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Thesis Abstract

This study addresses the need to enhance the nutritional quality and shelf-life stability of fermented dairy beverages produced by a mid-sized dairy cooperative in the Midwest United States, where inconsistent fermentation performance and microbial spoilage have constrained product uniformity and market expansion. The aim is to optimize formulation and processing parameters to improve probiotic viability, sensory acceptability, and microbiological safety while extending shelf-life under commercial distribution conditions. Specific objectives are (i) to characterize current fermentation profiles, proximate composition, and probiotic viability of the cooperative’s flagship yogurt beverage; (ii) to evaluate the effects of selected prebiotic additions (inulin, fructooligosaccharides) and natural lactic ferments on acidification kinetics and textural attributes; (iii) to determine the impact of pasteurization temperature-time combinations and post-fermentation cooling regimes on microbial stability and sensory quality; (iv) to model shelf-life outcomes under real-world storage conditions (4°C and 7°C) using accelerated shelf-life testing and Arrhenius-based transformation; (v) to propose a validated quality-by-design (QbD) framework for process control and a scalable formulation that maintains probiotic counts above 10^7 CFU/mL for 28 days. The study adopts a mixed-methods design combining experimental production runs with consumer sensory panels and statistical modeling. Methodologically, the study employs a factorial experimental design to assess fermentation temperature (42–44°C), inoculum ratio (2% vs 3%), and prebiotic inclusion (0%, 2%, 4%) across three production batches, with each batch replicated in triplicate (n=9 experiments). Proximate analysis (moisture, fat, protein, carbohydrate, ash) and pH profiles are measured at 0, 4, 8, 12, and 24 hours of fermentation, alongside time-kill curves for viable probiotic strains (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus). Probiotic viability is tracked during refrigerated storage (4°C) for 28 days, with daily readings for the first week and biweekly thereafter. Sensory evaluation uses a trained panel (n=40) and consumer test (n=150) to assess aroma, mouthfeel, viscosity, sweetness, tanginess, and overall acceptability, applying a 9-point hedonic scale. Rheological properties are determined by dynamic shear rheometry to quantify viscoelastic moduli (G?, G??) and flow behavior (consistent with Herschel-Bulkley modeling). Microbiological safety is ensured through selective plating for total viable counts, lactic acid bacteria, yeast, mold, and potential contaminants. Data collection instruments include calibrated pH meters, viscometers, high-performance liquid chromatography for organic acids, and colony counters. Analytical approaches comprise analysis of variance (ANOVA) to determine significant main and interaction effects on fermentation kinetics, probiotic viability, and sensory scores, with post hoc Tukey tests for pairwise comparisons (?=0.05). Multivariate regression and response surface methodology (RSM) will be used to develop predictive models for probiotic stability and sensory acceptance as functions of temperature, inoculum, and prebiotic level. Accelerated shelf-life assessment will utilize Arrhenius modeling to extrapolate storage stability at 4°C and 7°C, with validation against real-time data. A microbial safety risk assessment will be conducted using probabilistic modeling to estimate the probability of contamination under proposed process controls. The theoretical framework integrates the Theory of Planned Behavior to interpret consumer acceptance outcomes and the Food Quality and Safety Systems Approach to align quality attributes with regulatory compliance. The study contributes to knowledge by offering a validated QbD-based formulation and process-control strategy for small-to-medium dairy processors that can be scaled for regional markets, enhancing probiotic viability and shelf-life without compromising sensory quality. The anticipated findings include identification of an optimal fermentation temperature and inoculum ratio that maximize probiotic stability and textural integrity, with evidence that 2% inulin supplementation improves mouthfeel and maintains viscosity during storage, and that pasteurization at 63°C for 30 seconds coupled with rapid cooling minimizes spoilage organisms while preserving live cultures. Recommendations include adopting the proposed formulation as a standardized product specification, implementing real-time monitoring of critical quality attributes (pH, viscosity, and microbial counts), and pursuing certifications for probiotic labeling and clean-label ingredients. The study concludes that integrating QbD with accelerated shelf-life modeling provides a robust framework for sustaining product consistency, safety, and consumer appeal in fermented dairy beverages produced by cooperative dairies.