Factors that contribute to raisin browning

Matthew FidelibusViticulture and Enology, UC Davis

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

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

Caftaric acid

Caftaric acid is the most abundant hydroxycinnamate found in grapes and wine.

Hydroxycinnamates are the primary non-flavonoidphenolic compounds in grapes.

When oxidized, brown pigmentsare formed.

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.

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)

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

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

Factors that promote Maillard reaction

• High temperatures• Low moisture content• Long storage time

Factors that minimize Maillard reaction

• Low temperature drying and storage• SO2 fumigation

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