Chemistry of MOON CAKES

Happy Mid-Autumn Festival! New infographic: Chemistry of MOON CAKES

Mid-Autumn Festival (中秋节) is a traditional Chinese festival celebrated on the 15th day of the 8th lunar month each year (a full moon night in September). It started as anagricultural tradition (like harvest festival in western cultures) around 1000 BC in the Zhou Dynasty, and was formally acknowledged as a festival during the Northern Song Dynasty (between 960 and 1279 AD).

Today, Mid-Autumn Festival is celebrated with moon cakes, family reunions and three days off work. Moon cakes are circular to represent the full moon that always occurs on the Mid-Autumn Festival. Watch the video below to learn about the story behind the festival:

Moon cakes consist of crust, filling and an egg wash. The crust is made from flour, thepolysaccharides in which bind together at oven temperatures to form a strong, intricate network (also including proteins) that allows the moon cake to keep its all-important circular shape.

The crust also contains invert sugar syrup, which is chemically similar to both honey and golden syrup. Invert sugar syrup is made by hydrolysing sucrose into its constituentmonomers, glucose and fructose. The result is a sweeter-tasting, gooey liquid that doesn’t crystallise during cooking. This gives the moon cake a smooth mouthfeel.

Peanut oil (a blend of mostly monounsaturated triglycerides) is added to the crust for two reasons. First, it is a non-volatile liquid at room temperature, which prevents the moon cake from drying out. Second, the peanut oil molecules disrupt the protein matrix in the crust and give it an even smoother texture (not a doughy texture).

Maillard reactions are caramelisation reactions involving the removal of two hydrogen atoms from a sugar aldehyde or ketone. The resulting compounds are yellow/brown in colour because they contain carbon-carbon double bonds (C=C), which absorb violet and UV light (λmax ≈ 190 nm). The moon cake is usually also given an egg wash, which provides extra protein necessary for Maillard reactions to occur. More egg wash will provide a deeper brown colour to the dough.

Alkaline water (枧水) is a common ingredient in Guangdong-style cuisine. Chemically, it’s a ~0.020 molar solution of potassium carbonate and can be considered as the ‘opposite of vinegar’. It raises the pH in the moon cake, which accelerates the Maillard reaction, which is favoured by alkaline conditions. Alkaline water thus makes the crust more brown!

Finally, the fillings can be very diverse. Lotus seed with salted duck egg yolks is a common filling, but “five kernels”, red bean and green tea (with beans) are also quite popular. Lotus seed filling, for example, is made by soaking dried lotus seeds in alkaline water, pulverising and adding sugar. The resulting paste is then cooked with more oil and sugar before being used to fill a moon cake. ●

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