| Autor: |
Giuseppina Barra, John R. Mitchell |
| Rok vydání: |
2013 |
| Předmět: |
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| Zdroj: |
Current Nutrition & Food Science. 9:15-25 |
| ISSN: |
1573-4013 |
| Popis: |
The rheology of caramel was determined as a function of processing temperature and hydrocolloid additions. As the processing temperature increased the water content decreased and the caramel viscosity increased. X-ray diffraction showed that although crystalline fat was present, for the most part the sugars were in the amorphous state. The exception was the lowest water content caramel (7.9% water w.w.b.) which had been processed to a temperature of 122oC. This had a small amount of crystalline fructose. Caramel rheology was assessed by rotational and capillary rheometry. Rotational rheometry gave information on the steady shear viscosity, the dynamic parameters (storage and loss moduli and related functions) and the creep compliance and recovery response. Capillary rheometry gave shear viscosities at high shear rates and an extensional viscosity. It was found that caramel without added hydrocolloids had behavior which was close to a Newtonian liquid. The only exception to this was the values obtained for the Trouton ratio which ranged from 10 to 40. This was considerably higher than the value of 3 for a Newtonian fluid and may reflect the difficulties in making measurements on these relatively low viscosity systems in the capillary rheometer. The viscosities obtained from steady shear, oscillation and creep were combined and three approaches were used to model the data as a function of measurement temperature and water content. An empirical statistical model using a second order polynomial, an Arrhenius fit and a Williams Landel Ferry (WLF) model. The former and the latter gave a good fit to the data although the constants used in the WLF model varied with the water content of the caramel. Arrhenius plots showed curvature particularly at low water contents. Incorporation of the hydrocolloids carrageenan and gellan gum into the caramel made the material non-Newtonian and elastic. For carrageenan incorporation in particular the Trouton ratio increased with carrageenan concentration reaching a value ~500 at a strain rate of 100s-1 for the caramel containing 0.2% carrageenan It was demonstrated that incorporation of carrageenan could be used to prevent cold flow in caramels processed at relatively high water contents. Glass transition temperatures were measured by differential scanning calorimetry and calculated from the temperature dependence of the shift factors used to superimpose the oscillatory rheological data. Generally there was agreement between the two approaches although for some gellan gum containing samples the rheological Tg was about 10oC higher than the DSC value. Fragility calculated from the WLF constants for caramel was high as has been reported for sugars. The Tg for both caramel and sugar water mixtures calculated using the Couchman-Karastz equation in the water content of interest (9-15% w.w.b.) was some 30-40°C higher than measured. It is suggested that this disagreement could be related to the high fragility of the sugar water systems. Isoelectric point measurements using a streaming potential technique was shown to give information on the extent of the Maillard reaction and the presence of hydrocolloids. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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