Confronted by growing consumer demand for natural and healthy foodstuffs, food makers have increasingly been looking for 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.
Betanin, the main betacyanin compound found in red beetroots and listed in Europe as E162, 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.
The new 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.
The researchers, led by Florain Stintzing from the Institute of Food Technology, Hohenheim University, measured the retention of betacyanins in purple pitaya juices and pigments (Hylocereus polyrhizus (Weber) Britton and Rose) purchased fresh from Israel) when exposed to different pH levels and heat treatments, without or with the presence of the additives ascorbic acid, isoascorbic acid, or citric acid.
Stintzing and his colleagues looked at the effect of 0.1 or one per cent of each additive on the betacyanin retention at pH4 or pH6 after heating at 85 degrees C for one hour, followed by cool storage.
"Maximum pigment stability was obtained for purple pitaya juice at pH4 with a one per cent ascorbic acid supplementation, affording a pigment retention of 91 per cent after heating and cool storage," reported lead author Kirsten Herbach.
Increasing the pH to six led to a decrease in betacyanin retention, with the one per cent ascorbic acid additive falling to 80 per cent, but this represented a 35 per cent improvement in retention compared to the sample without an additive.
Again, the one per cent ascorbic acid, along with the isoascorbic acid, was the top performer when the researchers measured the retention of lightness under the study conditions, with a lightness value of 73.
"In short, heat-stability of betacyanins in purple pitaya juice stabilised with ascorbic acid is suitable for food colouring," concluded Herbach.
Medium and long-term storage stability of the purple pitaya juice with added vitamin C is currently being studied by the Hohenheim scientists, as well formation of other colours as a result of thermal treatment.
While this research may offer an alternative to the use of red beet-derived betalains, the same stability issues, particularly regarding heat treatment and pH, may be a limiting factor for the number of applications.