Agriculture Reference
In-Depth Information
Texture
Consumer surveys in the United Kingdom suggest that the overall accept-
ability of apples is principally related to their texture (Smith,
). Immature
fruits are too hard as well as having other undesirable characteristics. Fruits
soften during ripening in a progressive way that leads to unacceptability. Initial
firmness at harvest controls the effect of softening.
Softening can result from loss of turgor (Hatfield and Knee,
), degra-
dation of starch and, most importantly, cell wall degradation (Tucker,
)
and a weakening of the cohesive forces between cells.
Apple fruit cell walls consist mainly of cellulose and pectin, with some
hemicellulose and very small amounts of extensin. The cellulose is resis-
tant to degradation but the cell walls have a clearly distinguishable sec-
tion known as the middle lamella which separates adjacent cells and serves
as a bonding agent between cells. This section is rich in pectic poly-
saccharides and its cohesion is thought to depend on ionic bonds involving
calcium ions and uronic acids (Bartley and Knee,
). The breakdown
of the middle lamella may follow transport of exo-polygalacturonase and
possibly pectinesterase to it. Cell separation proceeds in the apples until
they develop a dry or mealy texture when the consumer's teeth pass be-
tween cells without breaking them, so that they fail to release juice (Knee,
).
The preferred texture of pears differs between species and markets.
European-type (
Pyrus communis
) cultivars are expected to have a tender skin,
soft, buttery, flesh and an absence of gritty stone-cells especially if the pears
are to be used to produce puree for baby food. Chinese and Japanese
pears (
P. ussuriensis
and
P. pyrifolia
) are gritty and a crisp, breaking, texture is
preferred.
Fruit firmness can be measured using the resistance of peeled fruit flesh
to the insertion of a plunger of known diameter, usually
mm for pear and
mm min
−
. The maximum force applied at each penetration at a depth of
mm for apple, with a uniform crosshead speed set between
and
mm is recorded in newtons or kilograms, the conversion factor from kg to N
being
).
More complexanalysis of results obtained with a commercial texture testing
machine can discriminate between apple cultivars with respect to their frac-
turability (which correlates with crispness), hardness, cohesiveness, springiness
and chewiness (Corrigan
et al.
,
×
.
(Smith,
). Tensile strength can also be measured as
the force required to pull a cylinder of apple tissue into two halves (Poovaiah
et al.
,
). Acoustic (non-destructive) measurements of firmness as a 'stiffness
factor' correlate well with compression and tensile rupture forces (Tu
et al.
,
).
