Effect of γ-PGA addition on the stability of guava juice

Cloudy juices, particularly from tropical fruits, are becoming a fast growing sector of the juice industry. Guava (Psidium guajava L.) juice is an important part of this sector, and claims a favored place over the other tropical fruits, by virtue of its high content of vitamin C, its delicious taste, its distinctive aroma and flavor. A main problem encountered in cloudy juice production is the maintenance of a stable cloud. Polysaccharides are commonly used in fruit juice industry to fulfill this goal. The objective of this study is an attempt to stabilize guava juice by mean of γ-polyglutamic acid (γ-PGA), a biodegradable polymer that is produced by Bacillus subtilis. It is water-soluble, polyanionic, biodegradable, and edible. Its high electronegativity gives it the potential use for cloudy juice stabilization. The high molecular weight sodium form (HMS, 841 kDa), the low molecular weight sodium form (LMS, 326 kDa) and the high molecular weight calcium form (HMC, 841 kDa) were used in this study at concentrations ranging from 0.05-0.20% and fixed pH 3.7. The results showed that all the γ-PGA tested could increase the stability of guava juice cloud according to their thickening capacity. At all concentrations, HMS appears as the best candidate in stabilizing juice cloud, followed by HMC and LMS. However, LMS depicted lesser particle aggregation after integrated light scattering measurements and HMC depicted the highest value of particle aggregation, due presumably to the interaction between pectin and calcium ions. The results also suggested that not only the viscosity, but also the ζ- potential influences the cloud stability in guava juice when various ii concentrations of different types of γ-PGA are added. HMS showed the highest ζ-potential thus, forming the most stable system, followed by HMS. HMC showed a ζ-potential pattern different from that of the other types of γ- PGA. The increase concentration of HMC decreases the ζ-potential and therefore, the stability, as showed by the highest shift of the particle sizes. Moreover, HMS retains more protein and total phenol in guava juice fine cloudy phase. However, all types of γ-PGA affected juice cloud Hunter’s L, a, b color parameters by small decreasing. The increase in concentration intensified the decrease in the values measured. The γ-PGAs used in this study were compared to carboxymethylcellulose (CMC) for the effectiveness to stabilize cloudy guava juice. At a concentration of 0.05%, juice added with CMC showed the highest viscosity, cloud stability, total phenol and highest color values. This study permitted to get more insights in the potential application of γ- PGA as cloud stabilizer in fruit juice industry. Like CMC, γ-PGA seems to hold its stabilizing effect by both its viscosity enhancing capacity and its high electronegativity.CHAPTER 1 INTRODUCTION.……….……………………………………….……1 2 LITERATURE REVIEW.………………………………..…………......5 2.1. Composition of guava...……………………………..……….….5 2.2. Processing of guava juice…………………………………..……..7 2.3. Composition of guava cloud………………………………...….…9 2.4. Mechanism involved in juice cloud loss………………………....14 2.4-1. Breakdown of pectin molecule………………………........14 2.4-2. Processing and cloud stability……………….....................18 2.5. Methods used for juice cloud stabilization……………………....20 2.5-1. Inactivation of pectinesterase……………………………..20 2.5-2. Use of food additives for cloud stabilization……………..22 2.5-2-1. Stoke’s law………………..…..............................22 2.5-2-2. Use of hydrocolloids….........................................23 2.6. γ-PGA…………………………………………………………....30 2.6-1. Chemical structure and biosynthesis of γ-PGA…………...30 2.6-2. Influence of solvent on electrolytes conformation…...........34 2.6-3. Potential applications of γ-PGA……………………..…….38 3 MATERIALS AND METHODS…………………………….….…….40 3.1. Materials…...………………………………………………..…..40 3.2. Chemicals…….....................................................................……41 3.3. Equipments......................................................................................41 3.4. Experimental design…………………………………………….42 3.4.1. Principle...............................................................................…42 3.4.2. Experimental protocol….........................................................44 3.4-3. Physical stability study….………………………………..45 3.4-4. Chemical analysis………………………………...............46 3.5. Methods……………………………………………………….…47 3.5-1. Sample preparation…………………….…………………47 3.5-3. Viscosity measurement…………………………………...48 3.5-2. Cloud stability measurement………………………..........48 3.5-5. Particle size analysis………………………………….…..49 3.5-4. Measurement of zeta-potential (ζ)………………………..49 3.5-8. pH Measurement…………………………………….........49 3.5-6. Color Hunter’s L, a, b measurement…..…………….…....50 3.5-7. Total soluble solids.............................................................50 3.5-9. Protein measurement.…………................................….…50 3.5-10. Total phenol measurement………………………….…...51 3.9-11. Statistical analysis…………………………………….…52 4 RESULTS AND DISCUSSION….……………………………....…...53 4.1. Effect of γ-PGA and CMC on guava juice physical stability………………………………………………………53 4.1-1.Effect of γ-PGA and CMC on guava juice viscosity……...53 4.1-2. Effect of γ-PGA on the stability of guava juice cloud…....56 4.1-3 Effect of γ-PGA on the particle size of guava juice……….58 4.1-4 Effect of γ-PGA on the ζ-potential of guava juice………...64 4.2. Effect of γ-PGA and CMC on guava juice chemical properties…………………………………………67 4.2-1. Effect of γ-PGA and CMC on guava juice pH………..........67 4.2-2. Effect of γ-PGA on guava juice Hunter’s L, a, b….……….69 4.2-3. Effect of γ-PGA on guava juice protein…………….……...71 4.2-4. Effect of γ-PGA on guava juice total phenol…………….....74 4.3. Comparative effect of CMC and γ-PGA on guava juice stability……………………………………………….76 5 CONCLUSIONS………………………………………………..…...78 6 REFERENCES……………………...……………………………….8