3. FOOD ChEMISTRy & bIOTEChNOLOGy 3.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Food Chemistry & Biotechnology
P93 INFLuENCE OF Ph ON ThE RhEOLOGy
PROPERTIES OF TOMATO KETChuPS
JARMILA LEHKOŽIVOVá, JOLAnA KAROVIČOVá,
JARMILA HOJEROVá, ZLATICA KOHAJDOVá and
IVAnA ŠIMOnOVá
Institute of Biotechnology and Food Science, Faculty of Chemical
and Food Technology, Slovak University of Technology,
Radlinského 9, 812 37 Bratislava, Slovak Republic,
jarmila.lehkozivova@stuba.sk
Introduction
A large part of the world tomato crop is processed into
tomato paste, which is subsequently used as an ingredient
in many food products, mainly soups, sauces and ketchup 1 .
Tomato ketchup is concentrated dispersion of insoluble matter
in aqueous medium 2 .
By definition, rheology is the study of the deformation
and flow of matter. So, rheological properties are based on
flow and deformation responses of foods when subjected to
stress 3 . The flow properties of food products significantly
influence not only technology processes during the production,
but the texture of final products, too 4 . Tomato ketchup
viscosity is very important from both engineering and consumer
viewpoints 5 .
The aim of this work was investigated the influence of
pH on the rheological properties of 3 samples of the formulated
tomato ketchups at pH levels 3.5, 4.0 and 4.5. This pH
range is most common for tomato ketchups. In recent years
rheological properties of food products have become increasingly
important in the formulation of food and the optimization
of food processing.
Experimental
Three samples of tomato ketchup were prepared in laboratory
conditions. The tomato paste with tomato soluble solids
30 % and guar gum cames from the company Kolagrex Int.,
s.r.o., Kolárovo, Slovak Republic. Some ingredients such as
Fig. 1. Flow curves of the studied ketchup samples at shear
rate 0–1,000 s –1 and temperature 25 ± 1 °C
s787
Isosweet and Resistamyl 348 were from Amylum Slovakia,
s.r.o., Boleráz, Slovak Republic and other were purchased
from local market.
T o m a t o K e t c h u p P r e p a r a t i o n
The 100 g of tomato ketchup samples were prepared
following the recipe: 35 g of tomato paste (TSS 30 %)
mixed with ingredients typically used in ketchup preparation
(15 g of isosweet, 11.5 g of vinegar, 1.5 g of salt, 2 g
of resistamyl 348, 0.1 g of guar gum and 34.9 g of water).
The pH level of the mixture was modified at level 3.5 (A1),
4.0 (A2) or 4.5 (A3) and followed by stirring for 5 min. The
mixture was heated and stirred constantly until temperature
between 80–85 °C reached. Ketchup samples were then stored
at ambient temperature (25 °C) for 24 h before analyses.
R h e o l o g i c a l M e a s u r e m e n t
The rheological measurements of this study were carried
out using the rotational viscometer (controlled rate mode)
HAAKE Viscotester VT 550 (Haake, Karlsruhe, Germany)
with coaxial cylinder measuring sensor systems MV DIn
53019. The Viscotester VT 550 was computer-operated with
the application software RheoWin Job Manager and data
evaluation software RheoWin Data Manager (ThermoHaake,
Karlsruhe, Germany).
Flow curves (the graphical correlation between shear
stress τ and shear rate D) were measured at a continuously
increasing shear rate ramp over 3 min from 0 to 1000 s –1 . The
apparent viscosity:
η a = τ/D (1)
The thermal stability of the studied samples was measured
at the low shear rate of 10 s –1 after 3 min of shearing.
Results
From the shape of the flow curves, which are demonstrated
on the Fig. 1., it is possible to get the important information
about rheological character of each ketchup sample.
All 3 samples behave as non-newtonian fluids, of which
shear stress and apparent viscosity was changed with rate of
dynamic strain. Rheograms confirmed their pseudoplastic
characters too.
Viscosity curves (Fig. 2.) were constructed by conversion
of values of flow curves. From their shape it is possible
to set viscosity of the product at specific shear rates to which
will be product exposed during dispersion, mixing, pumping,
and filling to container as well as during application
with consumer. The highest apparent viscosity during shear
rate range 10–1,000 s –1 was determined in ketchup sample
A1 with pH 3.5. Decreasing in the apparent viscosity was
followed by sample A2 with pH 4.0 and then by sample A3
with pH 4.5.The requirement putting on some food products
exposed to the specified temperature range is that its viscosity
changed minimally. The effect of temperature on viscosity of
ketchup samples is showed in Fig. 3. Ketchups should have