Point Of Maximum Stability Research Articles

Influence of the wall material on the moisture sorption properties and conditions of stability of sesame oil hydrogel beads by ionic gelation

Sesame oil was encapsulated by ionic gelation using matrices of sodium alginate and nopal mucilage as wall material. Moisture sorption isotherms of three different types of hydrogels beads formed by SA-NM (1:0 w/w), SA-NM (1:1 w/w) and SA-NM (1:1.5 w/w) were performed at 25, 35 and 45 °C. Experimental isotherms were described by means of the GAB model, showing sigmoidal shape. Pore radius values of beads ranged from 0.81 to 7.59 nm, corresponding to micropores and mesopores classification. The integral thermodynamic properties were estimated to define conditions of maximum stability of the hydrogel beads. The point of maximum stability, linked to minimum integral entropy, was in the range 3.31–5.59 kg H2O/100 kg d.s. (corresponding to water activity, aW, of 0.23–0.59) in the studied temperature range. Enthalpy-entropy compensation for the beads exhibited the presence of two isokinetic temperatures; one at low moisture contents (0–5.95 kg H2O/100 kg d.s.) controlled by variations of the water entropy, and a second given by enthalpy-driven mechanisms. Overall, the results showed that the hydrogel beads exhibited features of micro- and meso-porous biomaterials. Besides, the composition of the wall material has central implications for the characteristics of the sorption process.

T g and a w as criteria for the oxidative stability of spray-dried encapsulated paprika oleoresin

Moisture sorption isotherms and glass transition temperature (T g) of paprika oleoresin microcapsules were evaluated. Microcapsules were produced by spray drying using modified starch (Capsul®) as encapsulating agent. The differential and integral thermodynamic functions of enthalpy and entropy were estimated from the sorption data for paprika oleoresin microcapsules. T g of microcapsules conditioned at various water activities (a w’s) were determined by modulated differential scanning calorimetry. The critical water content or water activity (RHc) was estimated from T g values. Both a w and T g were used to determine the critical conditions for microcapsules storage. The GAB model provided a good fit to the experimental data. The point of maximum stability according to the minimum integral entropy (ΔS int)T was found at a w = 0.241 at 35 °C, and at this point the system was within the glassy state. RHc was founded at a w = 0.789 at 35 °C; all microcapsules stored at a w’s ≤ 0.627 were able to maintain their structural integrity without caking and stickiness occurring. The lowest carotenoid degradation of microcapsules during storage at 35 °C proceeded at a w = 0.742, which was near to RHc; however, microcapsules stored at this a w showed incipient caking and agglomeration.

Sorption isotherms, thermodynamic properties and glass transition temperature of mucilage extracted from chia seeds (Salvia hispanica L.).

Freeze-dried chia mucilage adsorption isotherms were determined at 25, 35 and 40°C and fitted with the Guggenheim-Anderson-de Boer model. The integral thermodynamic properties (enthalpy and entropy) were estimated with the Clausius-Clapeyron equation. Pore radius of the mucilage, calculated with the Kelvin equation, varied from 0.87 to 6.44 nm in the temperature range studied. The point of maximum stability (minimum integral entropy) ranged between 7.56 and 7.63kg H2O per 100 kg of dry solids (d.s.) (water activity of 0.34-0.53). Enthalpy-entropy compensation for the mucilage showed two isokinetic temperatures: (i) one occurring at low moisture contents (0-7.56 kg H2O per 100 kg d.s.), controlled by changes in water entropy; and (ii) another happening in the moisture interval of 7.56-24 kg H2O per 100 kg d.s. and was enthalpy driven. The glass transition temperature Tg of the mucilage fluctuated between 42.93 and 57.93°C.

Effects of Storage Temperature and Water Activity on the Degradation of Carotenoids Contained in Microencapsulated Chili Extract

The aim of this work was to evaluate the effects of storage temperature and water activity on degradation of carotenoids contained in microencapsulates of non-aqueous extracts from chili (NAEC). Total carotenoids content and adsorption isotherms of microencapsulated NAEC in a 1:1 weight ratio with gum Arabic-maltodextrin DE 20 (GA 50%–MD50%) were determined at 25, 35, and 40°C. The isotherms were fitted using the Guggenheim-Anderson-de-Boer model and their enthalpies and entropies, both differential and integral, were estimated by the Clausius-Clapeyron method. The minimum integral entropy was considered as the point of maximum stability at which water less readily participates in degradation reactions. Zones of minimum integral entropy were found between 7.56–8.30, 6.10–6.95, and 5.15–6.04 kg H2O/100 kg dried solids, corresponding to water activity (aw) of 0.210–0.239, 0.238–0.277, and 0.262–0.313 at 25, 35, and 40°C, respectively. Total carotenoids content (CT) degraded over time, but degradation of carotenoids was lower in microcapsules stored at 25°C than those stored at 35 or 40°C. The morphology of microcapsules was altered at aw > 0.6, including swelling of the polysaccharide matrix was presented, and possible subsequent dissolution of the wall material, which indicates a high rate of carotenoid degradation. When microencapsulated NAEC were storage between 0.2–0.6 of water activity, the highest glass transition temperatures were achieved. In this range, the wall materials of the microcapsules suffer less microstructural modifications, associated with the minimum level of degradation of carotenoids. Sometimes, in this water activity range, the zones of minimal entropy were observed.

Thermodynamic analysis of sorption isotherms of cassava (Manihot esculenta).

Sorption isotherms of cassava were determined experimentally using a static gravimetric method at 30, 45 and 60°C and within the range of 0.10-0.90 water activity. At a constant water activity, equilibrium moisture content decreased with increasing temperature. The equilibrium moisture content increased with increasing water activity at a given temperature. The experimental results were modelled using seven sorption models using non-linear regression technique. Results demonstrated that the GAB model adequately predicted equilibrium moisture content of cassava for the range of temperatures and water activities studied. The thermodynamic functions such as net isosteric heat of sorption, differential entropy of sorption, net integral enthalpy and entropy were evaluated to provide an understanding of the properties of water and energy requirements associated with the sorption behaviour. Net isosteric heat and differential entropy decreased with increasing equilibrium moisture content. The net integral enthalpy decreased while net integral entropy increased with increasing equilibrium moisture content. Net integral entropy was negative in value. All thermodynamic functions were adequately characterised by a power law model. The point of maximum stability was found between 0.053 and 0.154kg water/kg db for cassava.

Establishing the Most Suitable Storage Conditions for Microencapsulated Allspice Essential Oil Entrapped in Blended Biopolymers Matrices

The adsorption isotherms of allspice essential oil microencapsulated in biopolymers blend (whey protein concentrate [WPC], mesquite gum [MG], and maltodextrin DE10 [MD]) in different proportions (WPC17%-MG17%-MD66% w/w and WPC66%-MG17%-MD17% w/w) with wall-to-core material ratios of 4:1 were determined at 25, 35, and 40°C. The isotherms were fitted using the Guggenheim-Anderson-de Boer (GAB) model and the enthalpies and entropies, both differential and integral, were estimated by the Clausius-Clapeyron method. The minimum integral entropy was considered as the point of maximum stability where strong bonds between the adsorbate and adsorbent occurred, and water would be less available and likely to participate in spoilage reactions. The point of maximum stability was found between 13.79 and 15.11 kg H2O/100 kg d.s. (corresponding to water activity, a W , of 0.444–0.551) for the microcapsules with WPC17%-MG17%-MD66% w/w as wall material and 18.71–19.63 kg H2O/100 kg d.s. (a W = 0.591–0.713) for the microcapsules with WPC66%-MG17%-MD17% w/w as wall material in the temperature range studied.

Thermodynamic analysis of the sorption isotherms of pure and blended carbohydrate polymers

The adsorption isotherms of gum Arabic (GA), mesquite gum (MG), and maltodextrin DE 10 (MD), and a blend of the three gums (17%GA–66%MG–17%MD) were determined at 25, 35, and 40 °C. All isotherms were fitted using the GAB model and the thermodynamic properties (enthalpies and entropies, differential and integral) were estimated by the Clausius–Clapeyron method. The minimum integral entropy was considered as the point of maximum stability where strong bonds between the adsorbate and adsorbent occurred, and water is less available and likely to participate in spoilage reactions. The point of maximum stability was found between 12.24 and 14.68 kg H 2O/100 kg d.s. (corresponding to water activity, a w, of 0.32–0.57) for GA, 12.12–14.27 kg H 2O/100 kg d.s. ( a w = 0.33–0.55) for MG, and 11.37–13.84 kg H 2O/100 kg d.s. ( a w = 0.28–0.55) for the biopolymer blend, in the temperature range studied.

Formulation and evaluation of ophthalmic preparations of acetazolamide

The orally administered acetazolamide has a limited use in glaucoma due to the systemic side effects associated with its use. It has been reported to show little effect on the intraocular pressure (IOP) of human and rabbit eyes upon topical application, probably owing to its poor bioavailability and instability at pH>5.0. In order to enhance the bioavailability of the drug, contact time between the drug molecules and the ocular surface was increased using high viscosity, water soluble polymers (PVA, HPMC), and by incorporating acetazolamide into an in situ-forming ophthalmic drug delivery system. Moreover, a penetration enhancer (EDTA) was also used in these formulations to increase the extent of absorption of the drug. Acetazolamide at a concentration of 10% was used and the formulations (eyedrop suspensions) were evaluated for their in vitro release pattern. The effect of these formulations on the IOP in normotensive conscious rabbits was also investigated. These formulations were found to be therapeutically effective with a peak effect at 2 h. A fall in IOP of up to 46.4% was observed with repeated administration of one of the formulation containing PVA, EDTA and Tween 80 (MK-5). Results indicated that a topical effect of acetazolamide can be observed if the formulation, (a) contains a suitable polymer—to increase the residence time; (b) a penetration enhancer—as acetazolamide has a low permeability coefficient i.e. 4.1×10 −6 cm/s [Duffel, M.W., Ing. I.S., Segarra, T.M., Dixson, J.A., Barfknecht, C.F., Schoenwald, R.D., 1986. J. Med. Chem. 29, 1488–1494]; and (c) pH of the formulation is maintained at the point of maximum stability (pH≤5.0).

The critical gelling point in silica gels containing lithium, sodium and potassium

Silica suspensions (CabOSil) can be made to gel without loss of water by adjusting the ionic strength of the suspension with inert electrolyte ions. In this paper we examine this critical gelation concentration (CGC) for the nitrate salts of lithium, sodium and potassium. The pH of the suspensions is held constant at its point of maximum stability (pH = 5.85) and the ionic strength was varied (0.01–0.263 M) to determine the CGC. All of the systems gelled through water evaporation without loosing more than 10% of their volume. While this small volume change results in less sensitivity in determining the CGC, it can be said that lithium, sodium and potassium ions show a lyotropic trend. When lithium, which is the largest hydrated ion tested, is the counter-ion, the sol loses more water before gelling than in the presence of sodium which in turn has to lose more water than when potassium is the counter-ion. A similar lyotropic trend was found in the cleaning of these sols before gelation and in the ability of the gels to retain their salts upon drying and calcination to 200°C without evidence of salt segregation.

The Ecology of Resource Utilization

The paper begins with a review of the general attitudes of resource utilization over the past 25 years, and examines three main changes that have taken place during this time: first, the availability of fossil fuels to the western world, second, the increased public awareness of the environment, and third, the realization that the resources are essentially finite.The basic thesis of the paper is that our future standard of living, and indeed our very existence, depend upon us matching demand for the resources of food, fuel and wood fibre with our ability to produce these resources in the rural environment. As the demand for increased resources continues, we will become increasingly dependent upon the biological activity of our ecological systems as they utilize and transform the energy of the Sun reaching our world. We must, therefore, learn more about the efficiency of these ecological systems and the ways in which they work, in order to maintain them at their point of maximum stability. The rural environment, together with its soils, climate and ecological systems, needs itself to be accepted as an important resource, and conscious policies developed for its husbandry.

INACTIVATION OF CRYSTALLINE TRYPSIN

1. The rate of inactivation of crystalline trypsin solutions and the nature of the products formed during the inactivation at various pH at temperatures below 37 degrees C. have been studied. 2. The inactivation may be reversible or irreversible. Reversible inactivation is accompanied by the formation of reversibly denatured protein. This denatured protein exists in equilibrium with the native active protein and the equilibrium is shifted towards the denatured form by raising the temperature or by increasing the alkalinity. The decrease in the fraction of active enzyme present (due to the formation of this reversibly denatured protein) as the pH is increased from 8.0 to 12.0 accounts for the decrease in the rate of digestion of proteins by trypsin in this range of pH. 3. The loss of activity at high temperatures or in alkaline solutions, just described, is very rapid and is completely reversible for a short time only. If the solutions are allowed to stand the loss in activity becomes gradually irreversible and is accompanied by the appearance of various reaction products the nature of which depends upon the temperature and pH of the solution. 4. On the acid side of pH 2.0 the trypsin protein is changed to an inactive form which is irreversibly denatured by heat. The course of the reaction in this range is monomolecular and its velocity increases as the acidity increases. 5. From pH 2.0 to 9.0 trypsin protein is slowly hydrolyzed. The course of the inactivation in this range of pH is bimolecular and its velocity increases as the alkalinity increases to pH 10.0 and then decreases. As a result of these two reactions there is a point of maximum stability at about pH 2.3. 6. On the alkaline side of pH 13.0 the reaction is similar to that in strong acid solution and consists in the formation of inactive protein. The course of the reaction is monomolecular and the velocity increases with increasing alkalinity. From pH 9.0 to 12.0 some hydrolysis takes place and some inactive protein is formed and the course of the reaction is represented by the sum of a bi- and monomolecular reaction. The rate of hydrolysis decreases as the solution becomes more alkaline than pH 10.0 while the rate of formation of inactive protein increases so that there is a second point at about pH 13.0 at which the rate of inactivation is a minimum. In general the decrease in activity under all these conditions is proportional to the decrease in the concentration of the trypsin protein. Equations have been derived which agree quantitatively with the various inactivation experiments.