Wojtowicz conducted a study on the effects of plasticizers on starch, specifically the addition of water in amounts from 5 to 20%. Results showed that almost every recorded parameter during treatment in the Brabender Mixograph was affected by water addition. In mixtures of starch/glycerol/water, decreasing start melting temperatures were observed, ranging from 80°C for samples with limited water addition (5%) to 65-70°C for samples with 20% added water. The time needed for melting of samples also decreased with increasing water addition. These results suggest that water acts as a plasticizer for starch, which is consistent with previous reports.
The addition of water also had effects on torque values during mixing, with decreasing torque values observed with increasing water content in mixtures. However, torque values during mixing were quite low compared to those seen in the extrusion process and shear cell treatment. Potato starch displayed lower melting temperatures than wheat and corn starch with glycerol and water addition, but higher maximum torque values were reported during the melting tests. The lowest torque values were reached for corn starch samples, regardless of water addition.
During intensive thermomechanical treatment in the shear cell, shorter times necessary for the start of changes inside the structure were observed. This may be due to the much higher shear stress during shearing-heating in the shear cell. The starting melting temperatures for potato starch/glycerol mixtures without addition of water could not be achieved during heating in the water-heated Brabender device, but were found to be about 115°C at 100 rpm in shear cells. Mixtures with small amounts (5%) of added water showed the highest values of torque, and increases in water content led to lower torques in both types of treatment.
Overall, the addition of water as a plasticizer for starch has significant effects on its melting behavior and torque values during treatment, with varying effects observed depending on the type of starch used.
Wojtowicz and van der Goot noted that there were also visible differences in transparency and flexibility of samples produced after different types of treatment.
After treatment in the Brabender equipment, samples became elastic with foamy consistencies and non-transparent, milky white colors. Only for samples of potato starch without and with small amounts of water did added material become brittle, as well as partially transparent with 5% added water or completely transparent and glassy in appearance for potato starch without additives. This last sample, though, was much stickier after treatment then the others.
When they were warm, the samples were flexible and easily underwent elongation and formation into different shapes. After cooling at room temperature, the materials had become hard and were no longer flexible.
All the samples with added glycerol that were heat-sheared in the shear cell were of visibly transparent, glassy-like appearance and smooth surface; directly after processing, they were easily prone to elongation and showed rubbery properties even after cooling to room temperature.
Glycerol addition also had a strong effect on test results. Nashed et al reported DSC observations of glycerol behaving as an anti-plasticizer by hindering the gelatinization process, and linear increases in onset temperature with increasing glycerol content were observed during treatment of wheat starch/water/glycerol mixtures.
During thermomechanical treatment of starch/glycerol mixtures, it was clear that addition of larger quantities of glycerol resulted in decreasing melting or gelatinization times and temperatures, and torque also decreased during treatment.
The influence of shearing-heating treatment on mechanical parameters calculated on the basis of the torque was also studied. Comparison of data acquired from results during treatment of cornstarch and waxy cornstarch showed higher values of maximal shear stress than for measurements with potato starch without or with small amounts of water, and almost identical values for initial moisture content in the 30-34% range.
With increasing water addition, the maximal shear stress values decrease; increasing glycerol content also results in decreases in the values of this parameter. Van der Veen observed for potato starch/enzyme/glycerol mixtures (30% of moisture content (m.c.)) in a shear cell device at 50 rpm that the shear stress had increased after 30 s of shearing because of starch gelatinization, but that after 2 min of treatment it had decreased, due among other factors to degradation of starch.
Almost all parameters calculated for samples of starch/glycerol mixtures without water addition showed lower values than with 5% added water.
Much lower values in SME and in shear stress multiplied by time were noted. According to previous data, it may be concluded that increasing moisture content should lower values of maximal shear stress and should thus reduce the macromolecular degradation. This can be confirmed by intrinsic viscosity measurement.
There are some differences in the extrusion behavior of potato and of cornstarch, as reported by Della Valle et al. Molten potato starch under the same conditions exhibits higher melt viscosity, earlier melting inside the extruder, and higher energy requirements. The first may be the result of the higher molecular weight of potato starch, whereas early melting requires lower transition temperatures than needed for the cereal starches.
It has been found that increasing molecular degradation is observed with increasing extrusion temperature, but the influence of SME should also be taken into account. Increasing the moisture content influenced the lower maximal shear stress, reducing the macromolecular degradation, but here the influence of the temperature on starch/water mixtures is strong.
Della Valle et al reported that an increase in moisture content increases the SME values and therefore decreases the molecular weight as measured by intrinsic viscosity. The intrinsic viscosity of native potato starch was determined to be 369.8 mL g-1. Without addition of any plasticizer (water or glycerol), the starch seems quite thermostable.
By comparing those data with data reported by van den Einde, it can be concluded that potato starch is less thermostable than cornstarch. With addition of plasticizer, a significantly reduced thermostability of starch can be observed. All achieved results are much lower than for native starch, and variation of the values is between 111 and 219 mL g-1.
This means that high macromolecular degradation takes place during shearing-heating treatment in the shear cell. This is in accordance with the findings of Fujio et al for potato, corn, and wheat starches, of van den Einde for cornstarch, of Rushing and Hester for polymers, and also of van der Veen for corn starch with added glycerol and enzymes.
Intrinsic viscosity is very sensitive, especially to thermomechanical degradation. Low values of shear stress resulted in lower intrinsic viscosities of treated starch/glycerol mixtures. Larger amounts of glycerol, and also water, in the mixtures resulted in decreasing molecular degradation of starch granules whereas intrinsic viscosity values were a little higher, but these differences are not easy to explain.
Through comparison with native starch intrinsic viscosity values, this means that over 50% of residue starch molecules were broken by thermomechanical treatment. It has been suggested that increasing moisture content decreases the degree of starch degradation due to reduction of the stresses because the melt viscosity is reduced. On the other hand, water might be involved in the chemical reactions leading to thermal degradation and the effect of moisture content might be more complex [20] .
The microstructures of shear-heated samples differ with glycerol and water levels (Figure 5.9). Figure 5.10 shows the structures of starch/glycerol mixtures with 10% added water. The smaller starch granules, in comparison with Figure 5.9, are clearly visible. That means the starch gelatinization process has taken place. There are only a few visible starch granules in the smooth homogenous network inside the samples. They start to become transparent and the milky color disappears.
Lower levels of glycerol caused better homogeneity of the mixtures than was achieved with 25% glycerol. In this sample, individual starch granules are visible, but most of the product has been transformed into a compact mass with small amounts of gas bubbles.
Levels of water higher than 10% influenced starch gelatinization. Swollen starch granules in the gelatinized starch network were observed in samples with 15% of water added, but 20% added water resulted in smooth and homogenous structures in all the samples after the shearing process.