Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

HEAT TREATMENT FOR ENHANCING POSTHARVEST QUALITY 253
12
Days to tissue browning
10
8
6
4
2
0
0 47 48 49 50
Temperature of dip
Fig. 11.6 Cut iceberg lettuce dipped in chlorinated water at 4 ◦ C or in 47–50 ◦ C water for 20 s and then held at
4 ◦ C. The high-temperature-treated lettuce took longer to develop tissue browning. (Adapted from Delaquis et al.,
2004.)
using this 50 ◦ C dip with 20 ppm chlorine on Escherichia coli O157:H7 inoculated onto
fresh-cut iceberg lettuce. It was found that the pathogen population decreased when lettuce
was held at 5 ◦ C and increased when held at 15 ◦ C without respect to a chlorine dip
(Li et al., 2001a). Another study looked at E. coli O157:H7 and Salmonella enterica and
how a dip in alkaline-electrolyzed water at 50, 20, or 4 ◦ C affected pathogen populations
(Koseki et al., 2004). A treatment of 50 ◦ C for 1 or 5 min resulted in 2–4 log 10 cfu/g
reduction in both pathogens and showed no deleterious effects on the lettuce. However, in
this study no storage was performed. Other methods of sanitizing lettuce and preventing the
growth of human pathogens include the use of lactic acid or hydrogen peroxide combined
with hot water. A trial against E. coli O157:H7, S. enterica, and Listeria monocytogenes
looked at both lactic acid and hydrogen peroxide and found that the best treatment for reduction
of the pathogens and maintenance of sensory quality of the lettuce was 2% hydrogen
peroxide at 50 ◦ C for 60 s (Lin et al., 2002). When this treatment was tested in sensory trials
within supermarkets, the preferred lettuce was after antibacterial treatment (McWatters
et al., 2002).
Wounding caused by making fresh-cut lettuce causes accumulation of phenolic compounds,
which cause tissue browning, and treatments of 2.5 min heat at 45 ◦ Cor90sat
50 ◦ C will reduce this accumulation. It does this by preventing the increase in phenylalanine
ammonia lyase activity that accompanies wounding, and also by inducing the synthesis of
heat shock proteins (Saltveit, 2000; Loaiza-Velarde and Saltveit, 2001).
11.6 External damage
Damage can appear as peel browning (Kerbel et al., 1987; Klein and Lurie, 1992; Lay-Yee
and Rose, 1994; Schirra and D’hallewin, 1997), pitting (Jacobi and Gowanlock, 1995), or
yellowing of green vegetables such as zucchini (Jacobi et al., 1996) or cucumber (Chan and
Linse, 1989). One of the most common types of damage observed following a heat treatment
is surface scalding. “Manila” mangoes showed severe skin scalding when forced-air heated
at temperatures of 45 ◦ C or higher, slight skin scalding from heating at 44 ◦ C and no damage
at 43 ◦ C, indicating the presence of a threshold temperature for skin injury to develop