.548%,包括No.22、No.25、No.5、No.16等)會(huì)在某一溫度段內(nèi)(75~90 ℃)出現(xiàn)比熱的峰值,比熱呈現(xiàn)先上升后下降的趨勢(shì),并且90℃后比熱的上升速率要明顯高于75℃前的上升速率,這一現(xiàn)象與乳清粉比熱隨溫度的變化規(guī)律相似。乳清粉質(zhì)量分?jǐn)?shù)在6.250%~9.487%之間的6種配合飼料(No.11、No.4、No.28等)比熱雖不會(huì)出現(xiàn)上述的峰值,但也會(huì)出現(xiàn)增長速率的轉(zhuǎn)折點(diǎn),表現(xiàn)為低溫(25~90 ℃)下的比熱增長速率明顯較高溫(95~110 ℃)下的低。而(幾乎)不含乳清粉的7種配合飼料(包括No.1、No.17、No.21等)的比熱則不會(huì)表現(xiàn)出上述兩種現(xiàn)象。以上表明:配合飼料中含有少量乳清粉時(shí),乳清粉的自有比熱特性便可以在配合飼料中得到表達(dá),說明乳清粉的含量對(duì)配合飼料的比熱特性影響很大。
表4為33種配合飼料中部分有代表性的配合飼料的比熱回歸模型及統(tǒng)計(jì)信息。從表中可以看出,No.1、 No.2、No.17與No.21等(幾乎)不含乳清粉的配合飼料的比熱與溫度呈對(duì)數(shù)關(guān)系,而No.4、No.7、No.11、No.16、No.19、No.25、No.29、No.31等含有一定量乳清粉(質(zhì)量分?jǐn)?shù)≥6.25%)的配合飼料比熱可以用溫度的三次多項(xiàng)式來表示。以上回歸模型的R2≥0.994,P<0.000 1,RMSE≤0.025 kJ/(kg·K),e≤0.693%,表明模型預(yù)測(cè)精度較高。
圖2 配合飼料比熱隨溫度的變化曲線Fig.2 Specific heat curve of formula feeds with temperature
分別采用No.15、No.18和No.23配合飼料比熱的實(shí)測(cè)數(shù)據(jù)對(duì)No.11、No.25和No.21配合飼料的比熱預(yù)測(cè)模型進(jìn)行驗(yàn)證,結(jié)果如圖3所示,可以看出,預(yù)測(cè)值與實(shí)測(cè)值的關(guān)系可以很好地用一階方程來表示,且斜率接近1,截距接近0,表明預(yù)測(cè)模型可以對(duì)配合飼料比熱與溫度的關(guān)系作出較為真實(shí)的表達(dá)。從預(yù)測(cè)模型可以看出,(幾乎)不含乳清粉的配合飼料表現(xiàn)出與豆粕相似的比熱特性(比熱與溫度呈對(duì)數(shù)關(guān)系),而當(dāng)配合飼料中含有≥6.25%的乳清粉時(shí),其便會(huì)表現(xiàn)出與乳清粉相似的比熱特性(比熱與溫度呈三次多項(xiàng)式關(guān)系)。
2.2.2 原料配比對(duì)配合飼料比熱的影響
從圖2可以看出,同一溫度下,原料配比不同的配合飼料的比熱之間存在一定程度的差異??梢?,配合飼料的比熱除了受溫度的影響外,還受4種原料配比的影響。30~110 ℃下,配合飼料的比熱與4種原料質(zhì)量分?jǐn)?shù)的相關(guān)系數(shù)如表5所示??梢钥闯?,配合飼料的比熱與豆粕、魚粉的含量無顯著相關(guān)性,除110 ℃外,其與玉米的含量也無顯著相關(guān)性;而除70和90 ℃外,其余溫度下配合飼料的比熱均與乳清粉的含量顯著相關(guān),110 ℃下的相關(guān)系數(shù)更高達(dá)0.846。
表4 配合飼料比熱的回歸模型及統(tǒng)計(jì)信息Table 4 Regression equations and statistical information of specific heat of formula feeds
圖3 配合飼料比熱預(yù)測(cè)值與實(shí)測(cè)值對(duì)比圖Fig.3 Predicted specific heat versus experimental specific heat value of formula feeds
對(duì)110 ℃下配合飼料的比熱與原料質(zhì)量分?jǐn)?shù)的關(guān)系作線性逐步回歸,結(jié)果如下式所示:
式中Cp110表示配合飼料110℃時(shí)的比熱,kJ/(kg·K);X表示乳清粉的質(zhì)量分?jǐn)?shù),%。玉米、豆粕和魚粉因?qū)ε浜巷暳系谋葻嵊绊懖伙@著而被排除??梢?,溫度一定時(shí),配合飼料的比熱主要受乳清粉含量的影響。
表5 配合飼料比熱與4種原料質(zhì)量分?jǐn)?shù)的相關(guān)性分析Table 5 Correlation analysis between specific heat of formula feeds at different temperatures and mass fractions of four ingredients
2.2.3 配合飼料比熱關(guān)于溫度、原料配比的兩因素方差分析
采用SPSS軟件對(duì)影響仔豬配合飼料比熱的溫度、原料配比兩因素進(jìn)行方差分析,結(jié)果如表6所示??梢钥闯觯簻囟?、原料配比以及二者的交互作用均對(duì)配合飼料的比熱產(chǎn)生了極顯著影響(P<0.001);其中,溫度的影響最為顯著,原料配比次之。
2.3 乳清粉及高含量乳清粉的配合飼料的DSC熱特性分析
乳清粉在25~120 ℃升溫過程中的DSC熱焓曲線圖見圖4,其在90.23~115.25 ℃范圍內(nèi),出現(xiàn)了4.14 J/g(HΔ,峰面積)的吸熱峰,峰值溫度為109.79 ℃(見表7)。與此吸熱峰相對(duì)應(yīng)的是乳清粉比熱峰的出現(xiàn)(見圖1),比熱由起始時(shí)的1.745 kJ/(kg·K)上升到峰值2.798 kJ/(kg·K),又下降到低谷2.662 kJ/(kg·K)。而玉米、豆粕和魚粉3種飼料原料的DSC熱焓曲線在25~120 ℃范圍內(nèi)并未出現(xiàn)任何吸熱峰或放熱峰,這也表明乳清粉的熱穩(wěn)定性較上述三者差。
表6 溫度、原料配比對(duì)配合飼料比熱的主效應(yīng)方差分析Table 6 Analysis of variance for effect of temperature and ingredient proportion on specific heat of formula feeds
圖4 乳清粉和5種典型配合飼料的DSC熱焓曲線圖Fig.4 DSC thermograms of whey powder and five typical formula feeds
表7 乳清粉及高含量乳清粉的配合飼料DSC熱特性分析Table 7 DSC thermal properties of whey powder and formula feeds containing high content of whey powder
噴霧干燥生產(chǎn)的乳清粉中乳糖為亞穩(wěn)定的非晶狀態(tài)。Thomas等[31]的研究結(jié)果顯示:含10%乳清蛋白(β-乳球蛋白)和90%乳糖的乳清粉在180 ℃有個(gè)小的放熱峰,而純?nèi)樘窃?75 ℃時(shí)出現(xiàn),均為非晶態(tài)乳糖在DSC升溫過程中重結(jié)晶導(dǎo)致。可見,本文中乳清粉的吸熱峰與乳糖結(jié)晶放熱過程無關(guān),推測(cè)為乳清蛋白在升溫過程中受熱變性導(dǎo)致。乳清蛋白在加熱過程中的行為伴隨著熱特性的變化[32]。
Noisuwan等[33]的報(bào)道顯示10%(質(zhì)量分?jǐn)?shù))的乳清分離蛋白溶液的變性溫度為75.51 ℃。Khem等[18]的研究顯示:10%(質(zhì)量分?jǐn)?shù))的乳清分離蛋白溶液在pH值為7時(shí)的變性溫度為74.6 ℃,pH值為4時(shí)的變性溫度為86.0 ℃;乳清蛋白的變性程度隨著熱處理溫度、時(shí)間的增加而增大,75 ℃處理1 min,變性度為62.8%,而78 ℃處理10 min的變性度為89.6%。Duongthingoc等[19]的研究發(fā)現(xiàn)乳清蛋白溶液的濃度增大會(huì)導(dǎo)致變性溫度降低,而pH值降低則會(huì)導(dǎo)致變性溫度升高。Dissanayake等[20]用DSC測(cè)定了不同含量(10%、17.5%和25%,質(zhì)量分?jǐn)?shù))、pH值(4、5和6)的乳清蛋白溶液的熱特性,結(jié)果顯示:乳清蛋白的變性溫度在78.7~85.1 ℃范圍內(nèi)變化,起始點(diǎn)、終止點(diǎn)分別在61.7~79.7 ℃、80.0~91.7 ℃的范圍內(nèi)變化;變性溫度與溶液的pH值有關(guān),變性過程的起始點(diǎn)、終止點(diǎn)和熱焓HΔ均受濃度的影響;10%的乳清蛋白溶液在pH值4~6條件下的熱焓HΔ在3.9~4.6 J/g的范圍內(nèi)變化,與本文乳清粉(乳清蛋白質(zhì)量分?jǐn)?shù)為11.24%)的熱焓HΔ(4.14 J/g)較為接近。此外,Dissanayake等[21]的另一研究結(jié)果也證實(shí)了pH值對(duì)乳清蛋白變性的影響。
從上述研究報(bào)道中可以看出,溶液中乳清蛋白的變性受加熱條件、pH值和乳清蛋白質(zhì)量分?jǐn)?shù)的影響,其變性溫度主要在68~86 ℃的范圍內(nèi),起始點(diǎn)、終止點(diǎn)分別在61~80 ℃和80~95 ℃的范圍內(nèi)。相較乳清蛋白溶液而言,本文乳清粉中乳清蛋白的變性溫度(109.79 ℃)、起始點(diǎn)(90.23 ℃)和終止點(diǎn)(115.25 ℃)都要高得多。有研究顯示在低水分條件下,蛋白質(zhì)對(duì)熱變性更有抵抗力[34],這是由于含水率的降低會(huì)減少蛋白質(zhì)分子移動(dòng)的自由,從而阻止構(gòu)象的變化和變性[35]。而乳清粉為固態(tài)粉末,含水率較低,故其蛋白質(zhì)的變性溫度理應(yīng)較高。
圖4還呈現(xiàn)了No.12、No.15、No.22、No.26和No.7 5種典型配合飼料的DSC熱焓曲線,可以看出,不含乳清粉的No.12配合飼料與僅含6.25%乳清粉的No.15配合飼料的DSC曲線較為平滑,并未發(fā)現(xiàn)吸熱峰,而其余3種乳清粉含量較高的配合飼料則會(huì)在某一溫度段內(nèi)出現(xiàn)吸熱峰,且乳清粉含量越高,吸熱峰越明顯。
33種配合飼料中,20種高含量乳清粉的配合飼料(乳清粉質(zhì)量分?jǐn)?shù)≥14.548%,包括No.7、No.10、No.18、No.22、No.26等)均在DSC熱焓曲線上表現(xiàn)出了與乳清粉相似的吸熱峰,其起始點(diǎn)、峰值點(diǎn)、終止點(diǎn)溫度和HΔ均值分別為71.91 ℃、83.42 ℃、87.93 ℃和0.67 J/g,均較乳清粉的低(見表7)??梢姡谂浜巷暳现?,乳清粉在較低的溫度下便會(huì)發(fā)生蛋白質(zhì)的變性,說明與玉米、豆粕和魚粉的混合會(huì)導(dǎo)致乳清粉熱穩(wěn)定性降低。
配合飼料的熱特性參數(shù)與4種組分含量的相關(guān)性分析如表8所示。其熱變性的起始點(diǎn)、峰值點(diǎn)、終止點(diǎn)和HΔ均與乳清粉的含量呈顯著正相關(guān)關(guān)系(相關(guān)系數(shù)r分別為0.697,0.905,0.903,0.946,P<0.001),尤其以HΔ與乳清粉含量的相關(guān)性最為顯著。HΔ越小,表明變性需要的能量越少。可見,通過測(cè)定配合飼料HΔ的大小可以判定乳清粉含量的多少。
此外,配合飼料熱變性的峰值點(diǎn)、終止點(diǎn)和HΔ還與玉米的含量顯著相關(guān)(r分別為-0.568,-0.558,-0.495,P<0.05),而與豆粕、魚粉的含量無明顯的相關(guān)性。這可能是由于玉米的含水率較高,直接影響了乳清蛋白的變性溫度,而玉米在混合料中的配比間接影響了乳清粉的含量進(jìn)而影響了HΔ的大小。
2.4 乳清粉掃描電鏡圖像分析
乳清粉在經(jīng)不同溫度熱處理后的微觀結(jié)構(gòu)圖(放大1 000倍)如圖5所示??梢钥闯?,經(jīng)60 ℃和90 ℃加熱處理后的乳清粉顆粒與室溫20 ℃下的乳清粉顆粒一樣,大多數(shù)呈規(guī)則的球形,且表面比較粗糙,存在很多凸起和微孔,當(dāng)溫度升至110 ℃(接近乳清粉的變性溫度)時(shí),乳清粉顆粒的表面逐漸熔化,呈現(xiàn)較為光滑的狀態(tài),顆粒之間相互粘結(jié),到115 ℃時(shí),顆粒形狀已變得極為不規(guī)則。熱處理過程中發(fā)現(xiàn),當(dāng)溫度為90 ℃時(shí),乳清粉便會(huì)產(chǎn)生輕微的結(jié)塊現(xiàn)象;而當(dāng)溫度升至110~115 ℃時(shí),乳清粉則出現(xiàn)了嚴(yán)重的結(jié)團(tuán)現(xiàn)象,且顏色由原來的乳白變成了微黃,這可能是由于氨基酸與糖類發(fā)生了美拉德反應(yīng)導(dǎo)致的非酶褐變。這一反應(yīng)會(huì)降低蛋白質(zhì)的質(zhì)量、產(chǎn)生一些潛在的有害物質(zhì)丙烯酰胺、糖基化最終產(chǎn)物等[36],導(dǎo)致乳清粉的營養(yǎng)價(jià)值降低??梢姡榍宸墼诩訜徇^程中會(huì)發(fā)生蛋白質(zhì)的變性,并伴隨著顆粒表面結(jié)構(gòu)的變化,導(dǎo)致粘性增加。
在配合飼料的調(diào)質(zhì)、制粒兩個(gè)熱加工工段中,乳清粉的熱變性在一定程度上可以改善顆粒飼料的成型、降低含粉率與粉化率。但當(dāng)配方中乳清粉含量高于15%時(shí),制粒機(jī)就會(huì)很容易發(fā)生堵機(jī)的現(xiàn)象[8],降低顆粒飼料的生產(chǎn)效率。這與本文DSC的研究結(jié)果也是相呼應(yīng)的,只有當(dāng)配合飼料中乳清粉的質(zhì)量分?jǐn)?shù)≥14.548%時(shí),DSC熱焓曲線上才會(huì)產(chǎn)生明顯的熱變性吸熱峰。
綜上,在高含量乳清粉(或脫脂奶粉、代乳粉等乳制品)的仔豬配合顆粒飼料的加工過程中,為降低乳清蛋白的變性程度、減少環(huán)模制粒機(jī)的堵機(jī)現(xiàn)象,應(yīng)將調(diào)質(zhì)溫度降低至70 ℃以下為宜。
圖5 經(jīng)不同溫度熱處理后的乳清粉掃描電鏡圖像Fig.5 Scanning electron microscope of whey powders after different temperature thermal treatments. (Preheated at 20, 60, 90, 110, and 115 ℃ for 5 min and cooled to room temperature before scanning in SEM.)
1)玉米、豆粕、乳清粉和魚粉的比熱隨溫度的升高(25~120 ℃)而分別在1.614~2.705、1.844~2.529、1.355~2.911和1.592~2.464 kJ/(kg·K)的范圍內(nèi)增大。玉米、豆粕和魚粉的比熱分別與溫度(25~120 ℃)呈線性、對(duì)數(shù)和二次關(guān)系,而乳清粉的比熱與溫度(25~110 ℃)遵循三次多項(xiàng)式的關(guān)系。
2)在25~110 ℃的范圍內(nèi),33種仔豬配合飼料(由不同配比的玉米、豆粕、乳清粉和魚粉混合而成)的比熱均呈現(xiàn)隨溫度的升高而逐漸增大的趨勢(shì);(幾乎)不含乳清粉的配合飼料的比熱與溫度呈對(duì)數(shù)關(guān)系,而含有一定量乳清粉(質(zhì)量分?jǐn)?shù)≥6.25%)的配合飼料比熱與溫度遵循三次多項(xiàng)式關(guān)系??梢?,當(dāng)配合飼料中含有少量乳清粉時(shí),乳清粉自有的比熱特性便可以在配合飼料中得到表達(dá)。
3)配合飼料的比熱顯著受原料配比、溫度以及二者的交互作用的影響(P<0.001);其中,溫度的影響最為顯著,原料配比次之;而4種原料中,乳清粉的含量對(duì)配合飼料比熱的影響最為顯著。
4)差示掃描量熱儀(differential scanning calorimetry,DSC)熱焓曲線上,乳清粉在90.23~115.25 ℃范圍內(nèi),出現(xiàn)了4.14 J/g的吸熱峰,峰值溫度為109.79 ℃,為乳清蛋白的熱變性導(dǎo)致。與此過程相對(duì)應(yīng)的是,隨著溫度的升高,乳清粉顆粒由存在很多凸起、微孔的粗糙表面結(jié)構(gòu)逐漸過渡為光滑、粘結(jié)的狀態(tài),形狀也由規(guī)則的球形變得極為不規(guī)則。
5)與乳清粉的熱焓曲線相似,高含量乳清粉(含量≥14.548%)的配合飼料也會(huì)在某一溫度段內(nèi)出現(xiàn)吸熱峰,但其起始溫度(67.46~74.99 ℃)、峰值溫度(77.95~87.69 ℃)和終止溫度(81.47~91.72 ℃)均較乳清粉的低??梢姡c玉米、豆粕和魚粉的混合會(huì)導(dǎo)致乳清粉熱穩(wěn)定性的降低,促進(jìn)其乳清蛋白的變性。在高含量乳清粉的仔豬配合顆粒飼料的加工過程中,為降低乳清蛋白的變性程度、減少環(huán)模制粒機(jī)的堵機(jī)現(xiàn)象,應(yīng)將調(diào)質(zhì)溫度降低至70 ℃以下為宜。
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Thermal properties and conditioning temperature control of formula feeds containing high content of whey powder for weanling pigs
Kong Dandan, Fang Peng, Wang Hongying※, Chen Xiao, Yue Yan, Lü Fang, Jin Nan
(National R&D Center for Agro-processing Equipment, College of Engineering, China Agricultural University, Beijing 100083, China)
Whey powder is a kind of thermosensitive feedstuff, which is particularly prone to pyrogenation and protein denaturation in conditioning and pelleting processing of pellet feed for weanling pigs. Pellet mill can be blocked easily when the feed formulation is designed with high level of whey powder. Studies on thermal proprieties of whey powder and formula feed containing different levels of whey powder are required for parameter optimization in thermo processing of formula feed. As corn meal, soybean meal, whey powder and fish meal are the most common and important ingredients in the diet of weanling pigs, 33 kinds of formula feeds consisting of different levels of corn meal (22.6%-70%), soybean meal (13.7%-46%), whey powder (0-30%) and fish meal (0-10%) were obtained by D-optimal mixture design method in this work. Proximate composition of 4 ingredients including moisture, crude protein, crude fat, crude ash, crude fiber, starch, lactose contents was determined. The specific heat of these 4 ingredients at the temperature range of 25-120 ℃ and 33 kinds of formula feeds at 25-110 ℃ were measured by DSC (differential scanning calorimetry) at a programmed heating rate of 10℃/min, and the prediction models of specific heat as a function of temperature were established. Thermal denaturation of whey powder and formula feeds containing high levels of whey powder was also analyzed by DSC. The moisture contents of corn meal, soybean meal, whey powder and fish meal were 12.01%, 10.96%, 3.84% and 7.28% (wet basis) respectively, and the crude protein contents were 8.08%, 47.26%, 11.24% and 65.73% (wet basis) respectively. The specific heat of corn meal ranged from 1.614 to 2.705 kJ/(kg·K), soybean meal from 1.844 to 2.529 kJ/(kg·K), whey powder from 1.355 to 2.911 kJ/(kg·K), and fish meal from 1.592 to 2.464 kJ/(kg·K). Whey powder showed significantly lower values of specific heat at 25-100 ℃ compared to the other 3 ingredients, but significantly higher value at 120 ℃ (P<0.05). The specific heat of corn meal increased linearly with the increase in temperature, and that of soybean meal increased logarithmically with the increase in temperature. The specific heat of whey powder followed a cubic polynomial relationship with temperature, and fish meal displayed a quadratic polynomial relationship with temperature. The specific heat of formula feeds containing no less than 6.25% whey powder followed a cubic polynomial relationship with temperature, which was the same as that of whey powder. However, the specific heat of formula feeds containing no whey powder displayed a logarithmical relationship with temperature, similar with that of soybean meal. Analysis of variance showed that the specific heat of formula feeds was significantly dependent on temperature, ingredient proportion and the interaction of the 2 factors (P<0.001). Temperature had the most significant effect on specific heat, followed by the level of whey powder. An endothermic peak with an enthalpy of 4.14 J/g was observed on the DSC thermogram of whey powder, which may be caused by the denaturation of whey protein. The onset temperature, peak temperature, and termination temperature were 90.23, 109.79, and 115.25 ℃, respectively. During this thermal denaturation process, the specific heat of whey powder raised from 1.745 kJ/(kg·K) at onset temperature to 2.798 kJ/(kg·K) at peak temperature, and then declined to 2.662 kJ/(kg·K) at termination temperature. Whey powder particles exhibited a spherical shape with numerous bulges and micro pores on the surface after the thermal treatments of 20-90 ℃, but an irregular shape with smooth surface and agglutinate status after the thermal treatments of 110-115 ℃ based on micrographs obtained from the scanning electron microscope. Similarly, 20 kinds of formula feeds containing high levels (≥14.548%) of whey powder also displayed endothermic peaks on the DSC thermograms, but the onset temperatures (67.46-74.99 ℃), peak temperatures (77.95-87.69℃), termination temperatures (81.47-91.72℃) and enthalpy (0.38-1.00 J/g) were lower than that of whey powder. Mixture with corn meal, soybean meal and fish meal could decrease the thermal stability of whey powder and facilitate the denaturation of whey protein obviously. Consequently, in order to reduce the denaturation of whey protein and the blocking of pellet mill, the conditioning temperature of formula feed containing high level of whey powder for weanling pigs should be lower than 70 ℃. In addition, the onset temperature, peak temperature, termination temperature and enthalpy of formula feed were significantly positively correlated with whey powder content (r=0.697, 0.905, 0.903, and 0.946, respectively, P<0.001). This investigation provides fundamental theory and data for process optimizations of thermo processing such as conditioning, and pelleting of formula feed containing high level of whey powder for weanling pigs.
specific heat; temperature; physical properties; whey powder; formula feed; thermal denaturation; DSC(differential scanning calorimetry)
10.11975/j.issn.1002-6819.2017.16.039
S816.8
A
1002-6819(2017)-16-0299-09
孔丹丹,方 鵬,王紅英,陳 嘯,岳 巖,呂 芳,金 楠. 高含量乳清粉的仔豬配合飼料熱特性及調(diào)質(zhì)溫度控制[J].農(nóng)業(yè)工程學(xué)報(bào),2017,33(16):299-307.
10.11975/j.issn.1002-6819.2017.16.039 http://www.tcsae.org
Kong Dandan, Fang Peng, Wang Hongying, Chen Xiao, Yue Yan, Lü Fang, Jin Nan. Thermal properties and conditioning temperature control of formula feeds containing high content of whey powder for weanling pigs[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2017, 33(16): 299-307. (in Chinese with English abstract)
doi:10.11975/j.issn.1002-6819.2017.16.039 http://www.tcsae.org
2017-03-30
2017-07-28
公益性行業(yè)(農(nóng)業(yè))科研專項(xiàng)(201203015);中央高校基本科研業(yè)務(wù)費(fèi)專項(xiàng)(2017GX001)
孔丹丹,女(漢族),云南曲靖人,博士生,主要從事飼料加工工藝研究。北京市海淀區(qū)清華東路17號(hào)中國農(nóng)業(yè)大學(xué)東校區(qū),100083。
Email:dandank@cau.edu.cn
※通信作者:王紅英,女(漢族),教授,博士生導(dǎo)師,主要從事飼料加工工藝技術(shù)與設(shè)備及畜禽養(yǎng)殖技術(shù)與裝備研究。北京市海淀區(qū)清華東路17號(hào)中國農(nóng)業(yè)大學(xué)東校區(qū),100083。Email:hongyingw@cau.edu.cn
中國農(nóng)業(yè)工程學(xué)會(huì)會(huì)員:王紅英(E041200500S)