Actualizaci´ on de conocimiento

Safety and quality are at the heart of any successful aseptic operation. Kinetics of safety and quality factors strongly depend on temperature. Whether a process–product com- bination actually meets the specifi cations requires rigorous experimental validation. Process kinetics and process validation are therefore the topic of this section.

3.3.1 KINETICSOF SAFETYAND QUALITY FACTORS

Destruction of microorganisms and spores is achieved through the thermal process, though nutritional content and other quality factors are also affected. Traditionally, fi rst-order kinetics have been used to describe changes in safety and quality factors such as the destruction of microbes or nutrients. Such kinetics can be used to fi t kinetic data to the Bigelow or Arrhenius model. Both Bigelow and Arrhenius kinet- ics can be expressed as the relative rate of reaction Rr, which is the rate of change

of a microbial or quality factor at an instantaneous temperature T, relative to that at reference temperature Tr (Equation 3.7). Sensitivity of the process to tempera-

ture is accounted for through the z-value (the temperature difference required for a 10-fold change in reaction rate) and activation energy Ea for Bigelow and Arrhenius kinetics, respectively. Though Arrhenius kinetics are considered to be more accurate

in the high-temperature range (130°C–150°C) of ultrahigh temperature processing (Hallström et al., 1988; Datta, 1994), both models are extensively used.

a r r 1 1 r r Bigelow Arrhenius 10 10 E T T R T T Z R R ⎛ ⎞ − _{⎜ − ⎟} ⎜ ⎟ ⎝ ⎠ = ∨ = (3.7)

Integration of the relative reaction rate yields the equivalent process time te of the

heat treatment in seconds at the reference temperature Tr (Equation 3.8).

e r

0

d

t

t = R

∫

Several common names exist for te depending on the kinetic constants and the refer-

ence temperature chosen. For process safety, using a z-value of 10°C and a reference temperature Tr = 121.1°C (250°F) yields the well-known F0-value, or a “proteolytic

Clostridium botulinum” cook. In pasteurization processes, the equivalent time is

often referred to as the P_Tr-value, where a relevant reference temperature is chosen, normally in the 70°C–100°C range.

The time required at the reference temperature Tr to obtain a 10-fold reduction

(1 log) of the microorganism population is called the D_Tr-value. In safety calcula- tions, dividing the equivalent process time by the D_Tr-value yields the number of logarithmic reductions in the original microbial load N0 (Equation 3.9).

r 0 _log 0 T F N D = N (3.9)

To enable relevant safety or spoilage calculations, a critical target organism must be identifi ed and a number of log reductions chosen. Based on the D_Tr, Z, and Tr for said

microorganism, a minimum equivalent process time is then calculated.

Equations 3.7 and 3.8 can also be applied to quality factors such as color, texture, nutrient content, enzyme inactivation, or fl avor development (Holdsworth, 1992). The equivalent process time is referred to as C_Tr- or cook-value, when pertaining to textural change and using a reference temperature of 100°C. In that case the z-value depends on both the textural aspect and ingredient. Application of the decimal reduction time D_Tr to quality factors is also possible, and tables of the kinetic data are available in the literature.

It should be noted that Equations 3.7 through 3.9 apply to a liquid or solid element with a uniform temperature and no RTD. The intricacies of calculating safety and quality factors in nonuniform heterogeneous systems (both regarding temperature and residence time) are discussed in greater depth in Chapter 15.

3.3.2 VALIDATIONOF ASEPTIC LINES

Thermal processes need to be validated in order to establish that suffi cient thermal treatment has been applied. Legal requirements are different in each country: while

the EU states that each company is responsible for the safety and spoilage, the United States has a very stringent control organization in the Food and Drug Administration (FDA). FDA rules separate acid and low-acid foods, the latter have been compiled in the 21CFR113.3 and constitute a complex body of regulation where scientifi c valida- tion of the heat treatment is required. As product safety is obtained using a thermal process, validation focuses on the coldest part of the product either for liquids or for heterogeneous foods.

Traditional validation for retortable cans involves either the use of temperature sensors in the coldest spot or inoculation with viable thermally characterized micro- organisms (bacterial endospores). This was fi rst done in the 1920s by Bigelow and confi rmed by Ball in the 1950s. Spores of surrogate microorganisms were also inves- tigated, and one of the main concerns was to fi nd a microorganism with similar death kinetics to the microorganisms of concern (C. botulinum). For the juice indus- try the FDA defi ned a surrogate microorganism as “any nonpathogenic microorgan- ism that has acid tolerance, heat resistance, or other relevant characteristics similar to pathogenic microorganisms.” Clostridium sporogenes PA3679 is the recognized surrogate of C. botulinum. Pfl ug et al. (1980) demonstrated the use of Geobacillus

stearothermophilus and Bacillus subtilis as surrogate microorganisms. Spores of

these microorganisms are commercially available from several sources and have become the standard for validation of low-acid foods requiring sterilization.

In aseptic processing, products are processed separately from the fi nal container and the methods of validation are different. When homogeneous products are pro- cessed, the line must be designed to properly sterilize the part of the product which receives the least thermal treatment, i.e., fastest and coldest throughout the process. This is normally the center of the holding tube, where the velocity is highest and the temperature is lowest. In this case, temperature data are required at the inlet and exit of the holding section in the center of the tube. However, the response time of the thermo- couples must be short, and they should be suffi ciently small not to act as heat sinks.

Heterogeneous fl ows will generally have the coldest spot in the center of the slowest heating particle. There are several methods to monitor the temperature inside the particulates in real time. Moving thermocouples have been used in research though this method is not practical in an industrial environment (Balasubramanian, 1994; Ahmad et al., 1999). Magnetic resonance imaging can also be used to monitor the temperatures of moving particulates, but the cost is high and industrial relevance is low.

More practical off-line solutions have therefore been developed to monitor the integrated thermal effect and these include either by using real or simulated particles that carry spores of a known microorganism. Such time–temperature integrators (TTIs) can be of any of the following types: immobilized spores, chemical or bio- chemical markers, thermomagnetic markers, and combinations of the above (Tucker, 1999; Tucker et al., 2002).

Immobilized spores in calcium alginate beads have been used to evaluate the ultra high temperature (UHT) processes for several decades, and viable spores of

G. stearothermophilus and B. subtilis are inoculated into a sodium alginate solution

to a known concentration (Holdsworth, 1992). The alginate is then formed into par- ticulates of a certain size and shape and solidifi ed by immersion in a calcium-salt

solution. The alginate particulates can be made at any size or shape to simulate real food particulates or can be made as small alginate spheres (4 mm diameter), which are located into the center of existing food particulates (Tucker and Holdsworth, 1991).

Bioindicators are similar to the above, with the exception that the spores are inoculated in an indicator culture media contained inside small plastic pouches ( Brinley et al., 2007), thus reducing the risk of contamination to the products. Residual viability following heat treatment is indicated by a change in color of the culture media or by further culturing of the surviving spores.

Chemical markers are substances that exhibit irreversible changes due to the exposure to time–temperature such as luminescence, color changes, or following known kinetics of destruction or generation. Most chemical markers cannot be reused and only few chemical reactions with similar kinetics to spores have been identifi ed (Kim et al., 1996). Enzymes are probably the most promising of the chemical substances to be used as TTIs: inactivation kinetics are similar to those of microbes and food quality; they present low toxicity and can be obtained in high purity commercially (Tucker et al., 2002). Small quantities of enzyme can be located inside small tubes or pouches which are inside food or simulated particulates. A simple assay determines the remaining activity of the enzyme, which is related to the thermal treatment the enzyme received (Tucker and Holdsworth, 1991). How- ever, the stability of most enzymes above 100°C is unsatisfactory and their use has been limited to temperatures in the pasteurization range (Tucker, 1999; Tucker et al., 2002). Recently, high-temperature enzyme isolates from Pyrococcus furiosus have been identifi ed (Tucker et al., 2007) and also from B. licheniformis (Guiavarc’h et al., 2004). Applications of these are currently under development.

Thermomagnetic markers make use of changes in the magnetic fi eld of two magnets after such magnets are allowed to combine. To do so, glue with a well- defi ned melting point (e.g., eutectic alloys) is placed between two magnets. When the melting point is reached, the glue melts and allows the magnets to join, thus gen- erating disturbances in the magnetic fi eld that can be monitored by sensors located outside of the tube. Placing sensors in different locations of the heating and holding units yields an indication of the thermal treatment of particles along the system. The trajectory and residence time of each particulate can also be reconstructed by moni- toring the three-dimensional position of each particle in each sensing location. Since several magnet–glue pairs can be stacked inside one particle, a temperature history can be determined and used to optimize the thermal treatments (Simunovic et al., 2004). Recovery of magnetic particles at the end of the line is relatively simple: a metal detector can be used to recover only the particles containing the magnets.

Whatever the principle of TTIs, practical use is still far from straightforward in aseptic processing of heterogeneous food. A signifi cant number of particles are required for statistically meaningful results (the FDA requires at least 300 particu- lates) and these must be injected and retrieved intact and in-full from the system. In addition, size, shape, buoyancy, and thermal conductivity must be representative of the target real particles. Though progress in this fi eld is encouraging, the ideal would be miniature digital temperature loggers capable of surviving the aseptic process, which would be detectable throughout the line to establish particle fl ow trajectories.