M.A.J.S. van Boekel

On the use of the Weibull model to describe thermal inactivation of microbial vegetative cells

Abstract This paper evaluates the applicability of the Weibull model to describe thermal inactivation of microbial vegetative cells as an alternative for the classical Bigelow model of first-order kinetics; spores are excluded in this article because of the complications arising due to the activation of dormant spores. The Weibull model takes biological variation, with respect to thermal inactivation, into account and is basically a statistical model of distribution of inactivation times. The model used has two parameters, the scale parameter α (time) and the dimensionless shape parameter β. The model conveniently accounts for the frequently observed nonlinearity of semilogarithmic survivor curves, and the classical first-order approach is a special case of the Weibull model. The shape parameter accounts for upward concavity of a survival curve (β 1). Although the Weibull model is of an empirical nature, a link can be made with physiological effects. β 1 indicates that the remaining cells become increasingly damaged. Fifty-five case studies taken from the literature were analyzed to study the temperature dependence of the two parameters. The logarithm of the scale parameter α depended linearly on temperature, analogous to the classical D value. However, the temperature dependence of the shape parameter β was not so clear. In only seven cases, the shape parameter seemed to depend on temperature, in a linear way. In all other cases, no statistically significant (linear) relation with temperature could be found. In 39 cases, the shape parameter β was larger than 1, and in 14 cases, smaller than 1. Only in two cases was the shape parameter β=1 over the temperature range studied, indicating that the classical first-order kinetics approach is the exception rather than the rule. The conclusion is that the Weibull model can be used to model nonlinear survival curves, and may be helpful to pinpoint relevant physiological effects caused by heating. Most importantly, process calculations show that large discrepancies can be found between the classical first-order approach and the Weibull model. This case study suggests that the Weibull model performs much better than the classical inactivation model and can be of much value in modelling thermal inactivation more realistically, and therefore, in improving food safety and quality.

Effect of heating on Maillard reactions in milk

Heated milk is subject to the Maillard reaction; lactose and lysine residues in milk proteins (mainly casein) are the reactants. An overview is given of the early, advanced and final stages of the Maillard reaction as it occurs in milk. The early Maillard reaction is confined to the formation of the protein-bound Amadori product lactulosyllysine. Breakdown of the Amadori product leads to formation of all kinds of advanced Maillard reaction products such as lysylpyrraline, pentosidine, hydroxymethylfurfural, (iso)maltol, furfurals and formic acid. The content of these compounds in heated milk is, however, very low (with the exception of formic acid), and does not correspond to the breakdown of Amadori product in quantitative terms. The final stage, in which melanoidins (brown pigments) are formed and protein polymerization occurs, is largely unknown from a chemical point of view, let alone quantitatively. The conclusion can only be that not all important compounds are yet identified. Some experimental data for heated milk are given to illustrate the various stages of the Maillard reaction in heated milk. A kinetic analysis of the Maillard reaction is difficult because it is such a complicated reaction with many parallel and consecutive steps; in addition, one of the reactants, lactose, is also subject to another reaction, namely isomerization followed by degradation. The kinetics can be tackled by kinetic, multiresponse modelling, and this approach is illustrated. It appears that the temperature dependence of the (early) Maillard reaction is lower than for the simultaneously occurring isomerization reactions of lactose. The use of several components formed in the Maillard reaction to evaluate the heat intensity given to milk is discussed.

Long-term global availability of food: continued abundance or new scarcity?

Abstract During the 20th century hunger has become a problem of poverty amidst plenty rather than absolute food scarcity. The question is whether this will remain so or whether the hunger of the poor will once more be exacerbated by rising food prices. In this paper we discuss biophysical conditions, social forces and non-linear interactions that may critically influence the global availability of food in the long term. Until 2050, the global demand for primary phytomass for food will more than double, while competing claims to natural resources for other purposes (including biobased non-foods] will increase. A sober assessment of the earths biophysical potential for biomass production, which recognizes competing claims and unavoidable losses, suggests that this is in itself still large enough for accommodating this rising demand. However, the exploitation of this biophysical potential proceeds through technical paradigms that set a relative maximum to food production. In addition, socio-economic mechanisms make the food economy run up against a ceiling even before this maximum is reached. As a consequence, current developments may well entail a new trend change in international markets. These developments include the depletion of land and water reserves, the stagnation of the potential yields of major crops, the rise in energy prices, and the way in which systemic socio-economic factors lead to a strong underutilization of production possibilities in the developing world. Given these conditions, the avoidance of steep rises in food prices may depend on the timely relaxation of socio-economic constraints in developing countries and on timely breakthroughs in sustainable yield increases, biorefinement and non-farm production systems. Myopic expectations make it doubtful whether spontaneous market forces will provide the necessary incentives for this, which may be reason for societal actors to consider the need for more active policies.

Baobab Food Products: A Review on their Composition and Nutritional Value

Several authors have published about baobab food products. Data on macronutrients, micronutrients, amino acids, and fatty acids were collected from literature for pulp, leaves, seeds, and kernels of the baobab tree. The results show that baobab pulp is particularly rich in vitamin C; consumption of 40 g covers 84 to more than 100% of the Recommended Daily Intake (RDI) of pregnant women (19–30 years). The leaves are particularly rich in calcium (307 to 2640 mg/100 g dw), and they are known to contain good quality proteins with a chemical score of 0.81. The whole seeds and the kernels have a relatively high lipid content, 11.6 to 33.3 g/100 g dw and 18.9 to 34.7 g/100 g dw, respectively. The pulp and leaves exhibit antioxidant properties with a higher activity in the pulp than in the leaves. Reported nutrient contents of different baobab parts show a large variation, which may have arisen from various factors. Three recommendations are given for future research: 1. More attention should be given to accuracy and precision of analytical methods, 2. Research about digestibility and bioavailability of baobab products is needed, 3. The effect of storage and processing on the nutritional value of baobab products needs to be assessed.

Heat Inactivation of Bovine Plasmin.

Abstract The thermal behaviour of the protease plasmin was studied in the absence and presence of milk components. A global unfolding of the isolated plasmin occurred at 50–55°C, but secondary structure of plasmin increased with increasing temperature, possibly due to aggregation. Plasmin was reversibly inactivated between 55 and 65°C and was not subject to autoproteolysis. Irreversible inactivation (obeying first-order kinetics) started above 65°C with a temperature dependence typical for protein unfolding. Interaction with cystein caused irreversible (first-order) inactivation of plasmin from 45°C upwards; kinetics suggested a mixed mechanism of unfolding and SH/SS interaction. In agreement with literature, casein protected plasmin from irreversible inactivation while β -lactoglobulin increased inactivation. Casein partly compensated the destabilizing effect of β -lactoglobulin. The high heat stability of plasmin in milk products appeared not to be due to a high conformational stability, but to protection by casein towards irreversible inactivation of the unfolded enzyme.

Testing of kinetic models: usefulness of the multiresponse approach as applied to chlorophyll degradation in foods

Cascades of reactions, in which several reactants and products take part, frequently occur in foods. This work shows that kinetic modelling of such reactions having parameters in common is much more powerful when using a multiresponse rather than a uniresponse approach (i.e. analysing more than one reactant/product at the same time rather than only one reactant or product). With multiresponse modelling proposed kinetic models can be tested rigorously for their validity (not really possible with the uniresponse approach), and the precision of the parameters estimated is much increased. Multiresponse modelling requires (in most cases) not minimization of least squares but that of the determinant of the so-called dispersion matrix, consisting of sums of cross-products of the responses measured. Kinetic models describing breakdown of chlorophyll in foods were taken from literature and multiresponse modelling was shown to lead to a better insight in, and indeed improvement of such kinetic models.

Pulsed electric field processing of different fruit juices: Impact of pH and temperature on inactivation of spoilage and pathogenic micro-organisms

Pulsed electrical field (PEF) technology can be used for the inactivation of micro-organisms and therefore for preservation of food products. It is a mild technology compared to thermal pasteurization because a lower temperature is used during processing, leading to a better retention of the quality. In this study, pathogenic and spoilage micro-organisms relevant in refrigerated fruit juices were studied to determine the impact of process parameters and juice composition on the effectiveness of the PEF process to inactivate the micro-organisms. Experiments were performed using a continuous-flow PEF system at an electrical field strength of 20 kV/cm with variable frequencies to evaluate the inactivation of Salmonella Panama, Escherichia coli, Listeria monocytogenes and Saccharomyces cerevisiae in apple, orange and watermelon juices. Kinetic data showed that under the same conditions, S. cerevisiae was the most sensitive micro-organism, followed by S. Panama and E. coli, which displayed comparable inactivation kinetics. L. monocytogenes was the most resistant micro-organism towards the treatment conditions tested. A synergistic effect between temperature and electric pulses was observed at inlet temperatures above 35 °C, hence less energy for inactivation was required at higher temperatures. Different juice matrices resulted in a different degree of inactivation, predominantly determined by pH. The survival curves were nonlinear and could satisfactorily be modeled with the Weibull model.

Effects of heat on physicochemical properties of whey protein-stabilised emulsions

The effect of heating has been studied for whey protein-stabilised oil-in-water emulsions (25.0% (w/w) soybean oil, 3.0% (w/w) whey protein isolate, pH 7.0). These emulsions were heated between 55 and 95 degreesC as a function of time and the effect on particle size distribution, adsorbed protein amount, protein conformation and rheological properties was determined. Heating the emulsions as a function of temperature for 25 min resulted in an increase of the mean diameter (d(32)) and shear viscosity with a maximum at 75 degreesC. Heating of the emulsions at different temperatures as a function of time in all cases resulted in a curve with a maximum for d(32). A maximum increase of d(32) was observed after about 45 min at 75 degreesC and after 6-8 min at 90 degreesC. Similar trends were observed with viscosity measurements. Confocal scanning laser micrographs showed that after 8 min of heating at 90 degreesC large, loose aggregates of oil droplets were formed, while after 20 min of heating compact aggregates of two or three emulsion droplets remained. An increase of the adsorbed amount of protein was found with increasing heating temperature. Plateau values were reached after 10 min of heating at 75 degreesC and after 5 min of heating at 90 degreesC. Based on these results we concluded that in the whole process of aggregation of whey protein-stabilised emulsions an essential role is played by the non-adsorbed protein fraction, that the kinetics of the aggregation of whey protein-stabilised emulsions follow similar trends as those for heated whey protein solutions and that upon prolonged heating rearrangements take place leading to deaggregation of initially formed large, loose aggregates of emulsion droplets into smaller, more compact ones

The effect of storage of whole potatoes of three cultivars on the patatin and protease inhibitor content; a study using capillary electrophoresis and MALDI-TOF mass spectrometry

The content and biological activity of patatin and the protease inhibitors of molecular size 20-22 kDa present in whole potato tubers were investigated as a function of storage time. The amount of buffer-extractable protein decreased gradually during storage of whole potatoes of the cultivars Bintje and Desiree for 47 weeks whereas, for Elkana, it increased after approximately 25 weeks. The patatin proportion of the extractable protein did not decrease significantly during storage, whereas the proportion of PP20-22 protease inhibitors decreased. All cultivars contained several different patatin isoforms. Bintje and Desiree showed patatin populations with two masses whereas for Elkana, only one molar mass was found. Patatin isoforms of the three cultivars examined showed no significant differences in stability towards degradation as was concluded from capillary electrophoresis analysis. No inactivation of patatin or protease inhibitors by partial degradation of these proteins was observed using matrix-assisted laser desorption/ionisation time of flight mass spectrometry (MALDI-TOF MS), whereas enzyme-activity assays suggested that the biological activity, especially in the cultivar Bintje, decreased markedly at the break of dormancy.

Effects of spray drying on physicochemical properties of milk protein-stabilised emulsions

The effect of spray drying and reconstitution has been studied for oil-in-water emulsions (20.6% maltodextrin, 20% soybean oil, 2.4% protein, 0.13 M NaCl, pH 6.7) with varying ratios of sodium caseinate and whey protein, but with equal size distribution (d(32) = 0.77 mum). When the concentration of sodium caseinate in the emulsion was high enough to entirely cover the oil-water interface, the particle size distribution was hardly affected by spray drying and reconstitution. However, for emulsions of which the total protein consisted of more than 70% whey protein, spray drying resulted in a strong increase of the droplet size distribution. The adsorbed amount of protein ranged from 3 mg m(-2) for casein-stabilised emulsions to 4 mg m(-2) for whey protein-stabilised emulsions with a maximum of 4.2 mg m(-2) for emulsions containing 80% whey protein on total protein, which means that for all these emulsions about one quarter of the available protein was adsorbed at the oil-water interface. The adsorbed amount of protein was hardly affected by spray drying. After emulsion preparation casein proteins adsorbed preferentially at the oil-water interface. As a result of spray drying, the relative amount of beta-lactoglobulin in the adsorbed layer increased strongly at the expense of alpha(s1)-casein and beta-casein. Percentages of alpha(s2)-casein and kappa-casein in the adsorbed layer remained largely unchanged. The changes in the protein composition of the adsorbed layer as a result of spray drying and reconstitution were the largest when beforehand hardly any whey protein was present in the adsorbed layer and hardly any sodium caseinate in the aqueous phase. Apparently, during spray drying conditions have been such that beta-lactoglobulin could unfold, aggregate, and react with other cystein-containing proteins changing the particle size distribution of the emulsions and the composition of the adsorbed layer. It seemed, however, that non-adsorbed sodium caseinate in some way was able to protect the adsorbed casein proteins from being displaced by aggregating whey protein