Graduation date: 1980

Graduation date: 1953

Graduation date: 1939

Graduation date: 1960

Sensory evaluation and instrumental methods were applied to the evaluation of strawberry juice concentrate (68°Brix) stored at 20°C which had been produced both commercially (C-SJC) and in a pilot plant (SJC). Sensory evaluation included taste and aroma ratings by intensity scaling and time-intensity of taste, and visual colorimetry by matching Munsell color chips. Instrumental analyses included tristimulus colorimetry (i.e. Hunter colorimeter), spectrophotometric colorimetry for pigment analyses, titrametric analyses for acidity (pH and titratable acidity) and free α-amino acids (formol number), and headspace gas analyses for CO₂ and O₂. During six days storage of C-SJC, a decrease in concentration of anthocyanins and increase in polyphenolics (tannin) was associated with an increase in astringency. Free α-amino acids were observed to decrease, while CO₂ was released. These changes were associated with an increase in musty/moldy and pungent aromas. Free sugars and titratable acidity did not change. The pilot plant SJC was processed from blanched and unblanched fruit to evaluate the relative importance of oxidase activity (i.e. polyphenoloxidase) prior to pasteurization. The blanching treatment increased the astringency and sourness in unstored SJC. These affects were associated with an increase in concentration of polyphenolics (tannin). During storage, the blanch treatment decreased the rate of anthocyanin loss and decreased the release rate of CO₂, yet degradation rates were still high. The 0₂ concentration in headspace did not change significantly during storage indicating that polyphenoloxidase (PPO) activity during storage was low. The musty/moldy and pungent aromas increased similarly to C-SJC. A chemical mechanism accounting for these changes is proposed where products from the oxidative degradation of ascorbic acid contribute directly or indirectly to the degradation of anthocyanins to yield browning. Further, high initial concentrations and subsequent decreases during C-SJC storage of free α-amino acids indicate that Strecker degradation is a participating mechanism. Associations of browning with the development of off-flavors suggest this chemical mechanism forms odor-active volatile compounds. / Graduation date: 1989

Methodology for characterization of the phenolic profile of apple, pear and grape juice was developed. It utilized High Performance Liquid Chromatography (HPLC) for separation of individual phenolic compounds and Diode Array Detection for recording the Ultra-violet (UV) spectrum of chromatographic peaks. The high resolution achieved and the ability of recording more than one wavelength simultaneously, allowed for quantitation of phenolic acids (cinnamics and benzoics), flavonol and dihydrochalcone glycosides, arbutin and hydroxymethylfurfural (HMF) with minimum sample preparation. Quantitation of procyanidins, however, required preliminary isolation and analysis in a separate run. Procyanidin isolation based on Sephadex IH-20 resulted in high recoveries and degree of reproducibility. The methodology was applied to study the influence of variety, post-harvest storage, SO₂, initial high temperature short time (HTST) heat treatment, enzymatic clarification, fining, bottling, concentration and storage on the phenolic composition of apple, pear and grape juice. Processing with SO₂ inhibited extensive phenolic oxidation during pulping and resulted in increased phenolic yields. Initial HIST treatment protected phenolics extracted in the juioe from degradation during processing. Cinnamic hydrolysis occurred during enzymatic clarification. The hydrolytic activity was removed with fining or bottling. There was no appcirent reduction of phenolics with fining. Procyanidins were sensitive to the heat applied during bottling and concentration. Storage of juice concentrates for nine months at 25°C resulted in formation of HMF, considerable degradation of cinnamics and dihydrochaloones and total loss of procyanidins and flavonols. The effect of diffusion extraction at different teirperatures on the phenolic composition of apple juice from different varieties was also studied. Up to a three-fold increase in cinnamics and a five-fold increase in dihydrochalcone glycosides were measured in diffusion extracted juice relative to juice extracted with conventional pressing. The increase in procyanidins and flavonols was even higher. / Graduation date: 1989

No description available.

No description available.

Conventional thermal kinetics of enzyme inactivation and microbial destruction in fruit juices were studied in the pasteurization temperature range (50 to 90°C). Pectin methylesterase (PME), as the most heat resistant enzyme, in orange juice and Saccharomyces cerevisiae and Lactobacillus plantarum, as the most common spoilage yeast and bacteria, respectively, in apple juice used as indicators were subjected to heat treatment in a well-agitated water bath. Based on gathered time-temperature profiles, effective portions of the come-up (CUT) and come-down (CDT) times (lags) were determined for inclusion in kinetic data handling. The inactivation/destruction kinetics followed typical first-order rate of reactions. / A continuous-flow microwave heating system was set up and evaluated for obtaining kinetic parameters under microwave heating conditions. The outlet temperature was characterized as a function of fluid flow rate, heating volume and initial temperature. / Kinetics of enzyme inactivation and microbial destruction at various temperatures under continuous-flow microwave heating conditions were then evaluated using the technique established above. The rates of inactivation/destruction varied depending on temperature. Taking into consideration the effectiveness of the CUT and contributory thermal inactivation during the CDT, the D-values were found to vary from 38.5 s at 55°C to 1.32 s at 70°C (pH 3.7) for PME, 4.75 s at 52.5°C to 0.378 s at 60°C (pH 3.4) for S. cerevisiae (ATCC 16664) and 14.1 s at 57.5°C to 0.327 s at 65°C (pH 3.4) for L. plantarum (ATCC 14917). / Some non-thermal microwave effects were hypothesized to exist and responsible for such differences between the two heating modes. Enzyme inactivation and microbial destruction were then studied further to evaluate the non-thermal effects. A continuous-flow microwave heating system was developed operating at full power while maintaining sample temperatures below 40°C by circulating a microwave-transparent liquid (kerosene) for immediate removal of heat produced in the juice during microwave exposure. / In order to explain and better characterize the faster rate of inactivation/destruction associated with microwave heating conditions observed in kinetic studies, additional tests were carried out using the second set-up described above, but without the cooling heat exchanger. The temperature of samples of different sizes were allowed to progressively increase under carefully controlled conditions. Inactivation of PME in orange juice (pH 3.7) and destruction of S. cerevisiae in apple juice (pH 3.4) were again used as parameters. The results once again clearly demonstrated superior inactivation/destruction effects under microwave heating which increased with temperature and decreased with sample size. (Abstract shortened by UMI.)

Graduation date: 1951