2008年--外文翻譯--加熱溫度和脂肪含量對凝固型酸奶質構的影響

1、<p>　　6380漢字，4220單詞，21920英文字符</p><p>　　出處：Xu Z M, Emmanouelidou D G, Raphaelides S N, et al. Effects of heating temperature and fat content on the structure development of set yogurt[J]. Journal of Food

2、 Engineering, 2008, 85(4): 590-597.</p><p>　　本科畢業(yè)設計（論文）</p><p>　　外文參考文獻譯文及原文</p><p>　　學 院 輕工化工學院 </p><p>　　專 業(yè) 食品科學與工程 </p><p><b>　

3、　2010年6月</b></p><p><b>　　目錄</b></p><p><b>　　摘要1</b></p><p><b>　　1、導論1</b></p><p><b>　　2、材料與方法2</b></p>&

4、lt;p>　　2.1 簡單準備2</p><p><b>　　2.2 儀器3</b></p><p><b>　　3、結果3</b></p><p><b>　　4、討論6</b></p><p><b>　　5、結論7</b></p

5、><p><b>　　致謝7</b></p><p>　　Abstract8</p><p>　　1. Introduction8</p><p>　　2. Materials and methods9</p><p>　　2.1. Sample preparation10</p>

6、;<p>　　2.2. Instrumentation10</p><p>　　3. Results11</p><p>　　4. Discussion20</p><p>　　5. Conclusions21</p><p>　　Acknowledgement21</p><p>　　Refe

7、rences22</p><p>　　加熱溫度和脂肪含量對凝固型酸奶質構的影響</p><p><b>　　摘要</b></p><p>　　用不同脂肪含量（0％，1.5％或3.5％）的牛奶通過牛巴氏殺菌制成凝固型酸奶結構，是利用天然酸奶作為發(fā)酵劑通過新型U型管流變儀測量剛性模數（G’）的動態(tài)變化的手段而制成的。結果表明，無論是牛奶的脂肪含量

8、和加熱預處理都顯著影響凝膠率以及最終剛性的凝膠實現(xiàn)。也就是說，使用的加熱溫度越高和脂肪含量越高，凝膠網格形成速度越快凝膠似乎也越堅固。該凝膠形狀曲線顯示了從牛奶制成酸奶的過程，不論有無脂肪，在80，85，90或95℃加熱1分鐘和45℃時發(fā)酵，顯示三個獨特的部分表明酸奶凝膠過程發(fā)生在連續(xù)的三個階段。推薦機制的解釋討論了這個結果，這可以解釋上述的發(fā)現(xiàn)。</p><p>　　關鍵詞：酸奶凝膠;設置酸奶流變學;動態(tài)U型流

9、變管</p><p><b>　　1、導論</b></p><p>　　酸奶是由嗜熱乳酸菌發(fā)酵牛奶而制成的，發(fā)酵乳酸細菌共生（嗜熱鏈球菌和德氏乳桿菌過磷酸鈣牛奶，保加利亞乳酸菌）。在牛奶酸化過程中形成一種凝膠，由于結構不穩(wěn)定的酪蛋白的聚合，給予總體一個三維的網絡結構。在這個網格里除了酪蛋白聚合物，乳清蛋白參與，以及那些作為填充料的脂肪球，并且通過互動脂肪球膜與酪蛋白基

10、質脂肪球交聯(lián)。</p><p>　　凝膠的形成是酸奶產品最重要的功能屬性。這種復合凝膠的流變特性受牛奶成分，溫度和牛奶加熱預處理時間，牛奶接種的發(fā)酵菌的類型和質量，發(fā)酵溫度和最終產品的儲存條件所影響。研究這些參數如何影響酸奶的凝膠結構和其中一些論述了凝膠采用動態(tài)流變學方法的大量調查已經完成。大多數研究者采用一個同心圓柱幾何振蕩流變儀在原址準備酸奶樣本，同時通過測量一些參數，如儲存和損失模量等來監(jiān)測酸奶的凝膠過程。

11、酸奶質地非常脆弱，特別是在發(fā)酵過程中，如果凝膠基質遭受了一定的機械干擾，結構組織很容易破壞。此外，酸奶生產的一個主要問題是乳清分離。盡管事實上某些研究人員聲稱，它在pH5.0以下時幾乎不存在的，然而，如盧西等其他研究人員稱在一些條件下相當數量的乳清分離可以發(fā)生。這些條件包括高孵化溫度，如45℃；牛奶過度的加熱預處理，例如在大于80℃下加熱30分鐘；低酸的產品，當凝膠仍然脆弱時擾亂，例如，pH值4.9而不是4.6和總固形物含量低，<

12、/p><p>　　除了正被考慮的問題，在酸奶行業(yè)中，脫水收縮成為一個從質量和消費者可接受點的觀點缺陷，肯定導致對凝固型酸奶流變學特性研究的問題。原因是在流變儀的測量元件的外壁與樣本之間存在一個液體薄膜，可能會引起的延誤，這可能導致錯誤的結果或變化。因此，相當少量的樣品包含在凝膠過程的早期階段與振蕩流變儀的測量元件相和可能滑移的影響可能引發(fā)的片刻記錄現(xiàn)象丟失的風險。</p><p>　　考慮到上

13、述提到的，目前已開始著手研究酸奶凝膠過程使用特制的動態(tài)流變儀，這特別是為構建結構非常脆弱的脫水收縮的凝膠。據認為，在各種溫度對牛奶加熱預處理的影響以及脂肪含量凝固型酸奶中剛性的影響需要用商用牛奶作為原材料用系統(tǒng)的方法進行研究，加工條件類似于一般乳品行業(yè)的牛奶。因此，獲得的結果和得出的結論可能會朝著凝膠機制有助于澄清這一方向，這可能是極為重要的酸奶制造工業(yè)的發(fā)展，特別是為質量控制的目的。</p><p><b

14、>　　2、材料與方法</b></p><p>　　巴氏滅菌的牛奶和均質牛奶是從本地購買的乳制品。牛奶按照三種類型的脂肪含量經過標準化的，即全脂牛奶（3.5％脂肪），低脂牛奶（1.5％脂肪）及脫脂牛奶（0％脂肪）。每個類型的牛奶，會準備5個樣品在不同溫度加熱處理。這就是說，除了在工廠得到的經過巴氏殺菌的被稱為未加熱牛奶之外，第一個樣品是使用沒有進一步采用熱處理的牛奶。其余的樣本分別在80℃，85℃

15、，90℃或95℃的溫度下加熱1分鐘。對于酸奶制劑作為發(fā)酵培養(yǎng)物，用牛奶制備得到的天然酸奶，是在它加入其它添加物作樣品前測定其酸度的。</p><p><b>　　2.1 簡單準備</b></p><p>　　大量被包裝在四柏紙箱（1升容量）的牛奶是在不斷攪拌的情況下在預定溫度加熱1分鐘。等加熱完畢，樣本冷卻后，在流動的冷水中降溫至4℃。然后取400克樣品，用相當于樣品

16、重量3%的發(fā)酵菌接種。取部分的接種牛奶（約30毫升），放在U形管流變儀的計量元件里面。樣品的自由表面披上了一層薄薄的液體石蠟，以避免可能的蒸發(fā)以及樣品通過流變儀設置的恒溫控制的加熱部件在45℃下培養(yǎng)直至其pH值下降到4.0。把振蕩頻率設定為0.1赫茲。每5分鐘，測量的儲存模量G'，損耗模量G''，應力，正切角，孵化溫度和pH值，被采取并通過流變儀的微處理器控制單元自動記錄和顯示數據，并在一臺與流變儀連接的電腦中儲

17、存。PH的測量結果是這樣得到的，用合成的PH電極插入到大量以上描述的放在燒杯中的樣品中，燒杯放在一個非常小的與流變儀的測量部件保持相同溫度的定制的孵化室。孵化溫度和pH值計量單元都受控于流變儀的微處理器。所有樣本重復3次，取得了明顯優(yōu)于2％的重現(xiàn)性。</p><p><b>　　2.2 儀器</b></p><p>　　酸奶制作過程中結構的形成過程是利用特制的動態(tài)氣動

18、U型管流變儀進行監(jiān)測的，該流變儀新穎的設計，開發(fā)和建設，已在我們工程車間的其他地方有詳細說明。 這個流變儀是基于桑德斯和沃德的U型管技術，其中一凝膠的剛性模量可透過一個已知的空氣壓力測量的抽樣和由此產生的變形決定。我們的電腦U型管流變儀能夠確定凝膠體或其他材料，如在靜態(tài)和振蕩模式下的泡沫剛性。與從普通U型管的設計不同，這種裝載樣品的U型管是由兩個完全相同肢管組成的。連接著U型管的兩端的是由傳感器控制壓力的密閉空氣室。連接到空氣室一邊是一

19、個往復活塞產生驅動壓力，使樣本發(fā)生變形，并因此使U型管的另一端的空氣室壓力增量。通過測量這些壓力可以確定樣品的流變特性。這設計的一個重要的優(yōu)點就是由一個反壓作用產生的應力的適應性質。這使得測量和水一樣薄的液體保持最大的張力，包括線性粘彈性在內。也就是說，在整個實驗過程張力不是保持不變，而是根據不同材料的硬度，測試，展品的測量時刻。因此，凝膠的形成過程可以從一開始樣本仍然處于液體狀態(tài)的時候監(jiān)測到一個真正的凝膠點形成。做到這一點，是因為這儀

20、器提供了開關電磁閥的驅動壓力值自動選擇的選項，在凝膠形成的最后階段允許測</p><p><b>　　3、結果</b></p><p>　　大多數研究人員使用脫脂奶粉來制作酸奶樣品，來嘗試克服在本土牛乳成分季節(jié)性變化的嚴重的問題。雖然，現(xiàn)在看來，這種做法似乎解決了問題，然而，由作者的初步實驗表示，用奶粉制作的酸奶，至少從流變行為的角度來看，與用巴氏殺菌和均質的體態(tài)奶相

21、比是不同的。</p><p>　　為了在乳制品行業(yè)盡可能多的模擬制作酸奶的加工條件，我們決定以天然酸奶代替一些研究人員使用的葡萄糖酸-D-內酯作為酸化媒介物。此外，盧西和辛格報告說，在用葡萄糖酸-D-內酯酸化和細菌培養(yǎng)酸化之間牛奶有不同的酸化率，因為葡萄糖酸-D-內酯迅速水解為葡萄糖酸，而在加入發(fā)酵培養(yǎng)菌之后pH值通常會發(fā)生變化，在開始時，很緩慢。最終的pH值時獲得葡萄糖酸-D-內酯凝膠，這是牛奶的初始添加物的一

22、個大集合，然而發(fā)酵細菌可以繼續(xù)產酸，直到達到一個非常低的PH值pH<4.0。此外，用葡萄糖酸-D-內酯制的凝膠的流變學特性和物理性質不同于來自于發(fā)酵的凝膠，尤其是高凝膠溫度的。</p><p>　　為了使牛奶成分避免季節(jié)性的差異，我們非常謹慎，在連續(xù)兩個月內完成所有試驗，這證明，足以確保我們所用的牛奶成分沒有發(fā)生明顯變化。由于我們想要測試的U型管流變儀的靈敏度范圍，以及評估一個非常簡短牛奶的加熱是否可能導致

23、形成的酸奶凝膠硬度的影響，我們決定在預定溫度預熱牛奶1分鐘，這被認為是在這項工作中預熱溫度之間所采用的使乳清蛋白發(fā)生重大變性的一個相當短的時間。在酸奶制造行業(yè)通常的做法是在95℃的溫度下預熱5分鐘，因此乳清蛋白變性是100％。其他研究人員總是把牛奶樣品在預定溫度加熱15~30分鐘。至于我們在選擇這項工作中使用的發(fā)酵溫度，例如45℃，這通常被認為是非常高的，并在此溫度下酸奶凝膠自發(fā)經過乳清沉淀，即脫水收縮。然而，這種孵化溫度為我們提供一個

24、很好的理由來測試流變儀的處理脫水凝膠的能力。最后，我們決定不要pH值一下降到4.6（酪蛋白的等電點）就停止測量剛性模量，這是在世界文獻的一般的做法，而進一步進行下降到4.0，以便能夠發(fā)現(xiàn)在凝膠結構中由于總重排或結構斷裂，可能發(fā)生的變化。</p><p>　　圖2列出了沒有經過加熱或發(fā)酵前在80℃，85℃，90℃或95℃溫度下加熱的（a）脫脂奶，（b）低脂奶（1.5％），和（c）全脂牛奶（3.5％）三種奶樣品的時間

25、與G’模量之間的關系?？梢钥闯觯Ｄ碳訜犷A處理對酸奶凝膠結構的形成起著至關重要的作用。雖然在預定溫度只是加熱1分鐘，但剛性模量（G’）增加，當牛奶在95℃下加熱時，顯示出無論脂肪含量是多少、幾倍與未加熱的牛奶有關（圖3）。其他研究人員報告了類似的評論。此外，無論脂肪含量是多少，加熱處理的所有樣品曲線形狀都有個有趣的特征。其曲線顯示在它的初期部分有個峰，而且隨加熱溫度升高變得更明顯。事實上證明曲線由三部分組成，這表明酸奶凝膠動力學比以前一

26、直認為的要復雜得多。</p><p>　　此外脂肪含量也在第一和第三階段的形狀大小發(fā)揮作用，如在每一階段都顯示出脂肪含量越高，G’模量值越大。另一個值得注意的發(fā)現(xiàn)是，凝膠的開始既要預熱處理又依賴脂肪含量，即不加熱的樣本和沒有脂肪或低脂肪含量的樣品比在較高溫度下預熱和全脂牛奶做的樣品開始凝膠需要更多的培養(yǎng)時間。雖然其他研究人員，如李和盧西，使用不同溫度加熱再造脫脂奶粉報告了相當類似的結果，他們的樣本達到開始凝膠的時

27、間比我們目前的工作報告超過至少有兩到三倍的時間。事實上，這是我們所有研究發(fā)表工作的一個規(guī)則，在酸誘導牛奶膠凝動力學，無論酸化是否是由葡萄糖酸-D-內酯或發(fā)酵培養(yǎng)菌的添加而誘導的。此外，我們的結果表明，脂肪含量與預熱處理在獲得最大剛性模量上發(fā)揮同樣重要的作用；因為即使在沒有加熱處理的樣品中脂肪含量為3.5％的樣品獲得G’模量最大值比脂肪含量為0％和1.5％的樣品獲G’模量最大值得達一倍以上。這是直接與Lucey et al. (1998)

28、的調查結果相反。它指出用添加脂肪的再造脫脂奶粉制作的脫脂和全脂酸奶實際上G’模量最大值沒有變化。此外，Lucey et al. (1998)報告說，未加熱的牛奶樣品中脫脂牛奶樣品的凝膠時</p><p>　　謹記上述，我們認為，如果研究人員無法觀察到我們在這項工作中敘述的凝膠過程中的特點，這是由于他們所使用的流變儀內在缺陷，在開始出現(xiàn)凝膠時發(fā)現(xiàn)片刻的現(xiàn)象時凝膠系統(tǒng)實際上是液體，疏漏有重要影響。此外，我們認為，其他

29、研究人員在形成凝膠時滯后大量時間的原因是酸奶網絡的非常脆弱的性質。也就是說，正如我們前面提到的，即使輕微的機械擾動應用于凝膠樣品可能導致牛奶系統(tǒng)中交聯(lián)過程的延遲?？紤]到在商業(yè)流變儀幾乎所有可用的樣品表面在頻繁振蕩張力都用在流變儀的測量單位，很容易明白為什么開始形成凝膠會延遲太多。然而對于U型管流變儀來說，其應用壓力的樣品表面大約只有1.54cm²而樣本體積為30mL。</p><p>　　值得指出的是，

30、該時期在凝膠開始前已過去，我們在實驗中的記錄是非常接近通常酸奶制造工業(yè)出現(xiàn)的。</p><p>　　圖4顯示培養(yǎng)時間的函數pH值在減少?？梢钥闯觯瑹o論脂肪含量是多少在較高的預熱溫度下pH值迅速下降。其他研究人員也報道了相似的結果。不加熱的樣品顯示出的最慢的pH值降低率。眾所周知牛奶的預熱，特別是高溫預熱，例如95℃，引起pH值非?？焖傧陆颠@由于由于乳糖分解和刺激發(fā)酵培養(yǎng)菌形成的甲酸等有機酸而引起的。</p&

31、gt;<p>　　損耗角正切在膠凝體系的提高，是由凝膠過渡到液體的跡象。圖5表明，沒有加熱的樣品損失正切顯示比那些在高溫加熱處理的樣品達到最大值需要更長的時間。此外，不加熱樣品從開始發(fā)酵到出現(xiàn)損失正切最大值的持續(xù)時間是與脂肪含量成反比的，即樣品脂肪含量越高出現(xiàn)損耗角正切最大值的時間越短。</p><p>　　表1顯示了在開始凝膠和得到最大損失正切值時的pH值?？梢钥闯鲇捎谠谳^高的溫度下進行預熱處理的

32、牛奶會在更高的pH值開始凝膠。另外，高脂肪含量和高溫的預熱導致凝膠組合出現(xiàn)在一個非常高的PH值，6.0以上。Ronnegard and Dejmek (1993)指出，脫脂牛奶樣品在高溫下預熱，如90~100℃和在44℃下培養(yǎng)，在pH 5.5時開始凝膠。Lee and Lucey (2004)則稱，改造脫脂奶粉制成的牛奶，在82.5℃加熱30分鐘和在45.7℃下培養(yǎng)，發(fā)現(xiàn)在pH為5.59時開始凝膠。Dalgleish, Alexande

33、r,and Corredig (2004)報道，采用光譜擴散方法，出現(xiàn)的證據表明，未加熱脫脂牛奶的蛋白質顆粒開始出現(xiàn)類似的聚集。pH為5.5時凝膠酸的形成，即表面上在pH 5.0時分子大小的蛋白質開始爆炸式增長。</p><p><b>　　4、討論</b></p><p>　　人們普遍認為，凝膠過程開始于酪蛋白與乳清蛋白的結合，特別是β-乳球蛋白，假如它們被加熱變性

34、到一定程度，在加熱到90℃和保持相當長的時間（1小時），乳清蛋白和酪蛋白膠束之間會發(fā)生兩個主要的反應。那就是，一個是通過K-酪蛋白結合，β-乳球蛋白與-酪蛋白膠束直接反應，另一個是，通過兩個乳清蛋白形成膠束的解決辦法，在＠-乳白蛋白和β-乳球蛋白的膠束反應。因此，變性乳清蛋白與酪蛋白膠束的交聯(lián)與連接導致蛋白顆粒數量和交聯(lián)強度的增加。我們的研究結果證實，由于加熱溫度的升高，凝膠開始的pH值是越來越高的，這幾乎是獨立的，無論系統(tǒng)中是否存在有

35、脂肪球?；谏鲜隹紤]，圖2的預熱樣品的凝膠曲線出現(xiàn)峰，這歸因于變形乳清蛋白與血清中酪蛋白膠束的結合。然而，和不加熱的牛奶樣品證明的例子一樣，初步形成網絡的剛性模量也受脂肪含量的影響。這意味著均質牛奶的脂肪球起作用是由于磷脂和蛋白質在脂肪球表面的存在可以聚集而發(fā)生相互作用，甚至在和乳清蛋白或酪蛋白膠束發(fā)生相互作用之前，形成交聯(lián)。我們使用高的凝膠溫度可能使這種現(xiàn)象加強了，因為主導吸引力被認為是疏水作用。預熱樣本凝膠曲線中的第二階段是，曲線升

36、高穩(wěn)定時出現(xiàn)在波峰獲得剛性模量</p><p>　　從凝膠曲線得到的另一個重要的觀察結果是，不論樣品預熱與否和是否含有脂肪球所有的凝膠網絡依然十分穩(wěn)定，即使pH值下降到低于4.0。因此，當它不受干擾時，酸奶凝膠的結構似乎沒有發(fā)生倒塌現(xiàn)象，而明顯的脫水收縮是在pH值低于4.5才觀察到。</p><p>　　最近酸奶凝膠的研究使用共焦激光掃描顯微鏡表明對于不加熱樣本直到pH值5.3時沒有明顯可

37、見的結構，盡管可觀察到酪蛋白以及乳清蛋白顆粒分散良好，而pH值范圍在5.0到5.3酪蛋白出現(xiàn)聚集，存在的空隙間充滿水相乳清蛋白。pH值在4.95以下，酪蛋白和不想要的乳清蛋白都參與在凝膠網絡中。至于加熱（90℃和95℃）的牛奶樣品，在凝膠點附近，可見小結構，但后來酪蛋白和乳清蛋白都可看到明顯的結構，如我們所希望的一樣。然而，要求很高的加熱樣品的凝膠結構中，令人失望的是，比在較低溫度下加熱的樣本的乳清蛋白少得多。</p>&

38、lt;p>　　從上面的討論，顯而易見，仍然有許多工作需要做，以便明確闡明酸奶凝膠機制。</p><p><b>　　5、結論</b></p><p>　　酸奶凝膠結構的形成是受牛奶的脂肪含量和預熱處理影響的即使加熱時間短至1分鐘。預熱脫脂牛奶或含有脂肪的牛奶的凝膠曲線都由三個不同部分組成，這表明凝膠機制相當復雜。即使酸奶系統(tǒng)的pH值下降到小于4.0，其凝膠網絡依

39、然保持穩(wěn)定。</p><p><b>　　致謝</b></p><p>　　作者感謝歐洲聯(lián)盟希臘教育部“阿基米德”研究項目的財政支持。</p><p>　　Effects of heating temperature and fat content on the structure development of set yogurt</p

40、><p>　　Z.-M. Xu, D.G. Emmanouelidou, S.N. Raphaelides *, K.D. Antoniou</p><p>　　Food Process Engineering Laboratory, Department of Food Technology, A.T.E.I. of Thessaloniki, P.O. Box 141, Thessalon

41、iki GR-5740, Greece Received 23 February 2007; received in revised form 13 August 2007; accepted 30 August 2007</p><p>　　Available online 7 September 2007</p><p><b>　　Abstract</b><

42、;/p><p>　　The structure formation of yogurt gels made from bovine pasteurized milk of various fat contents (0%, 1.5% or 3.5%) and prepared using natural yogurt as a starter was monitored by measuring the rigidi

43、ty modulus (G’) development by means of a dynamic U-tube rheometer of novel design. The results indicated that both the heating pretreatment of the milk and its fat content significantly affect the rate of gelation as we

44、ll as the final rigidity attained by the gel. That is, the higher the heating te</p><p>　　2007 Elsevier Ltd. All rights reserved.</p><p>　　Keywords: Yogurt gelation; Set yogurt rheology; Dynamic

45、 U-tube rheometry</p><p>　　1. Introduction</p><p>　　Yogurt is produced by fermenting milk using thermophilic homofermentative lactic acid bacteria that live symbiotically (Streptococcus thermoph

46、ilus and Lactobacillus delbrueckii ssp. bulgaricus) (Tamime & Robinson, 1989). During milk acidification a gel is formed due to destabilization of casein micelles which aggregate to give a threedimensional network st

47、ructure. In the network apart from the casein aggregates, whey proteins participate as well as fat globules which act as a filler and interact</p><p>　　Gel formation is the most important functional property

48、 of yogurt products. The rheological characteristics of this composite gel are governed by milk composition, temperature and time of milk heat pretreatment, type and quantity of starter culture employed to inoculate the

49、milk, fermentation temperature and storage conditions of the final product (Tamime & Robinson, 1989). Numerous investigations have been carried out to study how these parameters affect the yogurt gel structure and a

50、number of</p><p>　　Syneresis, apart from being considered, in yogurt industry, as a defect from the quality and the consumer acceptance point of view, certainly causes problems in the study of the rheologica

51、l characteristics of yogurt gels. The reason is the existence of a thin film of liquid formed between the walls of the measuring unit of the rheometer and the sample which might cause slippage and it could lead to wrong

52、results or even artifacts (Barnes, 1995;Haque et al., 2001; Mellema, Walstra, van Opheusden,</p><p>　　Bearing in mind the above mentioned, the present work was initiated to study the yogurt gelation process

53、using a custom made dynamic rheometer which was constructed especially for syneresing gels of extremely fragile structure. It was thought that the effect of milk heat pretreatment at various temperatures as well as the e

54、ffect of fat content on the rigidity modulus evolution of yogurt gel needed to be studied in a systematic way using commercially available milk as raw material and processing </p><p>　　2. Materials and metho

55、ds</p><p>　　Pasteurized and homogenized bovine milk was purchased from a local dairy. The milk was standardized to three types according to its fat content, i.e. full fat (3.5% fat), low fat (1.5% fat) and s

56、kim milk (0% fat). From each type of milk, five samples were prepared differing in the temperature of their heat treatment. That is, the first was used without further heat treatment apart from that received at the facto

57、ry during the pasteurization process, termed as unheated milk. The rest of the sample</p><p>　　2.1. Sample preparation</p><p>　　Quantity of milk packed in tetra-pak cartons (1L capacity) was hea

58、ted at the predetermined temperature under constant stirring for 1 min. After the heating time was elapsed, the sample was cooled, in running cold water, down to 45℃. Then 400 g of the sample was taken and inoculated wit

59、h the starter culture using quantity corresponding to 3% of the sample’s weight. Part of the inoculated milk (approx. 30 mL) was placed inside the measuring unit of the U-tube rheometer. The free surface of the sam</p

60、><p>　　2.2. Instrumentation</p><p>　　The process of structure formation during yogurt making was monitored using a custom made pneumatic dynamic U-tube rheometer of novel design, designed, develope

61、d and constructed, at our engineering workshop, which has been described in detail elsewhere (Xu & Raphaelides, 2005; Xu, Raphaelides, Karapantsios, Tellos, & Bounarelis, 2004). The rheometer (Fig. 1) was based o

62、n the U-tube technique of Saunders and Ward (1953), in which the rigidity modulus of a gel can be determined by applying a known</p><p>　　3. Results</p><p>　　The majority of the researchers used

63、 skim milk powder to prepare their yogurt samples, in an effort to overcome the serious problem of seasonal variations in the composition of native milk. Although it seems that this practice solves the problem, neverthel

64、ess, yogurt prepared from powder milk differs, at least from the rheological behaviour point of view, from yogurt prepared from pasteurized and homogenized liquid milk as preliminary experiments as indicated by the autho

65、rs.</p><p>　　In an effort to simulate the processing conditions of yogurt making in dairy industry as much as possible, we decided to use natural yogurt as an acidification medium instead of glucono-d-lacton

66、e (GDL) which several researchers employed. Besides, Lucey and Singh (1998) reported that the rate of acidification is different between milk acidified with GDL and bacterial cultures, since GDL is rapidly hydrolysed to

67、gluconic acid whereas after the addition of starter cultures the pH usually changes, a</p><p>　　To avoid seasonal variations in milk composition we took care to carry out all our experiments within two conse

68、cutive months, which proved to be sufficient to ensure that no appreciable changes were detected in the milk composition we used. Since we wanted to test the limits of the sensitivity of the U-tube rheometer as well as t

69、o assess whether a very brief heating of the milk could cause any effect to the firmness of the yogurt gel formed, we decided to preheat the milk to the predetermined te</p><p>　　Fig. 2 shows the development

70、 of G’ modulus in relation to time for samples of (a) skim milk, (b) low fat (1.5%) milk and (c) full fat (3.5%) milk which were either unheated or heated at 80, 85, 90 or 95℃ prior to incubation. It can be seen that mil

71、k preheating plays a role of paramount importance to the structure development of yogurt gel. Although the heating at the predetermined temperature was only 1 min, the rigidity modulus (G’) increased, regardless of the f

72、at content, several times in rel</p><p>　　Moreover the fat content also plays a role on the size of the first and the third steps, i.e. the higher the fat content the higher is the value of G’ exhibited at t

73、he end of each step (Fig. 2a–c). Another noteworthy observation is that the onset of gelation is both preheating treatment and fat content dependent, i.e. the less heated samples and those without fat or low fat content

74、needed more incubation time to elapse before the commencement of gelation than the samples preheated at higher temp</p><p>　　Bearing in mind the above mentioned, we consider that if researchers are unable to

75、 observe the characteristics in the gelation process, which we reported in this work, it is due to inherent inability of the rheometers they employed, to detect minute phenomena occurring at the onset of gelation when th

76、e gelling system is virtually liquid and slip effects play a major role. Moreover, we believe that the significant time lags in gel formation experienced by other researchers were due to the very fr</p><p>　

77、　It is noteworthy to point out that the time periods elapsed before the onset of gelation, which we recorded in our experiments, were very close to those normally occurred in yogurt making industry.</p><p>　

78、　Fig. 4 shows the reduction of pH as a function of the incubation time. It can be seen that regardless of fat content the pH dropped more rapidly as the preheating temperature was higher. Similar results were reported by

79、 other researchers (Ro¨nnega?rd & Dejmek, 1993) as well.The unheated samples exhibited the slowest reduction rate of pH. It is well known (Rasicˇ & Kurmann, 1978; Tamime & Robinson, 1989) that the prehea

80、ting of milk, especially at high temperatures, e.g. 95 _C, causes a fairly rap</p><p>　　The increase in loss tangent in a gelling system is an indication of the transition from gel to liquid (Ferry,1980). Fi

81、g. 5 shows that the loss tangent of the unheated samples showed a peak at a much longer time elapsed since the initiation of incubation than the heated samples of which those treated at higher temperatures exhibited a pe

82、ak loss tangent much earlier than the others. Moreover,the duration of the time elapsed from the initiation of incubation to the appearance of maximum loss tange</p><p>　　Table 1 shows the pH values at the o

83、nset of gelation and at the maximum loss tangent. As it can be seen gelation starts at a much higher pH value as the preheating of milk was carried out at a higher temperature. Moreover the combination of high fat conten

84、t and high temperature of preheating leads to the onset of gelation to occur at an unusually high pH value which is above 6.0. Ro¨nnega?rd and Dejmek (1993) reported that skim milk samples preheated at high temperat

85、ures, e.g. 90–100℃and incubat</p><p>　　4. Discussion</p><p>　　It is generally considered that the gelation process starts with the aggregation of whey proteins associated with caseins, especiall

86、y of b-lactoglobulin, provided they have been denatured to a certain degree by heating (Heertje,Visser, & Smits, 1985; Lucey et al., 1998, 1997). Corredig and Dalgleish (1999) reported that two main interactions take

87、 place between whey proteins and casein micelles after heating up to 90℃ and for fairly long time (1 h). That is,a direct interaction of b-lactoglobulin</p><p>　　Another important observation from the gelati

88、on curves(Fig. 2) is that regardless of whether the samples were preheated or not and whether they contained fat globules or not all gel networks remained perfectly stable even though the pH was dropped below 4.0. Thus,

89、collapse phenomena seem not to occur to the yogurt gel structure when it remains undisturbed, while appreciable syneresis was observed at pH values below 4.5.</p><p>　　Recent studies (Dubert-Ferrandon et al.

90、, 2006) on acid milk gelation using confocal laser scanning microscopy indicated that for the unheated sample until pH 5.3 no distinct structure was visible despite the observation of a fine dispersion of particles of ca

91、sein as well as of whey proteins, whereas in the pH range from 5.3 to 5.0 casein aggregates were shown, with the existing gaps filled with whey proteins in the aqueous phase. At pH 4.95 and below, gelation took place whe

92、re both caseins and u</p><p>　　From the above discussion it becomes apparent that still a lot of work is needed to be done in order for the yogurt gelation mechanism to be unequivocally elucidated.</p>

93、<p>　　5. Conclusions</p><p>　　Structure development during yogurt gelation is influenced by both the preheating treatment of the milk even if the heating time employed is as short as 1 min and its fat

94、 content.</p><p>　　Gelation curves of preheated milk either skimmed or containing fat consist of three distinctive parts, indicating that gelation mechanism is fairly complicated.</p><p>　　Yogur

95、t gel network remains stable even when the pH value of the system drops below 4.0.</p><p>　　Acknowledgement</p><p>　　The authors are grateful for financial support from the European Union-Greek

96、Ministry of Education “ARCHIMEDES” research program.</p><p>　　References</p><p>　　Barnes, H. A. (1995). A review of the slip (wall depletion) of polymer solutions, emulsions and particle suspens

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