A AVID VID VID VID VID B B B B B. M . M . M . M . MIN IN IN IN INABSTRA ABSTRA ABSTRA ABSTRA ABSTRACT CT CT CT CT: Riboflavin is r : Riboflavin is r : Riboflavin is r : Riboflavin is r : Riboflavin is relativ elativ elativ elativ elatively stable dur ely stable dur ely stable dur ely stable dur ely stable during ther ing ther ing ther ing ther ing thermal and nonther mal and nonther mal and nonther mal and nonther mal and nonthermal food pr mal food pr mal food pr mal food pr mal food processing and stor ocessing and stor ocessing and stor ocessing and stor ocessing and storage but is v age but is v age but is v age but is v age but is ver er er er ery y y y y sensitive to light. or a pr ype II mechanism) or a pr ype II mechanism) or a pr ype II mechanism) or a pr ype II mechanism) or a proo oo oo oo ooxidant for food components under light. P xidant for food components under light. P xidant for food components under light. P xidant for food components under light. P xidant for food components under light. Photosensitization of r hotosensitization of r hotosensitization of r hotosensitization of r hotosensitization of riboflavin iboflavin iboflavin iboflavin iboflavin causes production of reactive oxygen species such as superoxide anion, singlet oxygen, hydroxy radical, and hydro-causes production of reactive oxygen species such as superoxide anion, singlet oxygen, hydroxy radical, and hydro-causes production of reactive oxygen species such as superoxide anion, singlet oxygen, hydroxy radical, and hydro-causes production of reactive oxygen species such as superoxide anion, singlet oxygen, hydroxy radical, and hydrocauses production of reactive oxygen species such as superoxide anion, singlet oxygen, hydroxy radical, and hydrogen peroxide. Radicals and reactive oxygen species accelerate the decomposition of proteins, lipids, carbohydrates, gen peroxide. Radicals and reactive oxygen species accelerate the decomposition of proteins, lipids, carbohydrates, gen peroxide. Radicals and reactive oxygen species accelerate the decomposition of proteins, lipids, carbohydrates, gen peroxide. Radicals and reactive oxygen species accelerate the decomposition of proteins, lipids, carbohydrates, gen peroxide. Radicals and reactive oxygen species accelerate the decomposition of proteins, lipids, carbohydrates, and vitamins, and could cause significant nutrient loss in foods. Carbohydrates are less sensitive to riboflavin-and vitamins, and could cause significant nutrient loss in foods. Carbohydrates are less sensitive to riboflavin-and vitamins, and could cause significant nutrient loss in foods. Carbohydrates are less sensitive to riboflavin-and vitamins, and could cause significant nutrient loss in foods. Carbohydrates are less sensitive to riboflavinand vitamins, and could cause significant nutrient loss in foods. Carbohydrates are less sensitive to riboflavinphotosensitized oxidation than proteins, lipids, or vitamins. Riboflavin is an excellent photosensitizer for singlet photosensitized oxidation than proteins, lipids, or v...

The formation of singlet oxygen by riboflavin and the kinetics and mechanisms of riboflavin degradation in aqueous solution under light were determined. The singlet oxygen formation rate by riboflavin was 2.31 mole oxygen/mL headspace/h of serum bottle. The degradations of riboflavin were 66% in D 2 O and 40% in H 2 O, respectively, under light after 24 h. The results indicate that singlet oxygen is involved in riboflavin destruction under light. The riboflavin destructions were 94.0% and 15.7% with 0 mM or 160 mM ascorbic acid, respectively, under light after 96 h. The reaction rate between riboflavin and singlet oxygen was 1.01 × 10 10 /M/ s, which is a diffusion-controlled reaction rate. This explains the extremely fast degradation of riboflavin in foods under light. Ascorbic acid and sodium azide reduce the degradation of riboflavin under light with different quenching mechanisms. Ascorbic acid quenched both singlet oxygen and excited triplet riboflavin. Sodium azide quenched only the singlet oxygen in riboflavin solution with a quenching rate of 1.547 × 10 7 /M/ s. With the involvement of both the Type-I and Type-II mechanisms in the riboflavin degradation under light, singlet oxygen quencher alone could not protect the riboflavin from degradation completely. Addition of ascorbic acid can protect riboflavin oxidation in foods exposed to light.

Lumichrome and lumiflavin were formed from riboflavin under light. pH had a significant influence on the formation of lumichrome and lumiflavin from riboflavin. Lumichrome was the only major product from riboflavin under neutral or acidic pH values. Lumiflavin was also formed from riboflavin in basic pH. The maximum concentration of lumiflavin from 100 microM riboflavin at pH 8.5 was 30.9 microM, and it was reached after 2 h of exposure at 1500 lux. The maximum concentration of lumichrome formed from 100 microM riboflavin at pH 4.5, 6.5, or 8.5 was 79.9, 58.7, and 73.1 microM, respectively, after 8, 6, or 2 h of light exposure. The formation of lumichrome and lumiflavin from riboflavin was due to the type I mechanism of the riboflavin photosensitized reaction. Singlet oxygen was also involved in the photosensitized degradation of lumiflavin and lumichrome. The reaction rates of riboflavin, lumiflavin, and lumichrome with singlet oxygen were 9.66 x 10(8), 8.58 x 10(8), and 8.21 x 10(8) M(-1) s(-1), respectively. The headspace oxygen depletion and headspace volatile formation were significant in soy milk containing lumichrome or lumiflavin under light (p < 0.05) and were insignificant (p > 0.05) in the dark. Ascorbic acid could inhibit the total volatile changes of soy milk under light. Soy milk should be protected from light to prevent the photodegradation of riboflavin and the oxidation of soy milk.

Soymilks with or without added riboflavin in serum bottles were stored under light or in dark at 20 °C. The headspace oxygen and volatile compounds were determined by gas chromatography. Riboflavin had significant effects on the headspace oxygen depletion and volatile compounds formation in soymilk under light (P < 0.05). Riboflavin did not have significant effects on the formation of volatile compounds and the depletion of headspace oxygen in dark (P > 0.05). The volatile compounds increased under light, but not in dark as the added riboflavin increased. Storage temperature at 4 °C or 20 °C did not have significant difference in the effect of riboflavin on the headspace oxygen depletion in soymilk under light. Hexanal, an important beany flavor compound, was identified as the major volatile compound in the riboflavin photosensitized soymilk. Singlet oxygen oxidation was involved in the formation of volatile compounds in soymilk under light. Hexanal could be formed by singlet oxygen oxidation. Ascorbic acid, a quencher for singlet oxygen and the excited triplet sensitizer, significantly inhibited the formation of hexanal and total volatiles in soymilk under light.

No abstract