Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 291

position was set to 100 mm below the interface of the coverslide in contact with
the liquid sample. The time course of phase separation was followed by taking a
series of images of frame size 512 512 pixels (i.e.,230 230 mm) with a scan
time of 6.2 s at intervals of 30 s.
Photographs of the tubes were taken with a digital camera after two days of
storage in the incubator (Material testing cabinet 2391, Ko¨ ttermann, Ger-
many). For all samples in which macroscopic phase separation into two layers
could be observed, the height of the top layer (H top ) and the total height (H col )
of the liquid were determined from the photographs using the millimetre scale
included in the photos, with analysis in Microsoft Photo Editor.
19.2.3 Gel Formation
Samples of the blends (100 g) were stirred in a beaker at 401C for 10 min using a
three-blade propeller-mixer operated at high speed. After addition of 3 g of
GDL (Sigma Chemical, USA) the mixtures were stirred for another 2 min at
401C and then loaded by means of a syringe (BD Plastipak, Drogheda, Ireland)
into test tubes for pH measurement, and into the sample cell of the rheometer
(19 mL) or into the tubes in which the samples for later microscopic analysis
were produced (11.5 mL). For all the experiments, pH curves were recorded in
parallel to monitor the evolution in sample pH during acidification. The pH
and gelation profiles were corrected for the initial time delay between GDL
addition (time ¼ 0) and the start of the measurement.
The pH was measured with a combination glass electrode with integrated
temperature probe (InLab 410, Mettler Toledo, Switzerland) connected to a pH
meter (MP 230, Mettler Toledo, Switzerland). The decline in pH due to GDL
hydrolysis was monitored by continuous acquisition of pH data using a Laptop
(Latitude C510, Dell, USA), connected to the pH meter via RS-232 link, and
software written by NRC-IT (NRC Lausanne) using Visual Basic 6.0 software
(Microsoft, USA).
Gel formation was monitored using a Paar Physica MCR 500 rheometer
equipped with a concentric cylinder geometry (CC 27) with thermal unit TEZ
180-C connected to water-bath Viscotherm VT2 (Anton Paar, Graz, Austria)
and equipped with solvent trap system. The rheometer was operated in con-
tinuous oscillation mode at a frequency of 0.1 Hz and 0.04% strain. Measure-
ments were recorded as a function of time at 30 s intervals (240 data points).
Under applied strain, the rheometer software calculated the stress and the
phase angle, from which the elastic modulus G 0 was calculated.
The gel samples for microscopic analysis were prepared in 14-mL round-
bottom polypropylene tubes (Falcon No. 352059, Becton Dickinson Labware,
USA). The tubes (closed with their caps) were kept in a water-bath to restore
sample temperature rapidly to 401C, and transferred to the incubator 10 min
after GDL addition. The samples were taken out of the incubator 60 min after
GDL addition, when the samples had reached pH 4.6, and sample preparation
for microscopy was started immediately at room temperature. Encapsulation in
agar gel tubes, and fixation and embedding, were performed as described