Food Biochemistry and Food Processing (2 edition)

BLBS102-c29 BLBS102-Simpson March 21, 2012 13:27 Trim: 276mm X 219mm Printer Name: Yet to Come

578 Part 5: Fruits, Vegetables, and Cereals

Table 29.8.Physicochemical Parameters of Tomato

Fruits and Processed Juice from Transgenic and

Control Tomato

Properties Control Transgenic

Fruit weight (g) 95.14±36.56a 34.87±21.57b

Firmness (N) 4.97±0.59a 5.99±1.03b

Redness (a+) 29.6±0.70a 31.70±1.67a

Acidity (%) 0.36±0.00a 0.38±0.05a

Brix 4.55±0.71a 4.75±0.71b

Dry matter-NSS (%) 3.54±0.05a 5.15±0.03b

Ash (%) 0.63±0.03a 0.089±0.13b

Vitamin C (mg/100g) 3.9±0.00a 10.4±0.016b

PPT (%) 15.70±0.33a 16.17±0.48a

Serum viscosity (mpa·s) 1.0919±0.04a 1.2503±0.010b

Brookfield viscosity (mpa·s) 1075± 35 a 1400 ± 35 b

Lycopene (mg/100g) 11.73±2.10a 17.47±0.58b

a,bThe values showing different superscripts are significantly different

atP<0.05.

aldehydes. Table 29.8 provides a comparison of various quality

parameters between a genetically modified tomato and a control,

which shows improvements in several quality parameters due to

the transformation.

Physicochemical Stability of Juices

There are two categories of juices, clear and comminuted. Clear

juice such as apple juice contains no visible vegetal particles,

whereas a comminuted juice such as tomato juice contains

mostly vegetal particles suspended in a liquid. To be stable,

a clear juice needs to remain clear (without sediment) during

its shelf life. On the other hand, a comminuted juice may not

separate into distinct phases during the shelf life of the product.

An important quality attribute of juices such as tomato juice is

the stability of their disperse system. In order to be stable, a juice

needs to be kinetically and physically stable.

Kinetic Stability

The ability of a poly-disperse system containing suspended par-

ticles to maintain its homogenous distribution without agglomer-

ation is called kinetic or sedimentation stability. Kinetic stability

depends on many factors, the most important of which include

size of suspended vegetable particles, viscosity of the disperse

medium and the intensity of the Brownian motion. In a liq-

uid medium, heavier or larger particles sediment faster than the

lighter ones in response to gravity. The sedimentation velocity

of any particle is described by the following Stokes Equation:

V= 2

/

9 r^2 (ρ 1 −ρ2) (1/η)g (2)

whereVis the sedimentation velocity, m/s (meter/second);ris

the radius of the suspended particles (m);ρ1 andρ2arethe

densities of the particles and the serum, respectively (kg/m^3 ;

serum=liquid medium in which particles are suspended);

Table 29.9.Sedimentation of Spherical Mineral

Particles in Water and in Juice With a Depth of 1 cm

Particle

Size (μ) Velocity (m/s)

Sedimentation

Time in Water

Sedimentation

Time in Juice

10 3.223× 10 −^4 31.03 s 2.29 min

0.1 3.223× 10 −^8 86.2 h 16 d

0.001 3.223× 10 −^12 100 yrs 436 yrs

ηis the viscosity of the juice (Pa·s; Pascal·second); and g is

the gravitational force (g=9.81 m/s^2 ; meter/second^2 ).

The sedimentation time of a given particle is about 4 times

longer in a juice than in water (Table 29.9.).

Particles larger than 10μm will sediment in a few seconds.

This is the reason why the sedimentation stability of juices,

especially comminuted juices such as tomato juice is a serious

processing issue.

The physical force that affects the sedimentation of particles

in a juice is called normal force and can be calculated by:

f′=mg, (3)

wheremis the mass (kg) of the suspended particle. In general,

a particle with a spherical shape has a mass represented by:

m= 4 / 3 πr^3 ρ1(4)

whereπ=3.14, r is the radius of the particle andρ1 is the density

of the particle. For particles with nonspherical shape (most of

suspended particles),ris equal to the nearest equivalent value

of a spherical particle with an identical mass and an identical

density.

During particle sedimentation, another important force, fric-

tion, also comes into play. Friction between particles results in

a reduction in their movement.

The frictional forcef′′= 6 πρrv (5)

whereηis the viscosity of the medium;ris the radius of the

particle in meters; andvis the velocity of the particle (m/s).

Friction between the particles increases depending on the den-

sity of the medium, higher the density, higher the friction. When

f′=f′′, sedimentation of the particles occur.

Kinetic stability of a heterogeneous dispersion system also de-

pends on Brownian motion. The most dispersed particles have

a very complex motion due to collisions from molecules in the

dispersed medium. Because of this, the suspended particles are

subjected to constant changes in their velocity and trajectory.

Molecular kinetics shows that dispersed particles with colloidal

size change their path 10^20 times/second. These particles may

also acquire a rotational Brownian motion. This is why col-

loidal particles have higher sedimentation stability than larger

particles. With an increase in the mass of the suspended parti-

cles, their momentum also increases. Particles smaller than 5×

10 −^4 cm in diameter that oscillate around a point do not sedi-

ment, whereas larger particles that do not experience as much

Brownian motion as smaller particles easily sediment. Thus,

when particles have reduced motion, they tend to aggregate