Kinetics of an Acid-Base Catalyzed Reaction (Aspartame Degradation) as Affected by Polyol-Induced Changes in Buffer pH and pKa Values

ABSTRACT:  The kinetics of an acid-base catalyzed reaction, aspartame degradation, were examined as affected by the changes in pH and pK_a values caused by adding polyols (sucrose, glycerol) to phosphate buffer. Sucrose-containing phosphate buffer solutions had a lower pH than that of phosphate buffer alone, which contributed, in part, to reduced aspartame reactivity. A kinetic model was introduced for aspartame degradation that encompassed pH and buffer salt concentrations, both of which change with a shift in the apparent pK_a value. Aspartame degradation rate constants in sucrose-containing solutions were successfully predicted using this model when corrections (that is, lower pH, lower apparent pK_a value, buffer dilution from the polyol) were applied. The change in buffer properties (pH, pK_a) from adding sucrose to phosphate buffer does impact food chemical stability. These effects can be successfully incorporated into predictive kinetic models. Therefore, pH and pK_a changes from adding polyols to buffer should be considered during food product development.     

Nonenzymatic Browning Kinetics in Low-moisture Food Systems as Affected by Matrix Composition and Crystallization

ABSTRACT: Nonenzymatic browning (NEB) in lactose, trehalose, and lactose/trehalose food model systems containing L-lysine and D-xylose (5% w/w) as reactants was studied at 4 different relative vapor pressure (RVP) (33.2%, 44.1%, 54.5%, 65.6%) environments at room temperature. Sorption isotherms of model systems were determined gravimetrically, and data were modeled using the Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-deBoer (GAB) models. Glass transition, T_g , was measured by differential scanning calorimetry (DSC). NEB was followed spectrophotometrically. Although the 3 model systems showed similar glass transition behavior, different crystallization properties were observed from loss of sorbed water. Mixtures of trehalose and lactose showed delayed crystallization of component sugars. The NEB rate was affected by sugar composition. At a low RVP (33.1%) environment, the NEB rate in trehalose system was higher than in the lactose/trehalose system, and the NEB rate in lactose system was the lowest. The NEB rate in different models seemed to be affected by component crystallization. The highest extent of browning in the trehalose matrix system seemed to be related to the formation of trehalose crystals in the system after crystallization at high RVP. The results indicated that the composition of the carbohydrate-based low-moisture real food systems should be considered in controlling NEB reaction.     

Aspartame Degradation in Solutions at Various pH Conditions

ABSTRACT: This study of aspartame degradation in phosphate-citrate buffer solutions of pH2,4,6, 7,8,10, and 12 using reverse-phase high-performance liquid chromatography (HPLC) showed different major degradation products for different pH ranges. The major degradation product found at pH 2 to 6 was L-phenylalanine methyl ester (PME). The major degradation product found at pH 7 to 10 was 3,6-dioxo-5-phenylmethylpiperazine acetic acid (diketopiperazine, DKP). L-aspartyl-phenylalanine (Asp-Phe) was the major product detected at pH 12.     

Sweetened Extruded Corn/Soy Product as Intermediate Moisture Food

An intermediate moisture food analog of sweetened condensed milk was developed based on soy protein, modified degermed corn, vegetable oil and sugar. Water solubility of corn was increased by extrusion-cooking followed by saccharification with glucoamylase in a 28% total solids (TS) soy protein-extruded corn suspension. Enzyme-modified corn/soy concentrates with 40–45% TS had a water solubility index (WSI) of 56–58% and a DE of 38–40. The final products had 73.5–78.4% TS, 71.5–72.7 WSI at pH 4.1–4.2, and a water activity of 0.82–0.83.     

Thiamin Stability in Simulated Intermediate Moisture Food

Four different intermediate moisture model systems (IMMS) were designed and prepared to simulate intermediate moisture foods. Stability of thiamin hydrochloride in IMMS as a function of temperature, oxygen, a_w, moisture content and composition of the model systems was studied. Analysis of kinetic data indicated that the rates of degradation of thiamin (1) can be represented by a first order model, (2) were not affected by oxygen concentration and presence of riboflavin and niacin, (3) obeyed an Arrhenius relationship for temperature dependence in the range 25-60°C, (4) increased as polarity of the reaction medium decreased, and (5) decreased as a_w increased from 0.65 to 0.85.     

Thermal Degradation Kinetics of Pyridoxine Hydrochloride in Dehydrated Model Food Systems

The thermal degradation kinetics of pyridoxine hydrochloride were examined using a dehydrated model food system designed to simulate a ready-to-eat breakfast cereal. This study was carried out to provide information useful in estimating the thermal losses of pyridoxine hydrochloride in the processing of breakfast cereals and other low moisture dehydrated food systems. Portions of the model system which were fortified with pyridoxine hydrochloride were toasted in a conduction oven at 155°, 170°, 185°, and 200°C for a minimum of six heating times at each temperature. Pyridoxine (PN) content was determined in the heat treated model systems by high performance liquid chromatography (HPLC). For each heat treatment, the loss of pyridoxine could be described by a first order kinetics model. The first order rate constants for 155°, 170°, 185°, and 200°C were 1.74 × 10^?2 min^?1, 5.22 × 10^?2 min^?1, 16.88 × 10^?2 min^?1, and 48.95 × 10^?2 min^?1, respectively. The calculated Arrhenius activation energy was 29.8 Kcal/mol. In comparing the HPLC method to the standard microbiological assay, the HPLC assay gave lower PN values for the toasted model system. To explain this discrepancy, HPLC fractions were collected and analyzed by the microbiological assay. No significant vitamin B₆ activity was found in any fraction other than that containing the PN peak. It is possible that the milder extraction procedure used in the HPLC assay is less capable of recovering forms of PN which may become bound during the toasting process. These potentially bound forms may or may not be biologically available.     

Accelerated Kinetic Study of Aspartame Degradation in the Neutral pH Range

The degradation of aspartame in solution as a function of temperature (70–100°C), buffer concentration (0.01–0.1M phosphate), and pH (6–7) was studied in order to estimate losses during thermal processing and storage of aseptic milk-based drinks. Prior data have been mostly on acid carbonated beverages. First order rate constants were obtained in all the conditions with activation energies in the range 14–20 kcal/ mole. An increase in both pH and buffer concentration caused an increase in rate of loss. These data were used to predict losses that would occur during pasteurization and sterilization conditions. Experiments at 4 and 30°C showed significant losses would occur during 4 and 30°C temperature storage and extrapolation from high temperatures predicted faster degradation rates than those found.     

Degradation Kinetics of Capsanthin in Paprika (Capsicum annuum L.) as Affected by Heating

ABSTRACT: Diluted juice of paprika was heated at 80, 90 and 100°C with holding times of 0, 2, 4, 8 and 16 min. Capsanthin in each sample was determined by HPLC. The degradation kinetics of capsanthin was studied by two groups of reaction models including elementary reaction rate models and those of modified square root-based pseudo (MRBP-). The MRBP-1st order reaction rate model was proven as an appropriate model in this study. The pseudo Q₁₀ (PQ₁₀) value was 1.045 and the predicted half-life (capsanthin 2,850 mg/L) was 27.47, 21.23 and 15.23 min, respectively, at 80, 90 and 100°C.     

Degradation Kinetics of Chlorophyll in Peas as a Function of pH

The kinetics of chlorophyll degradation in pea puree were determined in a specially designed reactor with on‐line pH control capability. Without pH control, the pH of the pea puree decreased continuously with heating due to acid formation; the pH was maintained within ±0.1 of the desired value with on‐line pH control. Chlorophyll (both a and b) degradation followed the first‐order reaction model. The temperature dependence of the rate constant was adequately modeled by the Arrhenius equation. The activation energy was independent of pH and was 17.5 kcal/mol and 17 kcal/mol for chlorophyll a and chlorophyll b, respectively. The degradation rate constant decreased log‐linearly as the pH was increased. A mathematical model was developed to predict the chlorophyll concentrations as a function of time, temperature and pH.     

A Method for the Determination of Diffusion Coefficients of Food Components in Low- and Intermediate Moisture Systems

A simple set-up to measure translational diffusion of components in low moisture systems is described. The tracer technique was tested on the diffusion of ¹⁴C-labeled tripalmitin and palmitic acid in a model system containing paraffin oil, microcrystalline cellulose and gum arabic. Different methods to evaluate the apparent diffusion coefficients from the experimental diffusion curve were compared. Good results were obtained using a curve-fitting procedure based on the sum of least squares. The technique appears to be suitable for measuring diffusion coefficients up to 10^?9 cm²/sec. The procedure offers the possibility to quantify the mobility of chemical components in dried foodstuffs in order to elucidate the mechanisms and kinetics of reactions occurring during storage.     

Moisture Retention and Textural Properties of Ground Chicken Meat as Affected by Sodium Tripolyphosphate, Ionic Strength and pH

Batches of ground chicken meat were mixed with 10% solutions containing 0.0–5.0% sodium tripolyphosphate and sufficient NaCl to adjust the ionic strength (IS) of the solutions from <0.7 to 6.0 at pH 6.5 to 9.5 in a 6 × 4 × 7 factorial experimental design. Patties prepared from the mixtures were evaluated for pH, cooking loss, and objective texture characteristics. Most effects of STPP were attributable to the direct effect of STPP on IS. However the ability of the phosphate to alter textural properties was greater at a pH value near the pK_a of the phosphate.     

Heat-induced Egg White Gels as Affected by pH

The functional properties of heat-induced egg white gels were investigated at five pH values. Textural characteristics were determined using the Instron Universal Machine. Hardness, elasticity, cohesiveness, chewiness, and fracturability were maximum at pH 11. Hunter L values were maximum at pH 5 and 7. Microstructure studied with electron microscopy was distinctly different at the five pH values. Alkaline gels showed a fine ordered network that might have contributed to excellent textural characteristics. Water-holding capacity (WHC) was high at alkaline pH, but decreased with addition of 2-mercaptoethanol, suggesting that disulfide bonds were important in egg white gels. Sodium dodecyl sulfate (SDS) improved WHC at pH 7 and 9. No significant correlation was observed between textural profiles and WHC.     

Surimi Gel Colors as Affected by Moisture Content and Physical Conditions

Colors (CIE L*, a*, and b*) of Alaska pollock (7′heragra chalcogramma) and Pacific whiting (Mecs productus) surimi gels were evaluated and related to compositional and physical conditions during preparation and measurements. Ten samples were used in each treatment. Water addition, sample size, species, cooking and test temperatures, and freeze/thaw affected lightness (L*) and yellowness (b*) values, while outside light conditions at measurement did not affect colors. Greenness (-a*) values were less affected by all conditions.     

Extrusion of Soybean and Wheat Flour as Affected by Moisture Content

The effect of moisture content (MC) on texture and properties of extrudates with varied ratios of soybean flour (SF) and wheat flour (WF) was studied. A single-screw extruder was used at screw speed 200 rpm. MCs of blends were 16, 17 or 18%, w.b. The properties of extrudates depended on flow rate of the material during extrusion. The flow rate revealed a nonlinear dependence on the blend composition and the MC at the same volume of filling of the screw feeding section. The expansion ratio of WF or SF extrudates increased with lowering of the MC. Unexpectedly, the expansion ratio decreased with lower MC for the composite extrudates. Optimal extrudate properties at 16% (w.b.) MC corresponded to 80, 90 or 100% (d.b.) WF and 20, 10% or no SF, respectively. At 17 and 18% MC, optimal properties were found for products containing 80 or 90% WF and 20 or 10% SF, respectively.     

Carrageenans in Beaker Sausage as Affected by pH and Sodium Tripolyphosphate

We investigated effects of meat pH or sodium tripolyphosphate (STPP) on performance of carageenans (kappa, iota or lambda) in low-fat beaker sausage model systems. Adding STPP or using high-pH meat affected performance of the meat batter in similar ways. High-pH meat batters or those containing STPP had lower force values when extruded, lower cooking losses, and were firmer when cooked. The addition of lambda-carrageenan produced the softest texture. Kappa- and iota-carrrageenan improved water retention of low-pH meat batters. After staining, light microscopy revealed different structural effects of the carrageenans.     

Strength and Frictional Properties of Popcorn Kernel as Affected by Moisture Content

Strength properties and dynamic coefficient of friction of popcorn kernels were evaluated as a function of moisture content in the range of 7.78–16.72 g/100 g dry solids. In order to determine strength properties of the kernels that are in terms of deformation at rupture point, rupture force, energy absorbed, hardness, and toughness of the popcorn kernels were quasi-statically loaded between two parallel plates. To determine frictional properties, friction tests were performed on aluminum, steel, and fiberglass surfaces. The values related to the strength properties, in general, decreased as the moisture content increased. On the other hand, the dynamic coefficient of friction of the popcorn kernels increased with increasing moisture content.     

Pectic Substance Degradation and Texture of Carrots as Affected by Pressurization

Heated pectin was degraded by transelimination (β-elimination) above pH 5 and by hydrolysis below pH 2, but pressurized pectin did not degrade. Thus cooked carrots decreased in firmness, but pressurized carrots did not. Pressurizing above 200 MPa slightly increased rupture strain. Galacturonic acid levels decreased in carrots cooked for 30 min. Total pectin in pressurized carrots was the same as in cooked for 3 min. However, with increased pressure, the amount of high methoxyl pectin in carrots decreased while low methoxyl pectin increased. Thus, the effects of pressurization on pectin degradation (transelimination) and texture of carrots were different from those of heating.     

Quality Decay Kinetics of Semi-preserved Sauce as Affected by Packaging

ABSTRACT: Semi-preserved tomato sauces packaged in polymeric containers have gained in popularity and market share, as opposed to traditional fully preserved tomato sauces. As a consequence of the different technologies applied, microbiological, enzymatic, and chemical problems are considered and compared with those related to traditional processing. This research represents a comparative study of the influence of different packaging (glass, polyethylene terephtalate [PET], PET added with an oxygen scavenger, and polypropylene [PP]) on the quality decay kinetics of a semi-preserved sauce. Results suggest that the packages chosen for our experiments can be used interchangeably because of the scant influence of the packaging materials on most of the quality indices. Two exceptions are represented by lycopene content (whose decrease was faster in PET and PP than in glass and PET containing the oxygen scavenger) and the peroxide data (which reached the highest values for PET containers).     

Triglyceride Distribution in a Low Moisture Food Model System

A coherent view on the distribution of triglycerides in a multicomponent multiphase food model system is set forward. Upon freeze-drying only a minor part of triolein and nearly no tripalmitin is encapsulated within the matrix. The level of encapsulated triolein decreases when the system is stored at higher a_w's. There is an absolute increase but a relative decrease in the level of encapsulated triolein with increasing initial triolein concentration. Free triglycerides are present as small, thin aggregates on the surface of the freeze-dried matrix or floating on the water film of the hydrated matrix. It is shown that by simple hexane extraction it is possible to distinguish between free and encapsulated lipid. Only free lipid diffuses. Lipid encapsulation and volatiles retention are compared.