(香港中文大學知識轉移項目基金Knowledge Transfer Project Fund──驅動社會影響項目Social Impact Driven Project 特稿七)
相信大家都有吃過果醬，甚至有自行製作的經驗。自製果醬的材料和步驟看似簡單，只需要把水果、糖和檸檬汁加熱煮至濃稠，入樽後冷卻就成了凝膠狀的果醬。果醬之所以能成形主要靠果膠(Pectin)，它是由半乳醣醛酸(Galacturonic acid)組成的多醣聚合物，是構成植物果實細胞壁的結構及硬度的物質。由於它不能被人體消化吸收，所以也屬於膳食纖維。煮果醬時，加熱會破壞細胞壁以釋出果膠，而糖會吸走水份，之後加入酸性檸檬汁以促進果膠聚合成立體網狀、內含液體的彈性凝膠──果醬。
水果果膠種類多 品種熟度皆影響
果醬是早期人們用以保存水果的方法，早在16世紀時已有記載。而果膠卻在近二百年前(1825年)被法國化學家發現，多年來都被用作天然食物添加劑。在水果未熟時，果膠還是不溶的原果膠(Protopectin)前體，令水果較硬。隨著水果成熟，原果膠會被轉化成水溶、能凝固成「啫喱」狀的果膠，令水果變得較軟及有彈性。但當水果過熟時，果膠會分解為果膠酸(Pectic acid)，令水果變得更軟、失去彈性、「出水」等。原果膠能在煮果醬時轉化成果膠，但果膠酸卻不能變回果膠，故做果醬的水果宜生不宜熟。有些水果如蘋果、紅莓、杏等，果膠含量較多，能直接用於製作果醬。低果膠水果如車厘子、提子等要製成果醬，則需要加入高果膠水果同煮，或直接加入已提煉的果膠。蘋果肉渣及柑橘皮分別含15-20%和30-35%果膠，這些製作果汁的副產品通常被物盡其用來提煉果膠。
果膠通常以脂化度(Degree of esterification)作分類。高脂化度果膠需要高濃度及較酸性環境(pH<3.5)才能凝膠，凝膠較挺身及脆，多用作製作啫喱、糖果、甜品、果醬等。低脂化度果膠凝膠只需適當濃度的鈣，所需的pH幅度亦較寬(pH2-6)，凝膠較軟而多被加入飲品增稠，或製作低糖果醬。果膠除了用作改善食品質感外，不少研究指出，果膠對穩定血糖、血脂，以至腸道健康都有所裨益。
果膠控糖降血脂 益菌利腸助抗炎
歐洲食品安全局(European Food Safety Authority, EFSA)在審視多份不同地區、不同時間的研究報告後指出，成年人每天攝取至少6克果膠，有助維持血液膽固醇正常，而每餐攝取至少10克果膠則能減低餐後血糖波動。
荷蘭有研究邀請27名輕微高血脂患者，每天多攝取15克果膠或纖維素(Cellulose)。四週後，吃果膠組別的血液總膽固醇(Total cholesterol, TC)、三酸甘油脂(Triglycerides, TG)及低密度膽固醇(LDL-C)都有明顯下降，而服用高脂化度及高份子量(Molecular weight)果膠組別的效果更為明顯。果膠和大部分水溶性纖維一樣，能吸收膽汁酸(Bile acid)，與之結合並透過排便帶出體外，引發身體運用儲存的膽固醇製造更多膽汁酸，從而降低血液的低密度膽固醇(壞膽固醇)及總膽固醇水平，而不會影響高密度膽固醇(好膽固醇)。有實驗分別測量高分子聚糖如果膠、幾丁聚醣(Chitosan)及海藻酸鈉(Alginate)與膽汁酸的結合能力，發現果膠和腸道膽汁酸脫氧膽酸(Sodium deoxycholate) 有特異性結合能力(Specific binding)，即是果膠對於清除這種膽汁酸特別有效，從而加強改善血脂的效果。
果膠除能改善血脂外，亦可穩定血糖。有研究以藥物令小鼠患上糖尿病，再以不同份量的橘皮果膠餵食四週。四週後，服食中至高劑量果膠小鼠的胰島素抗性指數(HOMA-IR)及餐後血糖都明顯降低，而果膠的劑量越高，穩定餐後血糖的效果越明顯。研究團隊指出，果膠能降低餐後血糖的效果，不只因它能在吸收水份後和食物混合並增加稠度，從而減慢小腸吸收糖份，更因它能調控胰島素及血糖代謝，從而穩定血糖。果膠亦能直接調控腸道中的免疫細胞、減少炎症細胞因子如TNF-a、IL-1、IL-6等分泌，有抗炎作用。
丹麥有研究以健康人士的腸道菌，以果膠培植24小時。發現腸道益菌如青春雙歧桿菌 (Bifidobacterium adolescentis)及普拉梭菌 (Faecalibacterium prausnitzii)在果膠培植下都有明顯上升，而在年輕受試者的腸道菌中更為明顯。有益短鏈脂肪酸如丙酸(Propionic acid)和丁酸(Butyric acid)在果膠及年輕受試者腸道菌的培植下，水平明顯上升。另有研究以果膠或纖維素餵飼小鼠四週，發現果膠組別血液中的短鏈脂肪酸乙酸明顯較多，而大腦中的腦源性神經營養因子 (Brain-Derived Neurotrophic Factor, BDNF)亦有明顯上升。研究指出，服用果膠能影響腸道菌，令血液中的短鏈脂肪酸增加，從而增加大腦BDNF水平，改善腦神經健康。
一般新鮮水果蔬菜平均含約1%果膠，故每天需吃約500-600克才能達至維持血液膽固醇水平健康的效果，當然亦可食用營養補充劑作補充。新鮮水果蔬菜所含的有益物質不只有果膠一種膳食纖維，敬請繼續留意我們的介紹。
參考資料/延伸閱讀:
＜益生第一關＞2024/03/27--『蘋果最好連皮吃 四季常吃保健康』
https://hskgene.com/blogs/probiolife-first/apple?_pos=5&_sid=c40b35bad&_ss=r
＜益生第一關＞2024/08/07--『夏日新興凝膠食品 飽腹利腸有助健康』
https://hskgene.com/blogs/probiolife-first/fiber_jelly?_pos=1&_sid=b17cfb72b&_ss=r
＜益生第一關＞2021/01/22--『膳食纖維種類多 腸道菌最佳拍檔』
https://hskgene.com/blogs/probiolife-first/dietaryfiber_microbiome?_pos=3&_sid=2c182256e&_ss=r
Easy Reading:
果醬製作掌握4關鍵！青蘋果、橘子最適合新手，謹記「基本公式」比例是美味秘訣|Yahoo News| 12-04-2022
為何做果醬沒有「酸」不行、棉花糖竟然不是素的？ |食力foodNEXT| 08-05-2022
https://www.foodnext.net/science/knowledge/paper/5098734120
Pectin recipes | BBC Food |
https://www.bbc.co.uk/food/pectin
The science and magic of jam-making | Biochemistry and molecular biology | The Guardian |10-03-2013
https://www.theguardian.com/science/blog/2013/oct/03/science-magic-jam-making
The short, sweet, and sticky history of jam| National Geographic | 04-26-2024
https://www.nationalgeographic.com/history/article/history-of-jam-meghan-markle-lifestyle
Google Images:
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