Research targets better sweet potato pigments for foods

Gongjian Fan and co-workers from Nanjing Agricultural University investigated the extraction of the pigment from purple sweet potato, and tap into the trend to source natural alternatives to artificial food colours such as Sunset Yellow, Tartrazine and Quinoline Yellow.

Market figures confirm the trend.

While the European colouring market faces an annual growth rate of just 1 per cent between 2001 and 2008, the colouring foodstuffs market is ripping ahead on growth of 10 per cent to 15 per cent.

"Anthocyanidins extracted from fermented purple sweet potato were more pure than chemistry extraction," wrote the researchers in the journal LWT-Food Science and Technology.

"The research result has a positive implication for using purple sweet potato anthocyanins (PSPAs) as natural colourants in food industry," they added.

Anthocyanins are the source of the blue, purple and red colour of berries, grapes and some other fruits and vegetables.

These pigments also function as antioxidants, believed to protect the human body from oxidative damage that may lead to heart disease, cancer and ageing.

Hundreds of different anthocyanins exist in nature, all with slightly different chemical compositions.

Fan and co-workers compared the extraction of the pigments from purple sweet potato culture fermented by Suzhou wine starter (Rhizopus 3.851, 3.866 and Saccharomyces cerevisiae ), with those extracted chemically (hydrochloric acid-ethanol, 5:95 per cent).

They report that five major anthocyanins were detected from the extracts, using high performance liquid chromatography (HPLC).

The major anthocyanidins were found to be cyanidin and peonidin, with over 80 per cent of the PSPAs present as acylated anthocyanins.

When tested under acid conditions (pH 2.0 to 4.0), excellent stability was reported for the pigments, while they were found to be less stable at higher pH (pH 5.0 to 6.0) "The anthocyanins solution was red at pH 2.0 and gradually turned to more red-lilac by raising pH to 7.0," wrote the researchers.

"It is believed that at low pH, anthocyanin exists as flavylium cation which is the most stable form.

This bright red form transforms into blue quinonoidal bases or colourless carbinol pseudobases as the pH-value increases."

They added that further study was necessary for the determination of pyroanthocyanins requires.

Natural red pigments have focussed on betanin, the main betacyanin compound found in red beetroots and listed in Europe as E162.

It is used in a variety of processed foods because it colours without changing the flavour profile.

However, stability issues of the betalains have so far restricted their use mostly to foods with short shelf lives.

The compounds are reported to be sensitive to heat, pH, light, moisture and oxygen.

Recently a study, published in the journal Food Research International (Vol. 39, pp.

667-677)

, looked at the pigment stability and colour of betacyanins extracted from the purple pitaya, a fruit reportedly attracting interest as an alternative to red beet.

Source: LWT-Food Science and Technology Published on-line ahead of print, doi:10.1016/j.lwt.2007.09.003 "Composition and colour stability of anthocyanins extracted from fermented purple sweet potato culture" Authors: Gongjian Fan, Y. Han, Z. Gu and F. Gu