Produce Color and Appearance 203
are present: the blue quinonoidal base, the red flavylium cation, the colorless pseudo-
base or carbinol, and the colorless chalcone (Brouillard and Delaporte, 1997).
Acidity of the system as expressed by pH has the dominant influence on the
color of produce containing anthocyanins. In the range of pH 2 to 4 the flavylium
cation is mostly responsible for the color, along with some contribution from the
quinonoidal base. With an increase of pH a large bathochromic shift occurs and the
quinonoidal base increasingly influences the color. With molecules containing more
than one OH group the existing equilibrium becomes more complex. An elegant
discussion of this topic may be found in a chapter written by Brouillard (1982).
Temperature significantly influences the degradation of anthocyanins, especially
in the presence of oxygen. This effect varies depending on the molecular structure
of the specific compounds. Anthocyanins containing more methoxy groups or sugars
are more stable than those containing more hydroxy groups (Mazza and Miniati,
1993). Three possible mechanisms have been suggested for the thermal degradation
of anthocyanins (Jackman and Smith, 1992). In the first, the quinonoidal base that
is in equilibrium with the flavylium cation leads to a coumarin derivative and B-ring
derivative. In the second, the flavylium cation transforms to a carbinol base, chalcone,
and, finally, brown polymeric compounds. In the third, the first two steps are similar
to those in the second mechanism but they result in low molecular degradation
products of chalcone instead of polymers.
Light significantly influences the stability of anthocyanins.
Numerous chemicals that are used in processing of foods containing fruits may
interact with anthocyanins. Ascorbic acid is frequently added to fruit juices to
increase its nutritional value. However, anthocyanins and ascorbic acid cause mutual
degradation.
Sulfites and sulfur dioxide added sometimes for preservation purposes are known
to cause loss of color of anthocyanins. Under some circumstances the color may be
restored (Francis, 1989). Anthocyanins are known to form complexes with metals.
These complexes may change the color of anthocyanins. The formation of complexes
may occur during processing in metal equipment or by adding some salts. Complexes
of metals with anthocyanins contribute to the wide variety of colors of plants in
nature. Molecules of anthocyanins may form complexes with molecules of other
organic compounds. This phenomenon is known as co-pigmentation and may involve
flavonoids, amino acids, proteins, pectin, carbohydrates, and polyphenols (Francis,
1989).
Vaccinium berries constitute an important group of small fruits containing bio-
logically active compounds (Prior et al., 1998; Kalt et al., 1999a,b; Moyer et al.,
2002). These compounds include anthocyanins and flavonols, most commonly as
glycosides, and acetylated glycosides. Taruscio et al. (2004) investigated Vaccinium
fruit samples from cultivated and undomesticated colonies within the northwestern
U.S. Total phenolics (TPH), total anthocyanins (ACY), and their antioxidant capacity
were determined. Total phenolics ranged from 0.81 to 2.84 mg/g of fresh weight.
Total anthocyanins were lowest in red huckleberry and wild cranberry (0.11 and 031
mg/g fresh weight, respectively), and the evergreen huckleberry and oval leaf blue-
berry that contained the highest TPH also showed the greatest ACY (3.64 and 3.07
mg/g of fresh weight, respectively). The selected bioactive compounds analyzed by