12.1 BASIC PHENOMENA IN MALAXATION
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join into the lipid phase. In conclusion, water, with its hydrogen-bonding ability, provides the force for separating the oil. The separation of the oil is more precisely described as an expulsion rather than an extraction phenomenon.
12.1.2 The time-temperature relationship Considering the temperature influence on the malaxing operation, it may be observed that: • coalescence of the oil increases with the increasing in the temperature and decreasing in viscosity; • diffusion of substances from the aqueous to the oil phase is also accelerated by an increase in temperature. On the other hand, an excessively high temperature may have detrimental effects on the oil quality due to: • enzymatic changes involving both lipolytic and oxidative degradation; • increase in the vapour pressure of the volatile compounds causes some loss of aromas. In the malaxing operation, time-temperature conditions should be chosen to maximize the desired and minimize the undesired effects. In general, the extent of a transformation depends on the reaction rate and time. The reaction rate is accelerated by an increase in temperature according to an exponential relationship, while a linear relationship links the extent of the transformation to time. A way to represent time/temperature relationships of technological operations is in a semi-log graph, with temperatures on the abscissa in a linear scale and times on the ordinate in a logarithmic scale. In this type of graph, the time-temperature conditions producing equivalent effects of transformations or reactions are represented by oblique lines. Based on literature information and practical experience, Figure 12.2 shows the range of time-temperature conditions that are suitable for optimal malaxation (Peri 2013). The temperature interval on the abscissa is between 20 and 35 ∘ C, whereas the times on the ordinate in logarithmic scale vary from 10 to 100 minutes. The two oblique lines indicate the conditions within which coalescence takes place at a suitable rate. The rectangular area traced in the centre of the diagram, between 25 and 30 ∘ C and between 20 and 50 minutes, is the area in which the best compromise between yield and quality can be found.