Manufacture of 'Xixona Turron': Important Properties for the Characterization
José Antonio Caballero
Departamento de Ingeniería Química. Universidad de Alicante
'Xixona Turron' is the product of cooking a mixture of milled and roasted almonds with caramel and honey with or without albumin. The product is manufactured in batch mode in a 'boixet' in where the paste, finely milled, is continuously mixed and heated until the final product is obtained. Traditionally, there are no objective criteria to determine the final point of the process, and it is the experience of the boixet operator that determines when the process is finished.
The process of cooking is the most important step in the manufacture and it is characterized by marked changes in texture and viscosity of the paste. The paste is initially fluid due to the oil released by the roasted and milled almonds. The caramel is particulated and separated from the almond cell fragments (cell wall fragments, starch granules, etc...). As the process progress (the paste is heated and stirred) the caramel softens, traps and includes the almond structure producing a phase inversion. At the beginning of the process the oil is the continuous phase; whereas at the end, a structure of caramel-almond flour dominates the product.
A large increase of the viscosity of the paste can be observed coupled with the phase inversion, and this can be used as an objective parameter to determine the final point of the process. The change in viscosity is directly related to the kinetics of the phase inversion.
The first reference to the phase inversion, although this nomenclature was not used, was by Galotto and Urra, who described qualitatively the process and carried out a study of the micro-structural changes. The influence of the variables was not studied in this work.
Galotto and co-workers presented rheological studies of the turron mass at different times in the "boixet mixer", but always before the final stage of the process where the main changes take place.
Marcilla and Martinez were the first to study the qualitative effects of the variables (temperature, heating rate, free oil etc). Latter, Marcilla et al proposed a semi-empirical mathematical model to explain the changes in the viscosity of the paste during the phase inversion. In that work viscosity was not measured directly.
Actually, the stirring torque was measured whereas the paste was heated in a jacketed mixer with constant jacket temperature. The temperature-time profile, could not be fixed but it was determined by the heat transfer through the jacket to the paste, and the heat absorbed by the paste during the phase inversion. In that study, a delay in the viscosity change with respect to the temperature profile was observed probably due to heat transfer problems.
On the other hand, the change in the visco-elastic properties that take place in the paste when the turron is heated can be studied using dynamic oscillatory tests, which are based in the application of a sinusoidal stress to the sample and the measure of the response of the system. This technique has been widely applied in the rheological characterization of a large number of products, most of them in the polymer technology: study of the gelation and melting of PVC plastisols studies of rheological behaviour of PVC gels, study of the crosslinking of PDMS (polydimethylsilonane) etc.
Detailed information about the evolution of viscosity during the cooking process of Xixona Turron can be found in:
Marcilla, A; Caballero, J.A.; Conesa, J.A.; Matínez, E.; "Study of the Rheological Properties of the Paste During the Phase inversion in the manufacturig of Xixona Turron". Journal of Food Engineering 26, 231-241 (1995).
Caballero, J.A.; Marcilla, A.; García, J.C.; "Mathematical Modelling of the Rheological Behaviour of the Paste of Xixona Turron During the Cooking Process". Journal of Food Engineering 34, 315-329 (1997)