The relationship between proteolysis and texture in Feta-type cheese
made from ultrafiltered cows' milk was studied using capillary electrophoresis,
chemical analysis and uniaxial compression. For this purpose cheeses were
made
with almost identical gross chemical compositions, but wide variations
in texture,
obtained by varying the amount of rennet and the coagulation conditions.
αs1-Casein
(CN) 8P, αs1-CN 9P and αs1-CN 8P-I
were degraded during ripening, while β-CN A1,
β-CN A2 and para-κ-CN were slightly
degraded at the highest rennet additions and
longest storage time tested (39 weeks). αs1-CN 8P,
αs1-CN 9P and αs1-CN 8P-I were
degraded even in cheeses made without rennet, and this was ascribed to
cathepsin D
activity. αs1-CN 9P disappeared faster than αs1-CN
8P,
which suggests that milk acid phosphatase was active during storage. During
ripening,
stress at fracture (σf), strain
at fracture (εf), the deformability modulus (E)
and work to fracture (Wf) all
decreased. Since the gross chemical composition was essentially constant
during
storage, these changes could be ascribed purely to proteolysis. Although
εf could be
predicted fairly well from capillary electrophoresis results (correlation
coefficient
0·85), there were no unique relationships between degradation of
casein components
and σf when both storage time and the amount of rennet
used were varied. In the
young cheese a high level of proteolysis could coexist with high values
of σf. This
suggests that high levels of rennet caused σf to increase,
via an effect unrelated to
general proteolysis, whereas general proteolysis caused the reverse effect.