Oven and freeze drying of guava promoted losses of soluble
phenolics in accordance with literature data for fruits
and vegetables Losses of phenolic compounds during drying processes may be mainly attributed to oxidative reactions. While during oven drying one might expectboth non-enzymatic and enzymatic oxidative reactions to take place, during freeze drying enzymatic oxidation by polyphenol oxidase and peroxidase is more likely to occur, due to the lower exposure to oxygen and to cell structure injury caused by ice crystals formation Consistently, freeze drying promoted a higher loss
in the contents of soluble phenolics (58% in comparison to the
fresh guava fruit) than oven drying (37%) This difference
may be explained by the formation of ice crystals within the tissue
matrix during freeze drying, leading to cell rupture and exposure of
phenolics to the aforementioned oxidative conditions Oven drying of pumpkin also led to higher contents of phenolic compounds than freeze drying However, Shih, Kuo, and Chiang (2009) observed that oven and freeze drying led to similar contents of phenolics in yellow sweet potatoes, and studies have showed that freeze drying of
grape skin, tomatoes, ginger and citrus fruits leads to higher contents
of phenolic compounds than oven drying Therefore, the choice between oven or freeze drying of a given fruit or vegetable aiming at phenolic compounds stability is difficult to predict based on data from other foods and needs to be supported by
experimental data.