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Graham H. Barry, William S. Castle and Frederick S. Davies

Juice quality of `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks was determined for fruit harvested by canopy quadrant and separated into size categories to ascertain the direct role of rootstock selection on juice soluble solids concentration (SSC) and soluble solids (SS) production per tree of citrus fruit. SS production per fruit and per tree for each size category was calculated. Juice quality was dependent on rootstock selection and fruit size, but independent of canopy quadrant. Fruit from trees on Carrizo citrange had >20% higher SSCs than fruit from trees on rough lemon, even for fruit of the same size. Large fruit accumulated more SS per fruit than smaller fruit, despite lower juice content and SSC. Within rootstocks, SS content per fruit decreased with decreasing fruit size, even though SSC increased. Rootstock effect on juice quality was a direct rather than an indirect one mediated through differences in fruit size. The conventional interpretation of juice quality data that differences in SSC among treatments, e.g., rootstocks or irrigation levels, or fruit size, are due to “dilution” of SS as a result of differences in fruit size and, hence, juice volume, is only partly supported by these data. Rather, accumulation of SS was greater for fruit from trees on Carrizo citrange than rough lemon by 25% to 30%.

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Robert L. Long, Kerry B. Walsh, David J. Midmore and Gordon Rogers

A common practice for the irrigation management of muskmelon (Cucumis melo L. reticulatus group) is to restrict water supply to the plants from late fruit development and through the harvest period. However, this late fruit development period is critical for sugar accumulation and water stress at this stage is likely to limit the final fruit soluble solids concentration (SSC). Two field irrigation experiments were conducted to test the idea that maintaining muskmelon plants free of water stress through to the end of harvest will maximise sugar accumulation in the fruit. In both trials, water stress before or during harvest detrimentally affected fruit SSC and fresh weight (e.g., no stress fruit 11.2% SSC, weight 1180 g; stress fruit 8.8% SSC, weight 990 g). Maintaining plants free of water stress from flowering through to the end of harvest is recommended to maximise yield and fruit quality.

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Carlos H. Crisosto, R. Scott Johnson, Juvenal G. Luza and Gayle M. Crisosto

The effect of irrigation management strategies on the quality and storage performance of `O'Henry' peaches [Prunus persica (L.) Batsch] was studied for two seasons. The deficit irrigation treatment induced a higher fruit soluble solids concentration and lower fruit weight. The excess irrigation treatment, compared to the optimum treatment, increased the rate of fruit water loss without altering fruit quality and storage performance. Scanning electron microscope observations indicated a higher density of trichomes on fruit from the deficit and optimum irrigation treatments than from the excess irrigation treatment. Light microscopy studies indicated that fruit from deficit and optimum irrigation had a continuous and much thicker cuticle than fruit from the excess irrigation treatment. These differences in exodermis structure may explain the high percentage of water loss from fruit from the excess irrigation treatment compared to the deficit and optimum irrigation treatments.

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Mau-Wei Lin, James F. Watson and James R. Baggett

Analysis of parents and progeny generations of bulb onion (Allium cepa L.) crosses among parents with differing content of soluble solids (SS) and pyruvic acid (PA) showed that SS and PA are expressed and inherited in a quantitative manner. Distribution of SS and PA in both parents and progenies covered a range of values. Generation means, frequency distributions, deviation from midparent value, and estimates of gene effects all indicated that inheritance of SS and PA was additive, except for small deviations from the additive hypothesis in several individual backcrosses. Estimates of broad-sense heritability ranged from 48% to 53% for PA and 8 % to 56 % for SS. Phenotypic correlations between PA and SS estimated from the F2 generations of two crosses, were moderate and positive (r = 0.50 and 0.42).

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K.H.S. Peiris, G.G. Dull, R.G. Leffler and S.J. Kays

A nondestructive method for measuring the soluble solids content (SSC) of individual processing tomatoes (Lycopersicon esculentum Mill.) was developed using NIR spectrometry. A diode array fiber optic spectrometer was used to measure NIR transmittance. Each fruit was scanned at two locations on opposite sides midway along the proximal-distal axis. After scanning, each fruit was processed and pureed, and SSC was determined using a refractometer. Multiple linear regression (MLR), partial least squares (PLS) regression, and neural network (NN) calibration models were developed using the second derivatives of averaged spectra from 780 to 980 nm. The validation results showed that NN calibration was better than MLR or PLS calibrations. The NN calibration could estimate the processed SSC of individual unprocessed tomatoes with a standard error of prediction of 0.52% and could classify >72% of fruit in an independent population within ±0.5% of SSC.

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K.H.S. Peiris, G.G. Dull, R.G. Leffler and S.J. Kays

A nondestructive method for measuring the soluble solids (SS) content of peaches [Prunus persica (L.) Batsch] was developed using near-infrared (NIR) spectrometry. NIR transmittance in the 800 to 1050 nm region was measured for four cultivars of peaches (`Blake', `Encore', `Red Haven', and `Winblo'), over a period of three seasons (1993 through 1995). Each fruit was scanned on both halves keeping the suture away from the incident light beam. Soluble solids contents of flesh samples taken from corresponding scanned areas were determined using a refractometer. Multiple linear regression models using two wavelengths were developed with second derivative spectral data and laboratory measurements of SS content. Multiple correlation coefficients (R) for individual cultivar calibrations within a single season ranged from 0.76 to 0.98 with standard error of calibration (SEC) values from 0.35% to 1.22%. Selected spectra and corresponding SS data in individual cultivar calibration data sets were combined to create season and cultivar calibration data sets to cover the entire range of SS contents within the season or within the cultivar. These combined calibrations resulted in R values of 0.92 to 0.97 with SEC values ranging from 0.37% to 0.79%. Simple correlations of validations (r) ranged from 0.20 to 0.94 and the standard error of prediction (SEP) ranged from 0.49% to 1.63% while the bias varied from -0.01% to -2.62%. Lower r values and higher SEP and bias values resulted when individual cultivar calibrations were used to predict SS levels in other cultivar validation data sets. Cultivar calibrations, season calibrations and the overall calibration predicted SS content of all validation data sets with a smaller bias and SEP and with higher r values. These results indicate that NIR spectrometry is suitable for rapid nondestructive determination of SS in peaches. Feasible applications of the method include packinghouse sorting of peaches for sweetness and parent and progeny fruit quality assessment in peach breeding programs. Using this technique fruit may be sorted into two or three sweetness classes. The technique may also potentially be extended to other fruit.

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David Del Pozo-Insfran, Christopher E. Duncan, Kristine C. Yu, Stephen T. Talcott and Craig K. Chandler

The effects of cultivar, harvest date, and production year on the soluble solids and antioxidant phytochemical levels of 22 strawberry (Fragaria ×ananassa Duch.) genotypes grown in a winter annual hill (raised bed) production system were investigated. Fruit harvested in Jan. 2003 and 2004 were characterized by low polyphenolic content, but high concentrations of soluble solids and ascorbic acid; whereas fruit harvested in Feb. 2003 and 2004 generally had elevated polyphenolic concentrations, but lower levels of soluble solids and ascorbic acid. Annual variation in soluble solids and phytochemical composition was also observed among nine strawberry genotypes, which was likely attributable to variations in solar radiation and air temperature. `Earlibrite' was among the highest for soluble solids concentration on three of the four harvest dates, while `Carmine' was noted for its high phytochemical concentrations across harvest dates and years. The breeder selection `FL 99-117' emerged as a promising selection in terms of producing fruit with high concentrations of soluble solids and antioxidant phytochemicals.

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T.K. Hartz*, P.R. Johnstone, M. LeStrange, J.J. Nunez and E.M. Miyao

Soluble solids concentration (SSC) is a major quality factor for tomatoes (Lycopersicon esculentum Mill.) grown for processing. The effects of early irrigation cutback were investigated in a series of drip-irrigated field trials in California from 2000-03. Irrigation cutback was initiated from 4-7 weeks preharvest, with irrigation volume reduced to 30% to 70% of reference evapotranspiration. Early irrigation cutback was compared to full irrigation until cutoff 2-3 weeks preharvest. SSC was monitored from the initiation of deficit irrigation until harvest, with breaker-stage fruit sampled at approximately 10-day intervals; additionally, early-maturing fruits were tagged on the plant at breaker stage and retrieved at harvest for SSC analysis. Fruit yield, overall SSC, and brix yield (Mg·ha-1 fruit solids) were evaluated at commercial maturity. Fruit SSC increased in response to soil moisture stress, with late-maturing fruit as much as 2.0 °Brix higher than fruit maturing before significant moisture stress. However, once a fruit reached the breaker stage of maturity, its SSC did not increase regardless of subsequent soil moisture stress. Across field trials, yield decline resulting from early irrigation cutback was matched by a corresponding increase in overall SSC, resulting in equivalent brix yield in all test fields. We conclude that the early irrigation cutback provides a flexible tool for SSC management and that °Brix monitoring of breaker-stage fruit can augment soil moisture monitoring to tune irrigation management to field-specific conditions.

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Frederick S. Davies, Glenn R. Zalman, Ed Stover and Scott Ciliento

EcoLyst, a formulation of N-N-diethyl-2-(4-methylbenzyloxy) ethylamine hydrochloride containing 1 g/floz [4.5 oz/gal (33.8 g·L-1)] a.i., is a plant growth regulator that has been reported to increase soluble solids concentration (SSC) in juice oranges by 0.6% to 1.2%. Our objectives were to determine the effectiveness of EcoLyst application for increasing SSC in Florida oranges (Citrus sinensis) and grapefruit (C. paradisi), and to identify the optimum rate and time of application. Experiments were conducted for three seasons using `Hamlin,' `Pineapple,' and `Valencia' sweet oranges; and for two seasons using `Flame,' `Marsh,' and `Ray Ruby' grapefruit, all in commercial groves. EcoLyst was applied at 6 and 12 floz/acre (0.44 and 0.88 L·ha-1) for oranges and 16 and 32 ppm (mg·L-1) [effectively 9 and 18 floz/acre (0.66 and 1.32 L·ha-1) in most sprays] for grapefruit, and included Silwet L-77 adjuvant at 0.05%. Applications were made at several stages of development from prebloom to initial fruit set. In all cases, SSC was determined as juice corrected SSC, by adjusting refractometer readings based on titratable acidity. In 13 trials with sweet orange only five displayed significant increases in SSC (P ≤ 0.05) resulting from EcoLyst application. Two additional trials produced SSC increases significant at P < 0.10. Even where significant increases in SSC occurred they were typically observed in only one harvest and at one time of application and were always relatively low in magnitude (highest increase over controls was 0.38%). No rate or timing of EcoLyst application was consistently associated with best response, although eight of nine SSC increases observed in orange occurred with applications ranging from prebloom to 25% open flowers. Only one significant increase in SSC was observed in five trials with grapefruit. In these studies, increases in SSC resulting from EcoLyst application were neither sufficiently consistent nor large enough to justify a recommendation for commercial use in Florida citrus.

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P.R. Johnstone, T.K. Hartz, M. LeStrange, J.J. Nunez and E.M. Miyao

Fruit soluble solids concentration (SSC) is an important quality factor for tomatoes (Lycopersicon esculentum Mill.) grown for processing. The use of drip irrigation often results in undesirably low SSC. The effects of late-season irrigation management on fruit yield and SSC was investigated in a series of drip-irrigated field trials in California from 2000–04. The effects of irrigation cutoff or deficit irrigation implemented 40 to 50 days preharvest (the period corresponding to the initiation of fruit ripening) were compared to a standard grower practice of irrigation cutoff 20 days preharvest. Irrigation cutoff 40 to 50 days preharvest increased SSC but resulted in substantial yield loss, with significantly reduced brix yield (Mg fruit solids ha-1). By contrast, deficit irrigation significantly increased SSC compared to the standard practice, with no significant loss of brix yield. In three commercial fields the effect of deficit irrigation on fruit SSC was investigated. Fruits were sampled on three dates: 1) 4 to 5 weeks preharvest, early-ripening, pink-stage fruit only, 2) about 1 week preharvest, both late-ripening, pink-stage fruit and early-ripening fruit now fully ripe, and 3) commercial harvest, composite of early- and late-maturing fruit. SSC increased in response to soil moisture stress induced by deficit irrigation, with late-maturing fruit as much as 1.6 °brix higher than fruit maturing before significant soil moisture stress. However, once a fruit reached the pink stage of maturity, its SSC was not affected by subsequent soil moisture stress. An additional five commercial field trials were conducted to compare growers' irrigation practices with greater degrees of deficit irrigation. In each field the grower's deficit irrigation regime was compared to a reduced treatment receiving 25% to 50% less water over the final 4 to 7 weeks before harvest. Across fields, applying 20% to 60% of reference evapotranspiration (ETo) over the fruit ripening period resulted in acceptable SSC without significant brix yield reduction. We conclude that deficit irrigation initiated during early fruit ripening provides a flexible tool for SSC management. Brix monitoring of earliest ripening fruit can help classify fields as to the severity of irrigation deficit required to reach desirable SSC at harvest.