BROWNING OF FRESH-CUT SWEET CORN KERNELS AFTER COOKING IS PREVENTED BY CONTROLLED ATMOSPHERE STORAGE

Abstract

Reduced O2 plus elevated CO2 (2% O2 + 10% CO2) was very beneficial in maintaining the visual quality of fresh-cut sweet corn kernels and reduced sugar and flavor losses during 10 days storage at 5 °C compared with storage in air. The main benefit of this controlled atmosphere (CA) was to prevent after cooking browning. Preliminary results indicate that after cooking browning is not associated with a Maillard reaction since 5-hydroxymethylfurfural (HMF), the characteristic intermediate compound produced during the Maillard reaction, is not present in cooked sweet corn kernels exhibiting browning. There were no significant changes in the total soluble phenolics content during storage in air or CA, but the soluble phenolic levels decreased with cooking, which suggests that the after cooking brown color may be due to as yet unidentified insoluble phenolic-protein complexes in the cooked sweet corn tissue The total aerobic microbe count increased with storage and the increase was significantly greater in air. This suggests that the browning could be a response of the sweet corn tissue to the microorganisms, or it may be associated with some product of microbial enzyme activity. INTRODUCTION Fresh-cut sweet corn is a very perishable product since the kernels have a very high respiration rate that increases the rate of deterioration compared to intact sweet corn. Previous work with fresh-cut sweet corn showed that this product can be stored for 10 days in low temperature (0-1 °C). Using atmosphere modification (2% O2 plus 10% CO2) allows storage even at higher temperature (5 °C) without much loss in visual or chemical quality (Brecht, 1999; Riad and Brecht, 2001). There are two main problems associated with this product: loss of sugar and flavor (dimethyl sulfide, the main component of sweet corn aroma), and browning that can occur after cooking when the cut kernels are stored in air at higher than optimum temperatures (Riad and Brecht, 2001). Blanchard et al. (1996) described a similar after cooking browning problem in air-stored diced onion, which was eliminated by storage in 10% CO2. After-cooking browning as a result of the Maillard reaction (a nonenzymatic reaction between sugars and free amino acids usually associated with thermal processing) is common in the food processing industry. This reaction produces 5-hydroxymethylfurfural (HMF) (Bayindirli et al., 1995; Gogus et al., 1998). In the case of sweet corn, the possibility of Maillard reaction would seem to be very high since sweet corn has high levels of free amino acids and sugars (Courter et al., 1988; Grunau and Swiader, 1991), and because the brown color does not develop until the kernels are cooked. Another possibility for this browning is the autooxidation of phenolic compounds that cause polymerization of polyphenolic compounds (Talcott and Howard, 1999). It is well known that phenolic acids increase after wounding (Babic et al., 1993a,b; Howard and Griffin, 1993; Ramamurthy et al., 1992). Brecht (1999) noticed that the browning is severe in cut kernels but there is no browning in intact kernels, which also suggests that the cutting or injured tissues in the kernels might result in elevated levels of phenolics that may play a role in the after cooking browning. Fresh-cut products may have high microbial loads (circa 10 to 10 CFU/g) and these populations may increase during storage and reach high levels (e.g., 10 CFU/g) in Proc. XXVI IHC – Issues and Advances in Postharvest Hort.