314 Produce Degradation: Reaction Pathways and their Prevention
Antibrowning inhibitors are usually applied to cut fruits, leaves, roots, or other
pieces of fresh produce by dipping or spraying. Sulfur dioxide can also be applied
in gaseous form, usually produced by burning sulfur, but its application can be
limited by the dosage of the dioxide in treated products. Individual inhibitors differ
in their ability to permeate into tissues. Sulfur dioxide and disulfites or sulfites are
most efficient and widely used. The inhibitory effect of sulfites is enhanced by their
ability to rapidly difuse into plant tissues. Health concerns have limited the use of
sulfites. It is permitted in most countries, with limitations. The potential induction
of allergic and pseudoallergic reactions in humans (Simon and Stevenson, 1998)
caused the Food and Drug Administration (FDA) to ban the use of sulfites in salad
bars in 1995 (Martinez and Whitaker, 1995). The presence of sulfite contents above
10 μg/kg is usually required to be declared and the additive can only be used for
selected foods. The potential health risk of sulfur dioxide and sulfites initiated the
development of alternative browning inhibitors, but while many other PPO inhibitors
are known, only a few of them have been considered as potential alternatives to
sulfites. Most antibrowning agents should be used in combination with other meth-
ods, particularly with handling and packaging methods that limit exposure to oxygen.
Most of the commercially available sulfite-free antibrowning agents are combina-
tions of various compounds and ascorbic acid (McEvily et al., 1992). Such combi-
nations usually include a chemical reductant (e.g., ascorbic acid), acidulant
(e.g., citric acid), and chelating agent (e.g., EDTA or phosphoric acid or pyrophos-
phate). Commercial mixtures often contain calcium as a firming agent. According
to Son et al. (1975), the use of oxalic acid as a browning inhibitor seems to be
promising, especially due to the synergistic effect when it is used in combination
with ascorbic, erythrobic, or citric acid. These commercial mixtures are applied by
direct addition to homogenated produce, dipping, or spraying, but recently some
authors have proposed the use of edible coating and wraps containing antibrowning
additives^ (Baldwin et al., 1995; McHugh and Senesi, 2000).^ An example of an edible
film is a combination that includes 61% apple puree, 23% beeswax, 7% pectin, 7%
glycerol, 1% ascorbic acid, and 1% citric acid. Such a coating prevented the color
changes of apple pieces stored for 12 days^ (McHugh and Senesi, 2000).
10.2.2.2 Blanching
Blanching is a thermal process to inactivate enzymes and remove intercellular gases.
The process usually utilizes temperatures in the range of 75 to 95°C for times of
about 1 to 10 min, depending on product requirements. Blanching is a necessary
pretreatment for many processed fruits and vegetables to achieve satisfactory quality
in dehydrated, canned, and frozen produce. It is also needed as a food processing
pretreatment process where there is a risk of a delay in enzyme inactivation, such
as in canning and freeze-drying. Elimination of intercellular gases is important for
the reduction of oxidation and for achieving suitable vacuum conditions within a
jar or can. Blanching can also contribute to microbial count reduction. Plant matter
tends to shrink because of loss of turgor, but texture can be improved. Undesirable
losses of nutrients may be caused during water blanching due to leaching (Selman,
1986).