Color Quality of Oregon Strawberries—Impact of Genotype, Composition, and Processing
T. NGO, R.E. WROLSTAD, AND Y. ZHAO
ABSTRACT: This investigation was to evaluate fruit color and study the effect of processing on color quality of strawberry products. Three color instruments with different viewing angles, viewing areas, and sample presentation geometrieswere compared for their effectiveness in measuring CIEL*C*ho color values for fresh fruits of 6 strawberry genotypes. There were significant differences between genotypes as well as between instruments. Fruits from the Totem genotype were frozen, canned, and made into jam. Color changes were measured along with the following compositional determinations: total monomeric anthocyanins (ACN), totalphenolic content (TPC), and percent polymeric color. ACN in fresh strawberries ranged from 37.1mg to 122.3 mg per 100 g of fresh fruit. Freezing resulted in an apparent increase in ACN and transfer of 70.2% of the anthocyanins from the berries into juice. Physical transfer of pigments to syrup also occurred with canning: there was approximately 70% loss in ACN, about 20% increase in polymericcolor, and 23.5% decrease in TPC. Pronounced color change and substantial losses in ACN and TPC of strawberry jams occurred during processing and 9 wk of storage. Storage of jams at 38 ◦ C compared to 21 ◦ C over a period of 9 wk resulted in marked losses of ACN and TPC. Keywords: canning, color, freezing, genotype, jam, strawberries
trawberries are a rich source of polyphenolics (Sun and others2002; Aaby and others 2005), with anthocyanin pigments being responsible for their appealing, bright red color (pelargonidin-3glucoside is the major pigment; Garz´ n and Wrolstad 2002). Studies o on strawberry extracts have shown that strawberry phenolic extracts have high antimicrobial activities (Nohynek and others 2006) and high levels of antioxidants (Aaby and others 2005; Rababah and others2005). The phytochemicals present in strawberry extracts have also been found to have a potent inhibitory effect in vitro on HepG 2 cell proliferation (Meyers and others 2003). Unfortunately, the attractive color of fresh strawberries does not normally prevail during processing and storage (Garz´ n and Wrolo stad 2002). Compared to other berries, strawberries are relatively low in pigment content(Skrede and others 1992; Clifford 2000), ranging in total anthocyanin (ACN) from 10 to 80 mg/100 g of fresh weight (Rein 2005). Thus degradation during processing and storage can have a major impact on the color of finished products. The freezing process triggers the formation of ice in cellular fruits, which increases the volume of the fruit (Fennema 1996) and damages the integrity of the cell,leading to fruit structure breakdown. Large drip loss found in the thawed product (Han and others 2004) will have a major effect on the appearance of the product. Another adverse consequence of freezing is that nonaqueous constituents become concentrated in the unfrozen phase (Fennema 1996). Thus, besides lowering reaction rate by lowering temperature, freeze-concentration can increase reaction rates,resulting in decreased anthocyanin and ascorbic acid contents in frozen stored strawberries (Larsen and Poll 1995; Sahari and others 2004). Processing strawberries by canning or manufacturing into jam necessitates high-temperature treatments that can alter and dam-
age color quality of the finished product. Normal commercial exhaust and sterilization procedures during canning have been shownto have little effect on anthocyanin degradation; the process, however, caused leaching of anthocyanins out of the berries into the syrup (Adams and Ongley 1973). During jam manufacture at atmospheric pressure, anthocyanin losses in the final product varied from 10% to 80% when boiling time ranging from 10 min to over 15 min (Garc´a-Viguera and Zafrilla 2001). Under vacuum pressure ı conditions,...