Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 36

formation of undesirable brown spots due to mechanical
injury during postharvest handling of olives (Segovia-
Bravo et al., 2007). In mango, PPO activity was found to be
4-12 times higher in the skin compared to the flesh at
various stages while that of total phenolic content was
11-29 times higher in the skin compared to the flesh of
mango (Ueda et al., 2000). Browning reaction due to PPO
activity has also been investigated in guava (Augustin et al.,
1985), papaya (Cano et al., 1998), and pineapple (Chutin-
trasria and Noomhorm, 2006).

The most common heat treatment method used is water or
steam blanching, which has been shown to inactivate PPO
in fruits (Cano et al., 1997; Arogba et al., 1998; Arogba,
2000; Chutintrasria and Noomhorm, 2006). However, tra-
ditional heat treatment methods cause adverse effects such
as color alterations, flavor damage, and vitamin and nu-
tritional losses (Soliva-Fortuny et al., 2002). Avocados, for
example, develop bitter off-flavors from thermal treatments
(Ben-et et al., 1973).
In the past, sulfites were utilized broadly in the food
industry as antibrowning agents because of their effective-
ness and low price (Eskin, 1990), but their use has since
been stopped because of safety concerns (Sapers, 1993;
Martinez and Whitaker, 1995). Ascorbic and erythorbic
acids are some of the best known alternatives to sulfites in
foods, including fruits. However, once ascorbic and erythor-
bic acids have been quenched or fully exhausted, the brown-
ing process proceeds (Sapers, 1993); therefore, a combina-
tion with other inhibitors (i.e. acidulants) is considered a
more effective method to control browning (Park and Luh,
1985; Eskin, 1990).
Combination treatments involving more than one strat-
egy to slow down the browning processes have been in-
vestigated. One such treatment combined heat and chem-
ical treatment in the preservation of banana puree, which
required a mild heat treatment plus addition of sodium
bisulphite, citric acid, and potassium sorbate (Garcia et al.,
1985). Other authors have also investigated the effect of
combination treatments on the quality of banana puree
stored at 30 ◦ C (Mowlah et al., 1982) or on intermediate-
moisture banana products (Ramirez-Martinez et al., 1977).

Controlling of browning in tropical
and subtropical fruits
Enzymatic browning of fruits catalyzed by PPO is one of
the most important causes of quality losses, resulting in a
decrease in their sensory properties and nutritional value
(Prota, 1988), and subsequent loss in consumer acceptance
(McEvily et al., 1992). In foods, the rate of browning
can be directly related to (1) PPO level, (2) endogenous
polyphenol content of the tissue, or (3) a specific combina-
tion of both factors.
Table 3.3 summarizes the role of PPO in the quality and
control methods in some tropical and subtropical fruits.
Some of the most effective methods used to inhibit PPO
activity during processing of fruits and vegetables include
addition of chemicals (Janovitz-Klapp et al., 1990), pH ad-
justment (Siddiq et al., 1992), exclusion of oxygen (Paulson
et al., 1980), refrigeration (Garcıa and Yousfi, 2006), and
thermal treatment (Vamos-Vigyazo, 1981).
In general, exposure of PPO to temperatures of 70 ◦ -90 ◦ C
destroys their catalytic activity (Vamos-Vigyazo, 1981).

Table 3.3. Role of polyphenol oxidase (PPO) in tropical and subtropical fruit quality and its
control strategies.

Role in Fresh Fruit and
Processed Products Fruit Inactivation or Control Options Reference

Heat inactivation (67 ◦ -84 ◦ C for up to 10 min),
pressure inactivation (450 MPa at pH 4 to
850 MPa at pH 8), chemical treatment
(EDTA and ascorbic acid, nitrogen, and
vacuum packaging)

Avocado Affects color quality G omez-L opez (2002);

Weemaes et al. (1998);
Soliva-Fortuny et al.
(2002)

Banana Affects color quality Steam blanching and freezing; chemical

treatment—metabisulphite, L-cysteine
ascorbic acid, and 4-hexylresorcinol.

Cano et al. (997)

Guava Affects color quality Chemical treatment—metabisulphite,

L-cysteine, and ascorbic acid

Augustin (1985)

Pineapple Affects color quality Chemical treatment—metabisulphite,

L-cysteine, and ascorbic acid

Das et al. (1997)