Thermal Food Processing

Time–Temperature Integrators for Thermal Process Evaluation 609
cube-shaped alginate artificial particles were developed by Brown et al., 16 who
observed discrepancies between the experimental sterilizing values and those
calculated by means of a mathematical model of temperature transfer. Another
approach was carried out by Sastry et al., 23 who developed an indicator inoculating
bacterial endospores suspended in an alginate gel and introducing it inside a
mushroom. The lack of homogeneity in the distribution of endospores observed
in this integrator hindered the interpretation of results, and in some cases, the
mushroom stalk came off the cap during the heating process.
Other problems present in TTIs developed in an alginate matrix are that some
devices cannot be stored for long 16 and the endospores can get lost by lixiviation
during the heating. Ocio 56 developed a TTI with alginate and mushroom puree
to which she added 4% starch. She observed that this allowed for the freezing of
the particle for a long period, and its later defrosting, while maintaining mechanical
characteristics similar to those of the particle that did not contain starch and that
had been stored for 7 days in refrigeration. Later on, Rodrigo 63 used an integrator
similar to the one developed by Ocio 56 to evaluate a thermal process under pilot
plant conditions. This author 63 did not find significant differences at a 5% level
of significance between the experimental lethality values and those calculated by
a mathematical model.
19.6 ALGINATE TTI ELABORATION
Alginate and food puree TTIs containing immobilized microorganisms are probably
the most versatile and the ones that approach best the physicochemical
characteristics of a real food particle (Figure 19.1). Therefore, the production
of this type of artificial particle is described below in further detail. A time–temperature
integrator based on alginate with immobilized spores inside requires a
series of previous studies related to the calibration of the sensor (microorganism,
enzyme, etc.) and the stability of the support or carrier (mechanical resistance,
capacity of retention of the sensing element inside, etc.).
19.6.1 THERMORESISTANCE OF THE SENSOR ELEMENT
Based on the example of the B. stearothermophilus spores or microbiological —
amylase, which are much used in this type of TTI, it is first necessary to consider the
thermoresistance characteristics of the microorganisms in the medium that will act
as carrier.
An important aspect in heat inactivation kinetic studies is the form of heating
and the method used for analyzing experimental data. Thermobacteriology studies
have conventionally been carried out in isothermal conditions. Determination of
kinetic parameters in these conditions is relatively simple, and it produces conservative
sterilization or pasteurization processes. However, during heat treatment
microorganisms are subjected to conditions that often differ substantially from
isothermal experimental conditions. An alternative is to apply nonisothermal
heating methods. These methods offer the advantage of subjecting the spores to