愛玉裡的化學！

夏日炎炎，又到了冰冷品的旺季。無奈這個夏天遇上了塑化劑風波，大部分的飲料都受到波及，因此筆者的母親今天特別跑去傳統市場買來了愛玉子，準備來做愛玉凍消消暑。

下午洗愛玉時，筆者突然對愛玉凍的性質感到好奇，為什麼愛玉凍放置久了之後會變成一鍋水？又為什麼將假愛玉（以洋菜為原料）加熱後會溶化，但真愛玉卻不會呢？

在網路上查了查資料後，才發現好吃的愛玉可是大有學問！

愛玉凍之所以會凝結是因為愛玉子含有果膠。在我們洗完愛玉後，有種叫做果膠酯酶（pectinesterase）的酵素會和有高含量（高於7w%）甲氧基（-OCH3）的果膠作用，果膠酯酶會將果膠脫去部分的甲氧基，使其成為低甲氧基含量的果膠。低甲氧基含量的果膠接著會和水中的鈣離子及鎂離子起交聯反應（cross-link）而成為凝膠。高甲氧基的果膠則須要和糖類才能產生交聯反應，因此果膠酯酶可說是愛玉成凍的關鍵！

一般市售的愛玉現在皆使用洋菜中的紅藻膠（carrageenan）替代愛玉的果膠，紅藻膠屬熱可逆性凝膠，遇熱會溶化；而果膠則是熱不可逆凝膠，加熱後還是不會改變結構，因此不會溶化（當然，加溫到極高溫還是會溶化的）。

至於愛玉凍製成一段時間後，凍中之水份逐漸析出的情形，我們稱之為離水現象。愛玉中有另一種酵素叫做聚半乳糖醛酸酶（polygalacturonase），聚半乳糖醛酸酶會將果膠中的半乳糖醛基水解，因此果膠的結構就被"吃"掉啦！凝膠的狀態也就無法維持了。

延伸閱讀：
蔡仲華：《愛玉子專題報告》於《中央研究院高中生命科學資優生培育計畫專題研究報告》
李柏宏（民89）：愛玉子凝膠性質及愛玉凍品質之研究
石正中（民76）：愛玉凍離水現象之研究

突吻蝣蛇抓魚的詭譎技巧 Tentacled Snake Uses Sneaky Strategy to Catch Fish

http://www.scientificamerican.com/video.cfm?id=913760431001

Script: (No guarantee of accuracy.)
Tentacled snake, name for the two appendages perched on the tip of its nose, is a master of ambush. But this snake has caught something other than fish lately. It’s caught the eyes of scientists.
Its markable strategy for catching prey has become a subject to study.
In the wild, the snake lives in the rivers in Southeast Asia.
It camouflages itself by holding a pose that resembles a dead tree branch. As an unsuspecting fish swims nearby, the snake swings into action and swallow it whole.
It happened so quickly that it is impossible to capture with regular film. But when view at 2000 frames per second, about a hundred times the speed of standard film. The snake’s strategy becomes clear.
It uses a quick flick of its body to start on the fish, corralling it toward its head and into its mouth. The snake exploits reflex that normally helps fish it captures. Two large nerves on either sides of the body called Mauthner Neuron react pressure ways and involuntarily cause the fish to move in the opposite direction.
Most of the time, this mechanism works in the fishes’ favor. But the Tentacled snake creates false waves, causing the fish to the wrong direction and straight into its waiting jaws.
What about those unusual tentacles that give the snake its name? Those are actually highly-involved motion detectors perfectly tuned to the movement of fish. They let the snake detect food even in murky, silty water, its natural environment. They are features that strike fear and sometimes fangs into heart of fish everywhere.
To learn more about the Tentacled snake, read the article “Natural-Born Killer” in the April issue of Scientific American.