Thermal Food Processing

Thermal Processing of Vegetables 411
and hardness of texture. The LTB also caused increase in gumminess and springness
of canned sweet potato. 59
Many vegetables are washed, blanched, comminuted, and converted to puree
or paste. The puree/paste is considered a minimally processed food, as the product
is sometimes acidified (approximate pH = 4.0) and pasteurized. These products
retain color and flavor comparable to the fresh produce and are convenient to use.
The rheological characteristics of these products change during thermal processing.
Rheological parameters are significantly affected by process temperature. Pureed
food behaves as a non-Newtonian fluid with yield stress and is pseudoplastic in
nature. 119 The consistency index decreased with temperature, while the flow behavior
index increased. Rheological parameters obtained from a controlled rate rheometer
of some strained vegetable pureed foods during thermal processing are
presented in Table 13.4. Among various rheological models, the Hershel–Bulkley
model (Equation 13.19) adequately describes shear stress–shear rate data of vegetable
purees. The presence of yield stress ( o) of pureed food indicates application
of minimum force to initiate the flow of the product. Temperature has a significant
effect on o, and the yield stress decreased with temperature: 119
TABLE 13.4
Rheological Characteristics of Strained Vegetable Puree
Vegetable Type
Carrot puree
(7.4°Brix and
pH 4.88)
Pea puree
(7°Brix and
pH 5.92)
Corn puree
(14°Brix and
pH 6.46)
Sweet potato
puree (11.2°
and pH 5.27)
Wax bean puree
(5.1°Brix and
pH 5.05)
Temperature
Range (°C)
τ = τ + K( γ)
Yield
Stress (Pa)
Consistency
Index
(Pa·sec n )
Flow
Behavior
Index (–)
(13.19)
Apparent
Viscosity at
100 sec –1
(Pa·sec)
20–80 1.65–3.72 5.44–11.53 0.27–0.35 0.24–0.42
20–80 1.25–1.83 1.12–1.35 0.36–0.44 0.11–0.14
20–80 0.43–1.18 0.58–2.06 0.26–0.30 0.02–0.05
20–80 1.46–2.66 0.77–1.20 0.38–0.43 0.06–0.09
20–80 5.10–5.23 0.22–1.86 0.48–1.0 0.23–0.26
Source: Unpublished work of J. Ahmed and H.S. Ramaswamy.
o
.
n