Jet-cooked papermaking starches studied using ¹H NMR-relaxometry and viscometry

Materials

The studied starches were provided by Chemigate Oy, Finland. Raisamyl 70021 and Raisamyl 80051 are cationized, high molecular weight (MW) wet-end starches that have a degree of substitution (DS) of 0.042 and 0.044, respectively. Raisamyl 23151 is a cationized and oxidized surface sizing starch with a DS of 0.018 and a medium MW of approximately a few million Da. Raisamyl 01151 is an oxidized coating binder starch with a smaller MW of approximately a million Da. Raisamyl 70021 is a potato starch, whereas the other starches are barley starches. The starch slurry was prepared into cold (18 °C) tap water.

NMR-equipment

The ¹H NMR-relaxometry measurements were performed using a TD-NMR (time-domain NMR) system (Resonance systems ltd., 2019). The ¹H resonance frequency of the system was 26 MHz, and the temperature of the magnet was 30 °C. Spin-spin relaxation rates R2 were measured by applying the CPMG pulse sequence (Abragam 1961). The echo time was 6 ms, and there were 600 180° pulses in the sequence. The relaxation delay was 6 s, and there were four scans. The durations of the 90° and 180° RF pulses were 7 μs and 16 μs, respectively.

For the measurements done immediately after jet-cooking, the system was modified for flowing samples, and the sample was automatically pumped through the system and measured. For the other measurements, the samples were simply measured in test tubes.

The instrument error of the NMR-equipment has been previously determined to be 0.72 % for black liquor samples (Nikolskaya et al. 2019).

Cooking and measurements

The jet-cooker used in this study is similar to the cookers used in paper mills; the starch slurry is continuously pumped into the system where it is mixed with steam, the heat and shear forces break down the starch structure, and the starch becomes gelatinized. Finally, the starch is pumped into a steam separator and a sample is collected. The cooking temperature is controlled by adjusting the amount of steam and backpressure. The cooking time is adjusted by changing the pump’s speed. Due to the mixing of steam with the slurry, the solids content of the solution is reduced by approximately 25 % during cooking. Table 1 shows a summary of the studied starches’ characteristics and the starch slurry solids contents before jet-cooking and during measurement.

After cooking, one sample was collected for NMR measurement, one for viscosity measurement and three samples for storage at 40, 60 and 90 °C. In addition, the surface sizing and coating binder starches were diluted with boiling water from a consistency of approximately 25 % to 13 % and from 35 % to 21 %, respectively, before sampling. The NMR and viscosity samples were allowed to cool to room temperature at approximately 1 °C/min. The NMR relaxation was measured on-line according to the following procedure:

1. A sample was continuously mixed in a container.

2. The sample was automatically pumped through the magnet system.

3. When a new sample was introduced to the system, the pump was stopped.

4. The CPMG signal was automatically measured and fitted using a mono-exponential function, and the R2 value was solved.

5. The next sample was pumped.

The high-temperature retrogradation (HTR) sediment and supernatant were collected from a separate container, so as not to interfere with the NMR and viscosity measurements. The sediment and supernatant were separated by centrifuging at 5000 rpm for 5 min in a Hermle Z 200 A centrifuge. The HTR samples were then moved back into 90 °C for 30 minutes to prevent cooling from affecting the results, and finally their NMR relaxation was measured.

The relationship between starch solids content and NMR relaxation rate was determined by diluting the surface sizing barley starch solution with 27 °C water at the end of the NMR measurements done immediately after cooking, when the solution had cooled to 27 °C.

NMR relaxation and viscosity after jet-cooking

Figure 1 shows the R2 relaxation rates for the studied starches. The NMR curves representing the cooling of wet-end starches on the left are nearly identical to the curves observed for cationic potato and barley wet-end starches in a previous study, in which batch cooking at 95 °C was studied at a similar consistency (Raunio et al. 2018). The points at approximately T=20°C and R2=0.8s−1 correspond to water only.

Figure 1

NMR relaxation rates after jet-cooking. The surface sizing barley starch NMR relaxation curve is missing some data points due to measurement set-up problems.

In Figure 1, the relaxation rate of jet-cooked wet-end potato starch remains very low (close to pure water) even at low temperatures. This is interpreted to mean that the starch has been fully gelatinized, and that it does not retrograde even at lower temperatures (60–30 °C). Wet-end barley starch behaves in a completely different manner; its R2 value is quite low at a high temperature, but unlike in the case of potato starch, the R2 quickly rises with decreasing temperature until about 50 °C. Interestingly, there is a small decrease in the lowest temperatures (40–30 °C).

This result can be interpreted in two ways. 1) The barley starch is fully gelatinized during jet-cooking, but it starts to retrograde anyway immediately afterward as temperature decreases, perhaps due to the lipids present in cereal starches forming complexes with amylose (Fanta et al. 1999), or simply due to the formation of an amylose network (BeMiller and Maurer 2005). 2) The barley starch has actually not been fully gelatinized, and because of this, it retrogrades quickly due to ghost particles left into the solution, which act as seed crystals (Smits et al. 2003). The second option seems implausible due to the high temperature and shear forces present in jet-cooking, which should break apart the starch structure very thoroughly.

Changing the cooking temperature from 120 °C to 140 °C or the cooking speed from 1.6 kg/min to 1.0 kg/min had a very small effect on the post-cooking R2 values of the wet-end barley starch, which probably means that such a small difference in cooking temperature or time did not have a significant effect on the starch’s gelatinization or retrogradation.

Figure 2

Viscosities measured after jet-cooking.

In the case of surface sizing and coating binder (barley) starches, the shapes of the NMR cooling curves are a bit different from the wet-end starches, and the R2 values are larger. The higher R2 values for these starches can partially be explained by the higher solids content, which will be demonstrated later, but some of the increased relaxation rate is probably also related to the structure of the starch. The coating binder starch stands out from the measured starches in that its R2 value is very high even in T>70°C, whereas the R2 values of all the other starches at high temperatures (70–90 °C) are quite low (R≈1.0–1.5s−1).

At the beginning of this study, it was expected that jet-cooking would enable the barley starch to become more highly gelatinized than in the case of atmospheric batch cooking, which would be reflected in the amount of retrogradation and thus in the R2 values. In addition, it was expected that the cooking temperature and speed would be reflected in the NMR measurements. Based on this data, it seems that barley starch retrogrades quickly with decreasing temperature, despite being “well-cooked”.

The formation of amylose-lipid complexes after jet-cooking is likely an explaining factor for the differences between the otherwise similar wet-end potato and barley starch. In a study by Fanta et al. (2002), potato starch did not form these complexes, whereas wheat starch, which is a cereal starch like barley, did form such complexes. Another probable factor for these differences is that cereal starches are known to contain more amylose than potato starch, which causes them to retrograde faster due to an amylose network forming into the solution (BeMiller and Maurer 2005).

Figure 2 shows the viscosities measured from the starch solutions after jet-cooking. The starches cooked at the lower temperature have the highest viscosities. This was to be expected, because it is known that increasing cooking temperature can cause the starch chains to break down, which reduces viscosity (Lehtinen 2000). The barley starches cooked at 140 °C all have quite a similar viscosity behavior, even though they (wet-end, surface sizing and coating binder) are structurally different from each other. Changing the cooking speed of wet-end barley starch did not have an effect on the viscosity, just as it did not have an effect on the NMR relaxation rates. This probably means that such a small change (1.6→1.0 kg/min, i. e. 60 %) in jet-cooking speed did not have a meaningful effect on the starch properties.

By comparing Figures 1 and 2, it seems that the NMR curves and viscosity curves do not correlate with each other. This is most pronounced in the case of wet-end potato starch, where the post-cooking R2 value remained almost constant but the viscosity increased rapidly with decreasing temperature. To gain more information about the correlations between the R2 values and viscosity, linear correlations were calculated for each sample. The R2 and viscosity values used for the calculations were picked in such a way that the temperature difference at which they were measured was as small as possible (ΔT<1°C). This procedure yielded 4–6 comparable points per sample for the calculations.

As seen from Table 2, the R2 values and viscosities of the surface sizing and coating binder starch samples correlate highly. However, this correlation is not present in the wet-end starch samples. I. e., sometimes the R2 values correlate with viscosity and sometimes they do not. The reason for this is unclear.

Based on the inconsistent correlation of R2 values and viscosities, and based on Figures 1 and 2, it is interpreted that in these starch-water systems, the changes in R2 values are dominated by changes in the structural properties of the starch, and that the effects of temperature (Arrhenius effect) and viscosity on relaxation rates are not visible.

Such an interpretation of the relationship of viscosity and NMR relaxation comes into some conflict with other studies about similar topics, such as the study by Nivelle et al. (2019), where viscosity and temperature play a significant role in explaining the NMR relaxation results. One explanation for this difference could be that in our studies the solids content is much lower than in the case of bread-making studies, which has an influence not only on the viscosity but also on the molecular dynamics between water and starch.

NMR relaxation and viscosity during storage

The R2 and viscosity results for the wet-end potato and barley starches stored at 40, 60 and 90 °C are shown in Figures 3–8. The relaxation rate of potato starch (Figure 3) stored at all temperatures remained at a very low R2 value throughout the whole period, which likely means that there was no retrogradation occurring. The viscosity of the potato starch samples held at 40 and 60 °C stayed almost constant, indicating that no changes happened. However, at 90 °C, the viscosity decreased over time, which probably means that the starch chains degraded over time due to the high temperature.

For the wet-end barley starch (Figure 4), the relaxation rates remained rather constant over time, as they did for the potato starch jet-cooked at the same temperature. The only difference is that now the “starting points of R2” are different because of how the barley starch retrogrades after cooking. In the case of the sample stored at 60 °C, after 21 h of storage, the R2 value “peaked”, which might be a measurement error. Due to the small number of consecutive measurements (n=5), one measurement point is not enough to draw any conclusions. The viscosities remained quite constant. The results indicate that neither retrogradation nor chain degradation happened on a meaningful level at any storage temperature.

For the wet-end barley starch cooked at a higher temperature (Figure 5), the viscosity was much lower, as was described before (Figure 2), and the R2 and viscosity values remained quite constant over time, as was the case for the same starch cooked at a lower temperature.

Changing the cooking speed of the wet-end barley starch (Figure 6) did not have a significant effect on the viscosity or R2 values.

The surface sizing barley starch samples (Figure 7) might have undergone some retrogradation at 40 and 60 °C because their R2 values increased over time, but the changes were small. Viscosities remained very low throughout the storage for all temperatures.

Figure 3

NMR relaxation rates and viscosities for wet-end potato starch jet-cooked at 120 °C and stored at different temperatures.

Figure 4

NMR relaxation rates and viscosities for wet-end barley starch jet-cooked at 120 °C and stored at different temperatures.

Figure 5

NMR relaxation rates and viscosities for wet-end barley starch jet-cooked at 140 °C and stored at different temperatures. Some data points from viscosity at 90 °C are missing due to measurement set-up issues.

Figure 6

NMR relaxation rates and viscosities for wet-end barley starch jet-cooked more slowly at 140 °C and stored at different temperatures.

Figure 7

NMR relaxation rates and viscosities for surface sizing barley starch jet-cooked at 140 °C and stored at different temperatures.

After one day in storage, a sediment began to form in the surface sizing barley starch sample at 90 °C. This was interpreted as high-temperature retrogradation (HTR), which is known to happen for some starches during storage (Davies et al. 1980, Fanta et al. 2002). Cereal starches, such as barley, have more lipids than potato starch, and lipids are known to have an important role in the HTR mechanism (Fanta et al. 2002). The forming sediment is probably comprised of amylose-lipid complexes, which have been described in previous studies (Davies et al. 1980, Fanta et al. 2002, Nivelle et al. 2019, Putseys et al. 2009), and are known to form at 90 °C. The sediment formation was not visible in these R2 or viscosity results. This was perhaps due to the samples being mixed before measuring, which resuspended the sediment.

The coating binder barley starch (Figure 8) expressed much of the same behavior as the surface sizing barley starch (Figure 7): a sediment formed at 90 °C after one day of storage, viscosities remained stable in all temperatures, and the samples at 40 and 60 °C remained stable based on the R2 values. However, the sample at 90 °C exhibited a clear decrease in R2 value over time, which might be a result of the HTR sediment formation. Why this happened for the coating binder starch, but not for the surface sizing starch, is unknown. It is important to note that the scale of the R2 y-axis in Figure 8 is different from Figures 3–7.

Figure 8

NMR relaxation rates and viscosities for coating binder barley starch jet-cooked at 140 °C and stored at different temperatures.

To summarize, the relaxation rates and viscosities for most samples did not significantly change over time when stored at a constant temperature. The R2 and viscosity results for wet-end starch samples stored at 40, 60 and 90 °C (Figures 3–6) were quite similar for all of the samples. This was interpreted to mean that the wet-end starches generally remained stable at all temperatures and that significant retrogradation did not occur. The exception to this behavior were the surface sizing and coating binder barley starches stored at 90 °C, which exhibited formation of a sediment starting after 1 day of storage.

High-temperature retrogradation samples

Figure 9 shows the R2 values for the supernatant and sediment separated from the surface sizing and coating binder barley starch HTR samples. The solids content of the sediment and supernatant samples are shown in Table 3 along with the solids content before separating the phases. As could be expected, the consistency of the supernatant was lower than the consistency of the original solution after cooking, and the consistency of the sediment was higher.

Figure 9

NMR relaxation rates for the supernatant and sediment separated from the surface sizing and coating binder barley starches jet-cooked at 140 °C after 2 days of storage at 90 °C.

It can be seen from Figure 9 that the sediments of both starches have a much higher R2 value than the supernatants. As will be shown in the next subsection, consistency does have an effect on the R2 value but such a large difference as seen in Figure 9 can only be explained by a difference in the structure of the supernatant and sediment. I. e., the sediment had a large amount of retrograded solid material, and the supernatant had mostly soluble material. The supernatant probably contained un-retrograded, solubilized amylopectin, which is known to retrograde slowly (Wang et al. 2015), and the sediment contained retrograded amylose or amylose-lipid complexes, as has been reported previously for corn, rice and wheat starches (Fanta et al. 2002).

The reason why only surface sizing and coating binder barley starches formed a significant amount of HTR sediment at 90 °C storage, but the wet-end barley starches did not, although they should have approximately the same lipids content, could be that the lower molecular weight of the surface sizing and coating binder starches facilitates HTR. Shorter chains are known to predispose a starch to retrogradation via amylose-lipid complex formation (Putseys et al. 2009).

Effect of solids content on NMR relaxation

The starch solution’s solids content and the NMR relaxation rate have a linear relationship, as is evident from Figure 10. As the solids content decreases, the proportional amount of water interacting with the starch also decreases. This leads to increased overall proton mobility and thus decreased R2 values.

Figure 10

Relationship between the NMR relaxation rate and solids content measured from surface sizing barley starch at 27 °C.