# Last edited on 2016-12-20 01:30:47 by stolfilocal

Purple yam (/Dioscorea alata/) is a tuber similar to ordinary yam, but with
an intensely purple flesh. The color is due to anthocyanin, the same
substance that gives color to blue, purple, and lilac flowers, as well as
purple cabbage.

Actually, anthocyanin changes color depending on the pH of the
environment. It is purple when neutral (pH = 7), red when acidified (pH
< 7), and blue or green when alcaline (pH > 7). In very alcaline
environments it becomes colorless. Thus it is responsible also for the
color of many red and pink flowers (such as roses and dahlias). 

However, anthocyanin not responsible for the purplish-red color of beet
roots.  That is due to betanin, fairly different substance.

There are several variants of anthocyanin. Their molecular structured
are all derived from a common core called anthocyanidin, with different
groups of atoms attached to it.

Anthocyanin is has been proposed as a safe and natural food coloring
agent. The purple yam would be a convenient source, due to its high
concentration: 1 kg of yam contains about 12 g of anthocyanin [1].

However, the extraction method suggested in the literature [1] treats
dry powdered yams with a mixture of 80% ethanol, 5% HCl, 15% water at 60
C for 1 hour. Ethanol with that concentration is expensive or
unavailable in many parts of the world, and people may be tempted to use
methanol or fuel ethanol, which may contaminate the product. That
process also may render the residue unfit for other uses.

Here we show that most anthocyanin can be extracted from purple yams by
a method that is much faster, simpler, and cheaper than the acidified
ethanol process above. The process also yields pure edible starch as a
byproduct. The extraction proper requires no special training, and uses
only water and utensils that are found in most kitchens. Most
importantly, the residue left from the extraction is a fully edible
flour.

The main part of the process, to be carried out in a kitchen, is simply
to grind the peeled and cleaned yams with some cold water in a kitchen
blender for several minutes, and filter the resulting slurry through
dense cotton cloth, with pressure. The liquid is then allowed to rest
for a few minutes to deposit the starch, and decanted.

The extraction of the anthocyanin from the liquid can be done in 
a more "industrial" environment.  




[1] Jinwei Li, Lianfu Zhang, and Yuanfa Liu (2013) "Optimization of
Extraction of Natural Pigment from Purple Sweet Potato by Response
Surface Methodology and Its Stability" Journal of Chemistry, volume
2013, article ID 590512, 5 pages. doi:10.1155/2013/590512