Published sorption curves in the form of moisture vs time relationships of milk powder and rice, exposed to moist atmosphere or soaked In water, were fitted by a two parameter, nonexponential empirical model. The model enabled prediction of moisture contents after long exposure from experimental data obtained in relatively short time, i.e., well before the moisture level appeared to reach a plateau. The model implied that the moisture equilibrium was somewhat higher than that determined on the assumption that the sample reached a constant weight, but there was no conclusive evidence that this was really the case.
The heat inactivation of microbial spores and the mortality of vegetative cells exposed to heat or a hostile environment have been traditionally assumed to be governed by first-order reaction kinetics. The concept of thermal death time and the standard methods of calculating the safety of commercial heat preservation processes are also based on this assumption. On closer scrutiny, however, at least some of the semilogarithmic survival curves, which have been considered linear are in fact slightly curved. This curvature can have a significant effect on the thermal death time, which is determined by extrapolation. The latter can be considerably smaller or larger depending on whether the semilogarithmic survival curve has downward or an upward concavity and how the experimenter chooses to calculate decimal reduction time. There are also numerous reports of organisms whose semilogarithmic survival curves are clearly and characteristically nonlinear, and it is unlikely that these observations are all due to a mixed population or experimental artifacts, as the traditional explanation implies. An alternative explanation is that the survival curve is the cumulative form of a temporal distribution of lethal events. According to this concept each individual organism, or spore, dies, or is inactivated, at a specific time. Because there is a spectrum of heat resistance in the population--some organism or spores are destroyed sooner, or later, than others--the shape of the survival curve is determined by its distributions properties. Thus, semilogarithmic survival curves whether linear or with an upward or a downward concavity are only reflections of heat resistance distributions having a different, mode variance, and skewness, and not of mortality kinetics of different orders. The concept is demonstrated with published data on the lethal effect of heat on pathogens and spores alone and in combination with other factors such as pH or high pressure. Their different survival patterns are all described in terms of different Weibull distribution of resistances as a first approximation, although alternative distribution functions can also be used. Changes in growing or environmental condition shift the resistances distribution's mode and can also affect its spread and skewness. The presented concept does not take into account the specific mechanisms that are the cause of mortality or inactivation--it only describes their manifestation in a given microbial population. However, it is consistent with the notion that the actual destruction of a critical system or target is a probabilistic process that is due, at least in part, to the natural variability that exists in microbial populations.
Relaxation curves of Agar gel, apple, bologna sausage, bread, cheddar cheese, pear and potato specimens, at variods deformation levels, were normalized and fitted to the equation: [F, -F(t)J/F, = abt/(l + bt) where F, is the initial force, f(t) the force after time t and a and b constants. Unlike other equations (e.g. a series of exponential terms derived from a Maxwellian model), this equation contains only two constants and these are directly related to the curve shape features. This enables simple comparison between the shape characteristics of curves of different materials. Similarly, the equation facilitates quantitative evaluation of the effects of the straining history on the shape of the stress relaxation curves of solid foods.
Published sigmoid moisture sorption isotherms (0 < aw < ∼ 0.9) were fitted by the four parameter models m = k1awn1+ k2awn2 where m is the moisture contents (dry basis), aw the water activity and the k and n values are constants (n1 < 1 and n2 > 1). Not surprisingly, the model had the same or better fit than the GAB model. In contrast with both the BET and GAB models, the proposed model is not based on the assumption that there exists a well‐defined monolayer of absorbed water. At aw < ∼ 0.4 and n1 > ∼ 0.55, however, the model produces a practically linear aw/[m(1 – aw)] vs aw plot, of the kind used to calculate the monolayer moisture with the BET model. The proposed model can be a convenient means to catalog both sigmoid and nonsigmoid isotherms, and used to calculate the equilibrium water activity of dry mixtures with equations solving software.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.