Raisin browning
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This presentation briefly reviews the main causes of raisin browning, and how they may be managed.
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Raisin browning
- 1. Factors that contribute to raisin browning Matthew Fidelibus Viticulture and Enology, UC Davis
- 2. Raisin browning • Brown color is an important quality attribute in raisins • Thorough browning is desired for California’s “natural” raisins • Complete prevention of browning is desired for golden and green raisins
- 5. What causes raisins to brown? • Enzymatic oxidation of caftaric acid via polyphenol oxidase (PPO) – PPO is concentrated in plastids (including chloroplasts), in the skin, seeds, and seed traces – Enzymatic browning initiates at the berry periphery and center, rapidly progressing into the pulp • Maillard reaction
- 6. Caftaric acid Caftaric acid is the most abundant hydroxycinnamate found in grapes and wine. Hydroxycinnamates are the primary non-flavonoid phenolic compounds in grapes. When oxidized, brown pigments are formed.
- 7. Polyphenol oxidase (PPO) • PPO enzymes oxidize phenolic compounds • PPO are localized in plastids of living cells, physically separated from phenolic substrates which are located in the vacuole • Disruption of organelle membranes may occur during berry handling or drying, allowing the enzymes and substrates to interact, resulting in brown pigments.
- 8. Factors that favor PPO browning • High moisture content • Prolonged drying time • High PPO activity • High oxygen availability • Physical damage to tissues • Sunlight • Relatively high temperatures (very high temps inactivate PPO)
- 9. Factors that limit PPO browning • Rapid drying at cooler temperatures • Heating to 100 C to inactivate PPO • Pretreatment with drying emulsion • Fumigation with SO2 • Treating with other antioxidants • Exclusion of oxygen (vacuum drying) • Shade drying
- 10. Maillard reaction • A non-enzymatic chemical reaction between amino acids and sugars • May occur during high-temperature drying or during prolonged storage at room temperatures
- 11. Factors that promote Maillard reaction • High temperatures • Low moisture content • Long storage time
- 12. Factors that minimize Maillard reaction • Low temperature drying and storage • SO2 fumigation
- 13. Conclusions • Browning is minimized by rapid drying at cool temperatures • Protect fresh and drying fruit from physical damage • Use reducing agents and antioxidants to minimize oxidation
Editor's Notes
- The same fruit, 5 days later.
- PPO activity results in the familiar browning of cut fruits such as apples. Maillard reaction accounts for the browning of toast and other heated foods
- Oxidizes caftaric acid to quinones, which undergo oxidative polymerization to form melanin pigments
- PPO activity is temperature dependent, and grapevine varieties differ in their PPO activity. E.G. sultana marble, a variegated sport of thompson with low chlorophyll content thus has low ppo activity.