Novel Processing Technologies for Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 73

L ∗ value increased from a value of 53.9 to 58.2 after HP
treatment at 600 MPa. At 60 ◦ Cthe L ∗ value increased sig-
nificantly (62.2), even at 200 MPa, and leveled thereafter.
Treatment time has no significant effect on L ∗ .Thered
color value ( a ∗ ) decreased while pressure was increased at
both 20 ◦ and 60 ◦ C. A decrease in a ∗ value indicated that
pressure may be an effective means of minimizing the pink
discoloration sometimes observed in lychee.
The color of some fruit products becomes more attractive
after HP-treatment. The lightness ( L ∗ ) and redness ( a ∗ )of
HP processed jam enhanced after HP treatment (Watanabe
et al., 1991). Pressurized guava puree retained the original
color after HP treatment (600 MPa/25 ◦ C/15 min) and even
after storage at 4 ◦ C for 60 days (Yen and Lin, 1996).
Enzymatic browning in HP-treated (379-586 MPa/room
temperature/0.03, 5, 10, 15, or 20 min) mango puree has
been reported by Guerrero-Beltran et al. (2006). Additions
of ascorbic acid and cysteine were found to be effective
against browning by inhibiting the polyphenoloxidase ac-
tivity. However, HP treatment enhanced this inhibition.
Polydera et al. (2003) found discoloration (based on L ∗ , a ∗ ,
and b ∗ values) of pressure-treated (500 MPa/35 ◦ C/5 min)
reconstituted orange juice during storage (0 ◦ ,10 ◦ ,15 ◦ Cfor
120 days), and the degradation trend was not significantly
different between pressure and thermally treated juices.

than that of fresh samples. Chewiness significantly reduced
after HP treatment. However, the increase in pressure did
not result in a significant change.
Ahmed et al. (2005) systematically studied flow and dy-
namic rheology of pressure-treated (100-400 MPa/20 ◦ C/15
and 30 min) mango pulps obtained from two different va-
rieties ('Alphanso' and 'Chousa') and two different forms
of pulp (canned and fresh). The Herschel-Bulkley model
(equation 5.4) well described shear stress-shear rate data.
Differences were observed in the rheological behavior of
fresh and canned mango pulps treated with HP, both under
steady and oscillatory shear testing conditions. The mag-
nitude of yield stress (

τ o ) for mango pulps varied between
1.09 and 6.24 Pa after the HP treatment. The yield stress
consistently increased with pressure for 'Alphanso' pulp
and followed a linear relationship (equation 5.5):

K ( .

γ ) n

τ = τ o + (5.4)

τ o = 0 . 006 P + 3 . 61 (5.5)

For fresh pulp, the flow behavior index ( n ) decreased
with applied pressure and ranged between 0.25 and 0.34,
whereas n for canned pulp increased from 0.31 to 0.36
at similar condition. The consistency index ( K ) increased
significantly with pressure treatment for 'Chousa,' and a de-
creasing trend was observed for 'Alphanso' pulp. The rhe-
ological parameters are presented in Table 5.4 for 'Chousa'
pulps. Mango puree exhibited thixotropic behavior and it
retained after HP treatment. The area enclosed by the hys-
teresis loop signifies the degree of structural breakdown
during shearing. From a dynamic rheological point of view,
both elastic ( G ) and viscous ( G ) modulii of fresh pulp in-
creased linearly with angular frequency (0.1-10 Hz) up to
200 MPa for a treatment time of 30 min, while a steady
decreasing trend was observed for canned pulp.
A shelf life study on Navel orange juice revealed that
HP treatment (600 MPa/40 ◦ C/4 min) resulted in a higher
viscosity than thermal treatment (80 ◦ C/min), as reported
by Polydera et al. (2005). Furthermore, a limited cloud loss
and a small decrease in the viscosity of HP-treated juice
were observed during storage (0 ◦ ,5 ◦ ,10 ◦ ,15 ◦ ,or30 ◦ C
for 64 days). These studies indicate that the HP has less
effect on product viscosity/rheology than that of thermal
processing.

Texture/rheology
One of the major effects of the conventional processing is
the softening of the fruit products due to heat treatment.
Hardness or firmness is mostly considered as a texture pa-
rameter for whole and cut fruits and is certainly included
in the consumer acceptance criteria.
Basak and Ramaswamy (1998) studied the effect of HPP
(100-400 MPa/room temperature/5-60 min) on the firm-
ness on selected fruits (apple, pear, orange, pineapple). A
rapid firmness loss was noted during compression. During
the holding period (30-60 min), the firmness of fruits (pear,
orange, pineapple) either decreased further or recovered
gradually at 100-200 MPa pressure levels. Fruit samples
became soft above 200 MPa (room temperature for 5-60
min). At 100 MPa, pear was the most pressure-sensitive
fruit, followed by apple, pineapple and orange, while at
200 MPa, apple was more sensitive than pear.
The effect of HPP (50-700 MPa/25 ◦ C/10 min) on texture
of pineapple slices was reported by Kingsly et al. (2009).
The hardness of the pineapple slices reduced with an in-
crease in pressure, but the change was insignificant over the
100-500 MPa pressure range. There were no differences
in the cohesiveness of fresh and HP-pretreated samples,
whereas springiness of the HP-treated sample was lower

Flavor, nutritional, and sensory characteristics
Flavor is an important quality parameter for fruit products.
It is generally assumed that the fresh flavor of fruits is not
changed by HPP since the structure of small molecular fla-
vor compounds is not directly affected by high pressure.