María del Pilar Buera

Water activity, water glass dynamics, and the control of microbiological growth in foods.

Water is probably the single most important factor governing microbial spoilage in foods, and the concept of water activity (a(w)) has been very valuable because measured values generally correlate well with the potential for growth and metabolic activity. Despite some drawbacks (e.g., solute effect), the concept of a(w) has assisted food scientists in their effort to predict the onset of food spoilage as well as to control food-borne disease hazards in food products. In the last decade the concept of a(w) has been challenged. It has been suggested that reduced-moisture food products (e.g., low and intermediate) may be nonequilibrium systems and that most of them are in the amorphous metastable state, which is very sensitive to changes in moisture content and temperature. It has been proposed that the glass transition temperature Tg (temperature at which the glass-rubber transition occurs), is a parameter that can determine many product properties, the safety of foods among them. The concept of water dynamics, originating in a food polymer science approach, has been suggested instead of a(w) to better predict the microbial stability of intermediate-moisture foods. The usage of a(w) to predict microbial safety of foods has been discouraged on the basis that (1) in intermediate-moisture foods the measured water vapor pressure is not an equilibrium one, and because a(w) is a thermodynamic concept, it refers only to equilibrium; and (2) the microbial response may differ at a particular a(w) when the latter is obtained with different solutes. This review analyzes these suggestions on the basis of abundant experimental evidence found in the literature. It is concluded that nonequilibrium effects (e.g., inability of water to diffuse in a semimoist food) appear to be in many cases slow within the time frame (foods shelf life) of the experiments and/or so small that they do not affect seriously the application of the a(w) concept as a predictor of microbial stability in foods. The claims that a food science polymer approach to understanding the behavior of aqueous sugar glasses and concentrated solutions may be used to predict the microbial stability of food systems is not substantiated by experimental evidence. This approach does not offer, at the present time, a better alternative to the concept of a(w) as a predictor of microbial growth in foods. It is also recognized that a(w) has several limitations and should be always used carefully, and this must include precautions regarding the possible influences of nonequilibrium situations. This aspect may be summarized by simply saying that anyone who is going to employ the term water activity must be aware of the implications of its definition.

Color changes during storage of honeys in relation to their composition and initial color

Abstract The causes of darkening in honey have been attributed to Maillard reaction, fructose caramelization and reactions of polyphenols, however, no systematic studies exist on this subject. The influence of composition and initial color on the rate of darkening of several Argentine honeys submitted to storage at 37°C during 90 days was evaluated through spectrocolorimetric measurements. The most suitable color functions to evaluate darkening of honeys [lightness (Lab*), browning index (BI), metric chroma (Cab*), metric hue (Hab*) and 1/Z] increased linearly as storage time increased, after an initial induction period of very low browning development. The slope of the linear browning development zone with time was an index of browning rate, and it was analyzed in relation to the initial color and the composition of honeys (moisture content, total nitrogen, total lipids and polyunsaturated fatty acids, fructose and glucose content). Of the analyzed variables, the initial color was the parameter which better defined the rate of darkening of honeys.

Glassy state and thermal inactivation of invertase and lactase in dried amorphous matrices

The thermal stability of enzymes lactase and invertase in dried, amorphous matrices of sugars (trehalose, maltose, lactose, sucrose, raffinose) and some other selected systems (casein, PVP, milk) was studied. The glass transition temperature (Tg) was limited as a threshold parameter for predicting enzyme inactivation because (a) enzyme inactivation was observed in glassy matrices, (b) a specific effect of enzyme stabilization by certain matrices particularly trehalose was observed, and (c) enzyme stability appeared to depend on heating temperature (T) “per se” rather than (T − Tg) . For these reasons, a protective mechanism by sugars related to the maintenance of the tertiary structure of the enzyme was favored. A rapid loss of enzyme (lactase) activity was observed in heated sucrose systems at T > Tg, and this was attributed to sucrose crystallization since it is known that upon crystallization the protective effect of sugars is lost. Thus, the stabilizing effect could be indirectly affected by the Tg of the matrix, since crystallization of sugars only occurs above Tg. Trehalose model systems (with added invertase) showed an exceptional stability toward “darkening” (e.g., non‐enzymatic browning) when heated in the dried state to elevated temperatures and for long periods of time.

Stabilization of the restriction enzyme EcoRI dried with trehalose and other selected glass-forming solutes.

The stabilization of the restriction enzyme EcoRI by its incorporation into aqueous glass-forming carbohydrate or polymer solutions, followed by vacuum-drying to low moisture, has been studied. Glass-forming solutes included trehalose, sucrose, lactose, maltose, raffinose, maltodextrin DE 10, and poly(vinylpyrrolidone) (molecular weight 40,000, PVP). Among the solutes examined, trehalose and sucrose protected the enzyme most effectively during storage at 37 and 45 degrees C. The restriction enzyme dried with trehalose or sucrose maintained its activity without detectable loss for at least 20 days at 37 degrees C and 12 days at 45 degrees C. In contrast, the activity of the enzyme dried with maltodextrin or PVP was reduced during vacuum desiccation and also it decreased remarkably during storage at the same temperatures. Stored (37/45 degrees C) vacuum-dried trehalose and sucrose systems were either a dense paste or a very viscous syrup, and this indicated that they were not glassy. Moreover, no relationship was found between the glass transition temperatures (Tg) of the pure added solute and enzyme protection during storage, since, e.g., sucrose which has significantly lower Tg values protected the enzyme much better than either maltose, lactose, maltodextrin, or PVP. The trisaccharide raffinose offered good protection of enzyme activity, and its role as a novel excipient matrix for labile enzyme stabilization deserves further investigation. The stability of enzyme EcoRI was rapidly lost when the vacuum-dried trehalose and sucrose systems were humidified to 58% relative humidity and stored at 45 degrees C, and this was attributed to disaccharide crystallization.

A simple model for predicting the viscosity of sugar and oligosaccharide solutions

Abstract A model originally developed to predict the viscosity of concentrated electrolyte solutions was adapted to fit viscosity data of various sugars and sugar mixtures up to very high concentrations. The model was μ r = a exp( EX ), where μ r is the relative viscosity, a and E are parameters (in most cases a was very close to unity). The model was used to describe the concentration dependence of viscosity of sucrose, fructose, glucose, maltose, lactose and corn syrup (having different dextrose equivalent values) solutions up to very high solids concentration. The molecular weight of the sugar was a main factor in determining the value of parameter E , which on theoretical grounds may be somewhat related to the free energy of activation for viscous flow per mole of solute.

Adsorption isotherm of amorphous trehalose

The water adsorption isotherm at 25°C of freeze-dried amorphous trehalose and trehalose with the addition of maltodextrin (MD) DE 10.9 was obtained. Moisture uptake as a function of time at various relative humidities (RH) was also measured. At low relative humidities (RH) amorphous trehalose adsorbs moisture but at 44% RH and above the isotherm exhibits a plateau indicating crystallisation (formation of the dihydrate) of amorphous trehalose and this was reflected on the shape of the isotherm. Trehalose crystallisation was delayed in the system containing maltodextrin (50: 50) as compared with pure trehalose.

Glassy state in relation to the thermal inactivation of the enzyme invertase in amorphous dried matrices of trehalose, maltodextrin and PVP

Abstract The stabilization of invertase by its incorporation in aqueous trehalose and polymer solutions, followed by freeze-drying and desiccation to ‘zero’ moisture content, was studied. The dried amorphous preparations of trehalose, maltodextrin (MD; DE = 10.9), and poly(vinyl)pyrrolidone (PVP), molecular weights 360000, 40000 and 10000, greatly protected invertase—as compared with its behavior in liquid solution—from heat inactivation at elevated temperatures. Significant invertase inactivation was observed in heated PVP and MD matrices kept well below their glass-transition temperature. Under glassy conditions the extent of enzyme protection by MD and PVP systems was related to their glass-transition temperature (Tg) since systems of higher Tg afforded better protection. However, the data for trehalose deviated from this behavior since invertase stabilization was higher than expected on the basis of the results obtained with polymer matrices. Present results suggest that invertase inactivation in dried amorphous systems cannot be adequately explained by the glass-transition theory and this is particularly true for trehalose, for which some additional mechanism of enzyme protection is likely to operate.

Combined effects of trehalose and cations on the thermal resistance of β-galactosidase in freeze-dried systems

The purpose of this study was to investigate the combined effects of trehalose and cations on the preservation of beta-galactosidase in freeze-dried systems and their relationship to physical properties. Differential scanning calorimetry was employed to measure the glass transition temperature (T(g)) and the endothermal peak area, related to the amount of crystalline trehalose dihydrate present in the samples. In systems in which the trehalose matrix was humidified to conditions which allowed a high proportion of trehalose to crystallize, the enzyme was rapidly inactivated upon heating at 70 degrees C. In these conditions the addition of CsCl, NaCl and particularly KCl or MgCl(2), improved the enzyme stability with respect to that observed in matrices containing only trehalose. For a given moisture content, addition of salts produced very little change on the glass transition temperature; therefore the protective effect could not be attributed to a higher T(g) value. The crystallization of trehalose dihydrate in the humidified samples was delayed in the trehalose/salt systems (principally in the presence of Mg(2+)) and a parallel improvement of enzyme stability was observed.

A study of acid-catalyzed sucrose hydrolysis in an amorphous polymeric matrix at reduced moisture contents

Abstract The changes in viscosity and mobility which take place in the proximity of the glass transition affect the physical stability of amorphous foods and could also affect the rate of chemical reactions. The effect of glass transition on the rate of acid-catalyzed sucrose hydrolysis was investigated in an amorphous polymeric matrix of polyvinylpyrrolidone (PVP). Aqueous solutions of PVP-sucrose in a citrate buffer were freeze-dried in order to obtain the amorphous matrix. Samples were then equilibrated to several relative humidities and stored at several temperatures. The difference between the glass transition temperature ( T g ) and the storage temperature ( T ), T - T g was not a key factor determining the rate of sucrose hydrolysis, which was controlled by the water content of the system. The major effect on the rate of hydrolysis was related to changes in pH which occur when the system is dehydrated. Knowledge of the actual pH of a system, and the possible changes that may occur during concentration/drying are necessary for a better understanding of chemical changes in low and intermediate moisture foods.

Water properties of food, pharmaceutical, and biological materials

Water properties of food, pharmaceutical, and biological materials , Water properties of food, pharmaceutical, and biological materials , کتابخانه مرکزی دانشگاه علوم پزشکی تهران