Nonenzymic Browning and Corrosion in Stored Single-strength Grapefruit Juice

Nonenzymic browning and corrosion were monitored in commercially canned single-strength grapefruit juice stored for 15 weeks at 10°, 20°, 30°, 40°, and 50°C. Browning did not occur in juices stored at 10° and 20°C, but began to ap pear after 9 weeks in juices stored at 30°C. Browning was quite evident within 3 weeks for juices stored at 40° and 50°C. Plots of nonenzymic browning versus corrosion (tin and iron dissolution) showed significant correlations and equa tions that best fit these correlations were quadratic. At high storage temperatures, the protection exerted by tin against nonenzymic browning is diminished and juice discoloration was evident. Possible hypotheses for the protective effects of tin are presented. When the dissolved tin level exceeded about 60 mg tin/kg juice, noticeable browning occurred. The iron level of juice does not appear to manifest protective activ ity and, in fact, might be detrimental. Color is an important quality factor in the marketing of citrus juices. Change in color, primarily manifested by nonenzymic browning, reduces consumer acceptance and, therefore, is an important shelf-life variable. Although sig nificant differences in brown pigment formation appear more dependent on thermal processing and storage (6), the type of packaging container contributes to hue changes Florida Agricultural Experiment Station Journal Series No. N-00238. 274 and the overall brown perception of the product (3). Nonenzymic browning develops faster and more intensely in glass-packed juices than in juices packed in tinplate con tainers (10, 17). Packaging of citrus juices in tinplate containers offers many advantages over other types containers (glass, fiberboard, plastic, aseptic packages), namely, ease of packing, sterilizing, handling and transporting. However, a major disadvantage in the use of tinplate cans is potential corro sion. During the corrosion process, metallic off-flavors are introduced into the product and notable amounts of tin and iron are dissolved by the acidic juice. Extensive studies have been conducted to evaluate the influence of temper ature and storage time on corrosion in processed canned citrus juices (9, 11, 12, 18). However, no reported studies have been conducted on the relationship of nonenzymic browning to the dissolution of tin and iron during storage of processed canned citrus juices. To this end, we report the results of our investigations on reaction rate kinetics of nonenzymic browning and corrosion in canned singlestrength grapefruit juice. Materials and Methods Samples and Storage Treatments. Commercially processed single-strength grapefruit juice (SSGJ) was purchased from a local processing company. All samples were pro duced from the same batch of reconstituted concentrate, and were packed in tinplated cans with enamel-coated lids (177 ml; 48 cans per case). All samples were obtained dur ing the time of processing and immediately transported to the Citrus Research and Education Center. Canned SSGJ Proc. Fla. State Hort. Soc. 103: 1990. were placed in storage lockers (about 100 cans) at varying temperatures, namely, 10°, 20°, 30°, 40° and 50°C and stored for 3, 6, 9, 12, and 15 weeks. Three cases of grape fruit juice were placed at -22°C and were considered as "nontreated samples" (controls). Samples were randomly selected at specific time periods and analyzed for nonenzymic browning, and tin and iron contents. Browning Determination. Nonenzymic browning of SSGJ was determined by an improved method of Klim and Nagy (2) with slight modifications (10). Clarified, filtered sam ples were read at 420 nm with a single-beam Bausch and Lomb Spectronic 88 using 13-mm cuvettes (10-mm light path). The analog signal from the spectrometer was meas ured with a Fluke Model 75 SVfe-digit multimeter for im proved accuracy. Readings were recorded to three signifi cant figures. Absorbances of samples subjected to different storage temperatures and times were reported as absor bance difference data by subtracting the treated sample absorbance from freshly canned juice absorbance, namely, AA420 = A42o treated sample A42o nontreated sample Iron and Tin Measurements. Minerals in SSGJ were deter mined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The SSGJ sample was accurately weighed (12 to 12.5 g) into a Teflon digestion vessel (MDS81D; CEM Co., Indian Trail, NC). To this sample was added 1.25 g cone. HNO;i, 3.75 g cone. HCI and the mix ture brought accurately to a total weight of 25 g with deionized water. The Teflon vessel, containing a 7.03 kg/ cm2 pressure relief valve and threaded cap, was closed with a CEM capper. The vessel was placed in a programmable 600 W microwave oven with rotating turntable and prog rammed for 8 min at 100% power, then left for 5 min at 0% power and, finally, 8 min at 100% power. The diges tion vessel was cooled to room temperature, opened using the capping station, and the contents filtered through ashless filter paper (Whatman #42). Digestion of SSGJ sam ples in an enclosed Teflon vessel by microwave resulted in recoveries of greater than 99% for Sn and Fe as evidenced by ICP analyses. Samples were stored in polyethylene or Teflon bottles pending analyses. Spectrochemical analysis was carried out with a Jarrel-Ash Atomscan 2000 sequen tial and computer-controlled spectrometer equipped with an autosampler. Sample gas flow determines the velocity of argon through the nebulizer and was set to 0.6 L/min for high power (1.4-1.5 kw). The observation height was 16 mm above the coil. Sample uptake rate was 1 ml/min and the spectral resolution was 0.018 nm for 178-380 nm and 0.036 nm for 380-780 nm. Analytical lines used for these analyses were 238.20 nm (Fe) and 235.48 nm (Sn). Other Methods. Degrees Brix, percent citric acid and pH were determined by recognized analytical methods (15). Statistical evaluation of data was conducted with the SAS PC Version 6.03 (SAS Institute, Inc., Cary, NC). Results and Discussion The processed SSGJ employed in these studies posses sed the following properties: 10.2° Brix, 1.11% citric acid, pH 3.36, Brix/acid ratio 9.2. The tin and iron contents of these freshly canned samples averaged 17 mg/kg and 1.7 mg/kg, respectively. Mineral contents of these commercial grapefruit juices prior to canning were not ascertained be cause of flow throughput processes within a closed system during processing and canning. Nikdel (1985), however, reported tin contents of freshly prepared grapefruit juices (noncanned) to be below the detection limit of the ICPAES, namely, less than 0.030 mg/kg. The process of pack ing hot juice into a tinplated can cause immediate dissolu tion of the tin coating and, hence, the reason why a freshly packed product contains high levels of tin. Factors respon sible for these elevated levels were enumerated by Nagy et al. (12) and by Rouseff and Ting (19). Nagy (8) reported iron contents of noncanned SSGJ to range between 0.6 and 1.9 mg/kg. The initial level of iron found in these samples (1.7 mg/kg) indicated that there was minimal dis solution of the base plate during canning. However, iron does dissolve from the base plate during elevated temper ature storage of canned juices (9). Nonenzymic browning index values for SSGJ's were ob tained by measuring absorbance at 420 nm and correcting those values by subtracting from the absorbance value of nontreated processed juice. Nontreated canned SSGJ yielded an average A42() of 0.104 (CV = 2.9%). The cor rected absorbance values (AA420) are considered a reason ably true measure of browning changes occurring during storage. Table 1 lists the mean browning index values of SSGJ stored for varying periods and temperatures. As noted by Table 1, minimal browning change occurs in juices stored at 10° and 20°C (nonsignificant, p > .05). With 30°C-stored juices, noticeable changes were evident after the 9th week (p < .01). Major changes in browning occurred at the higher storage temperatures, namely, 40° and 50°C (p < .01). The relationship of corrosion (tin and iron dissolution) and nonenzymic browning has never been ascertained. Concurrent examination of corrosion and nonenzymic browning in randomly selected samples yielded data shown in Figures 1 and 2. As noted in Figure 1, tin dissolution versus nonenzymic browning, the equation which best rep resents this relationship is second order with respect to tin cone, (y = a + bx + ex2). The regression equation is y = -0.0091 + 0.000159 x + .000009x2 where x = mg tin/kg juice (R2 = .973). The equation and model indicate a high correlation (relationship) between detinning of the can and increase in nonenzymic browning absorbance. The data indicates that nonenzymic browning begins to increase when the level of dissolved tin exceeds about 60 mg/kg juice. Figure 2 shows nonenzymic browning absorbance changes versus juice iron contents. The best-fit equation is also quadratic but the correlation (R2 = .769) is not as high Table 1. Mean browning index values (AA42o) of canned SSGJ stored at varying temperatures and storage times.