PREDICTION OF NEUTRAL AND ACID DETERGENT FIBER CONTENT OF GREEN ASPARAGUS STORED UNDER REFRIGERATION...

PREDICTION OF NEUTRAL AND ACID DETERGENT FIBER CONTENT OF GREEN ASPARAGUS STORED UNDER

REFRIGERATION AND MODIFIED ATMOSPHERE

SPECTROSCOPY CONDITIONS BY NEAR-INFRARED REnECTANCE

A. GARRIDO’33, M.T. G. CAN02, D. PEREZ’ and C. LOPEZ’

‘Depament of Animal Production E.T.S. de Ingenieros Agrbnomos y de Montes

Alamea’a del Obispo s/n Universidad de Cbrdoba 14080 Cbrdoba, Spain

2Department of Food Technology E.T.S. de Ingenieros Agrbnomos y de Montes

Alamea‘a del Obispo s/n. Aptdo 3048 Universidad a2 Cbrdoba 14080 Cbrdoba, Spain

Accepted for Publication October 9, 2000

ABSTRACT

The influence of the cultivar, harvesting time and length of storage under refigeration and modified atmosphere conditions on the fiber content of green asparagus, grown in Hudor-Tajar, Granada, Spain, was studied. Neutral detergent (NDF) and acid detergent (ADF) fiber content evolution was measured afrer specifled storage conditions. Near-infrared (NIR) calibration equations were developed and validated for these chemical characteristics. NIR spectroscopy was accurate in predicting NDF and ADF, proving its potential as a QC analytical technique.

Direct inquiries to Dr. Gamdo E-mail: palgavaaQuco.es or Dr. Sinchez E-mail: bt 1 sapim@uco . es

Journal of Food Quality 24 (2001) 539-550. All Rights Reserved. OCopyright 2001 by Food & Nutrition Press, Inc.. Trumbull, Connecticut. 539

540 A. GARRIDO, M.T. SANCHEZ, G. CANO, D. P E E Z and C. LOPEZ

INTRODUCTION

Asparagus is a highly perishable product requiring fixed storage conditions for shelf-life extension: low temperatures (0-5C), and high relative humidity (85- 90%) (Bernalte ef al. 1995; Moure ef al. 1995). Refrigeration under modified atmosphere (MA) storage seemed to be the best method for white asparagus storage (Lill 1980). More recently, MA storage using polypropylene bags has been proposed to prevent water loss and enhance C 0 2 concentration in the ambient atmosphere (Zagory and Kader 1988).

Analytical techniques used at present in quality control, either for the chemical determination of the fiber fractions or physical techniques to find out the toughness or energy of the asparagus cuts are long and costly requiring qualified, trained personnel and they suffer from a lack of precision. Thus, it would be of interest to devise a method to incorporate a fast analysis with a low cost per sample, a high rate of repeatabilityheproducibility and be capable of furnishing analytical information about constituents.

The NIRS technique (Near-Infrared Reflectance Spectroscopy) is a promising contribution from this viewpoint since it fulfills the basic requirements demanded from these analytical techniques.

Early research conducted with NIRS for use in forage quality analysis indicated the potential of the technology for the prediction of chemical components. Authors have reported predictions of constituents such as dry matter, neutral detergent fiber, acid detergent fiber and acid detergent lignin (Martin ef al. 1996). NIRS has also proved to be useful for the prediction of fiber constituents in fresh forage, avoiding the need to dry or grind the forage (Dardenne ef al. 1996; Kennedy ef al. 1996).

At present, there is no research work available on the application of the NIRS technique to either white or green asparagus. It is therefore necessary to study its use in characterizing asparagus and to evaluate possible changes in the composition and quality of the product during storage under different circum- stances.

The overall goal of this work, therefore, was to develop NIRS methodology and correlation equations for evaluating the chemical characteristics of green asparagus as related to cultivar, harvest time and storage under refrigeration and MA conditions. Specific objectives were: (1) To relate the changes in chemical characteristics, neutral detergent fiber (NDF) and acid detergent fiber (ADF) content to the harvest time and storage length afforded by plastic film and refrigeration, and (2) to develop and evaluate NIR calibration equations for assessing ADF and NDF content in asparagus. The equations will further help to study the evolution of the fiber content in asparagus samples without other analytical reference data, eluding costly and time consuming analytical procedures, and make NIRS usable in QC programs.

FIBER CONTENT OF ASPARAGUS BY NIR SPECTROSCOPY 541

MATERIAL AND METHODS

Material and Sample Preparation

Green asparagus spears (Aspurugus oflcinulis, L., var. UC-157 and Taxara), cultivated within the Specific Quality Appellation “Asparagus of Huetor-Tajar” (Granada, Spain) area, were manually harvested and rapidly transported for processing to a pilot plant. The spears were harvested three times during the season of mid April, May and June.

Spears were washed, drained and cut to 22 cm in length, then packed by hand in polypropylene bags (30 pm thickness). The bags were kept under refrigeration conditions of 2C and 90% relative humidity during 1, 2, 7, 14 and 21 days.

For the analytical determinations, asparagus spears were cut into three portions, 0-6 cm, the bud; 6-14 cm, the middle portion; and 14-22 cm, the base. The change in their fibrousness was studied in each of them.

Chemical Analysis

The NDF and ADF analyses were performed following the method proposed by Robertson and Van Soest (1981) in all the samples of the middle portion (n = 36) and in the bud and base samples (5 and 10 samples, respective- ly). For the determination of fiber, a Selecta “Dosi-Fiber” extractor (J.P. Selecta, S.A. Barcelona) used samples dried in a forced-air circulation oven at 60C for 24 h and ground in a Culatti hammer mill (CULAlTI AG, Zurich) with a 1 mm screen. The analysis of the dry matter content of the samples was made in a forced-air convection oven at 105 f 1C for 24 h.

Spectroscopy Measurements

The spectral properties of the treated samples were measured in a scanning monochromator (NIRSystems 6500, Foss NIRSystems, Silver Spring, Maryland) controlled by the IS1 NIRS-3 software version 3.11 (Infrasoft International, Port Matilda, PA). Reflectance spectra between 400 and 2500 nm in 2 nm steps were obtained by twice scanning and averaging dried and milled samples in a ring cup. The spectra were recorded as log (UR), where R represented diffusely reflected energy. Spectral data were combined with the ADF and NDF data obtained analytically. Spectra corresponding to the total number of samples (108) were collected.

Calibration and Prediction Procedures

Once the spectra were collected, a mean-centering step was then made of the population as a preliminary step to calibration, and a subsequent selection

542 A. GARRIDO, M.T. SANCHEZ, G. CANO, D. PEREZ and C. LOPEZ

of the most representative samples of that population was made. These two operations were carried out by the algorithms Center and Select, both included in the software IS1 NIRS 3, as a function of the Mahalanobis “H” distance.

A Modified Partial Least Square (MPLS) regression procedure was used to develop the calibration equations. The correction processes for the “scatter” phenomenon, known as Standard Normal Variate (SNV) and De-trend, were employed in all the cases.

For each analytical parameter, calibrations were generated using different mathematical treatments. These corresponded to raw spectral data followed by the first and the second derivatives of log (UR). Mathematical derivations used were “0,0,1,1”, “1,4,4,1”, “1,5,5,1”, “2,4,4,1”and“2,5,5,1”, wherethefirst digit is the number of the derivative, the second is the gap over which the derivative is calculated, the third is the number of data points in a running average or smoothing, and the fourth is the second smoothing (Shenk and Westerhaus 1995).

The calibration accuracy was evaluated by the determination coefficient, 8, and the standard error of cross validation (SECV), since the latter is a good estimator of the standard error of prediction (SEP) and is preferred in order not to have to reduce the number of samples to be used for the calibrations (Shenk and Westerhaus 1996). Other statistics used were the RPD, or ratio of the standard deviation of the original data (SD) to SECV (SD/SECV), and the RER, or ratio of the range of the original data (maximum-minimum value) to SECV (Range/SECV). All the equations were obtained using the NIR spectral region (1 100-2500 D).

Statistical Analysis

N.C.). All data were analyzed using the SAS Statistical Analysis System (Cary,

RESULTS AND DISCUSSION

Change in Fiber Content During the Harvesting Period and During Refrigerated Storage Under MA

The study, by chemical methods, of the change in fiber content during harvesting, was conducted using the middle portion of the asparagus (6-14 cm), since this is (Hernhdez ef al. 1993) the most representative part of the asparagus. To corroborate these results, and based on the information supplied by the algorithm Select, the fiber content was determined by chemical methods in 5 samples of buds and 10 of bases, permitting the verification of the correlation equations.

FBER CONTENT OF ASPARAGUS BY NIR SPECTROSCOPY 543

The fiber content of the middle part, both NDF and ADF, was significantly higher (a = 0.05) in all cases in the Taxara variety than in the UC-157 variety (Table l), in agreement with the findings of Canalejo er al. (1998) in their study of the conduct of these varieties under refrigeration conditions. The change in the fiber content throughout harvesting showed an upward trend, the highest values for both varieties corresponding to the month of June, the last month of the harvesting period. For the NDF content, a significant increase (a = 0.05) occurred in the case of the UC-157 variety throughout the harvesting period. The Taxara variety showed the same behavior but the differences were not significant (Fig. 1).

TABLE 1 . COMPARISON OF THE NEUTRAL DETERGENT AND ACID DETERGENT FIBER

VALUES BY CHEMICAL. ANALYSIS AND BY NIRS

Chemical Analysis NIRS Analysis -

Spear portions Spear portions Fiber (8mJ s dmt Middle Bud Middle BaSe

Taxara UC-157 Taxara UC-157 Taxara UC-157 Taxara UC-157

NDF 18.13 16.20 14.79 15.32 14.50 13.75 28.43 24.91

ADF 13.50 12.76 13.12 13.00 11.31 10.89 19.91 17.47

The ADF content increased as harvesting advanced, from April to June, with significant differences found (a = 0.05) for both varieties at the harvesting times analyzed (Fig. 2).

The NDF and ADF content in the Taxara and UC-157 varieties increased, although not significantly (a = 0.05), throughout the storage period under MA conditions and refrigeration. This progression was repeated at the three times analyzed during the harvesting period and this phenomenon is widely corroborat- ed in the literature (Bernalte e? al. 1995).

NIRS Calibration Development

The NDF and ADF data obtained analytically were entered in the NIRS software simultaneously with the spectra of samples of the middle portions of the dehydrated asparagus in order to correlate the spectra with the NDF and ADF information for all the samples studied.


I

TAXARA - UC-157

I

FIG. 1. CHANGE DURING HARVESTING PERIOD OF NEUTRAL DETERGENT FIBER CONTENT IN THE MIDDLE PORTION OF THE ASPARAGUS

Table 2 shows the statistics obtained for the NIR equations for each of the constituents studied. The 3 values for all the constituents were very high (> 0.9). The 3 value indicates the portion of the variation in the reference data explained by the model obtained. She& and Westerhaus (1996) indicated that equations over 0.90 3 provide excellent quantitative information.

The SECV values obtained for NDF and ADF are low in comparison with those usually reported for forage (Martin el al. 1996). Generally speaking, asparagus has lower NDF and ADF values than forage. A more accurate way to evaluate the SECVs would be by using the ratios of RPD/RER.

Williams and Sobering (1996) indicate that the RPD should be at least 3 and RER higher than 10 for reliable quantification. Table 2, referring to the best equations obtained for NDF and ADF, gives RPD (7.81, 6.32) and RER values (25.26, 20.99), which comply in excess with the recommended values.

Data Predicted by NIRS for Fiber Content Evolution

With the best prediction equations shown in Table 2, fiber data predicted by NIRS were obtained with which to begin the study of the fiber content change throughout the harvesting period and during refrigerated MA storage.

The NIRS data on the NDF and ADF content always gave higher values in the Taxara variety than in the UC-157 variety, these differences were significant (a = 0.05) in the middle portion and in the base, confirming the chemical data (Table 1).

FIBER CONTENT OF ASPARAGUS BY NIR SPECTROSCOPY

('UP 001.15) dav


TABLE 2. CALIBRATION STATISTICS FOR THE EQUATIONS OBTAINED FOR THE PREDICTION OF NDF AND ADF FOR GREEN ASPARAGUS STORED UNDER REFRIGERATION AND

MODIFIED ATMOSPHERE CONDITIONS

Ropertics N Math SEVC 8 Means Range SD Term RPD RER Tredhncnt

NDF 46 O,O,l,l 0.91 0.98 18.58 11.80-33.12 6.59 8 7.19 2327

48 1,4,4,1 1.07 0.97 18.56 11.80-33.12 6.45 8 5.99 19.81

48 IJ.5.1 1.07 0.97 18.56 11.80-33.12 6.45 8 6.01 19.88

46. 2.4.4.1 0.84 0.98 18.58 11.80-33.12 6.59 8 7.81 25.26

46 2,5,5,1 0.84 0.98 18.58 11.80-33.12 6.59 8 7.80 25.23

ADF 50 0,0.1,1 0.64 0.97 13.57 9.45-22.17 3.79 4 5.91 19.84

51 1,4,4,1 0.62 0.97 13.65 9.45-22.17 3.79 6 6.07 20.35

51 1.5.5.1 0.62 0.97 13.65 9.45-22.17 3.79 6 6.07 20.35

50' 2,4,4,1 0.60 0.97 13.63 9.45-22.17 3.83 6 6.32 20.99

50 2,5J,I 0.60 0.97 13.63 9.45-22.17 3.83 6 6.38 21.20

N: R a l ample numberr for ulibmtion SECV: Sundad crmr of avm validation r' : Coefftcient of detennkatmn S D mndd deviation Term: Number of PLS ficton RDP SDBECV RER RangdSECV *: Best equation

Throughout the harvesting period, it was observed that the contents of NDFN,, (Fig. 3) and ADFNIR (Fig. 4) from the middle and base portions of the Taxara variety increased significantly (a = 0.05) from April to June. For UC- 157, there were no significant differences (a = 0.05) in the NDF,,, content in the middle portion. However, the ADFNIR content increased throughout the harvesting period with significant differences, the same result as was found when analyzed by the chemical method in the study of the middle portion.

There were no significant differences (a = 0.05) between the values of NDF,,, and ADF,,, for any of the two varieties in the three portions of the asparagus during MA refrigerated storage at the three harvesting times analyzed. This confirmed that refrigerated storage combined with MA is a technique which maintains the quality of the asparagus as measured by the NIRS technique.


BUD MIDDLE BASE

Spear zone

FIG. 3. EVOLUTION OF NEUTRAL DETERGENT FIBER IN TAXARA VARIETY THROUGHOUT THE HARVESTING PERIOD

The results of NDF and ADF predicted by NIRS indicate that the fiber content of the base portion increased with storage, although the differences between these values were not significant. The bud and middle portions had very similar values, and the bud nearly always showed higher fiber values than those of the middle portion of the asparagus.

CONCLUSIONS

Storing asparagus under refrigerated and MA conditions seems to be an effective method for extending the shelf life of green asparagus. Spectral reflectance was sensitive to asparagus chemical changes (NDF, ADF) across the NIR analytical range (1100-2500 nm). NIR spectroscopy appears to have excellent potential for evaluating the two chemical properties tested. The high precision of the calibration equations obtained for the green asparagus population studied, confirmed the expectations aroused by the NIRS technique for quality control in agrofood industries, especially when a large number of samples need to be analyzed.

548 A. GARRIDO, M.T. SANCHEZ, G. CANO, D. PEmZ and C . LOPEZ


ACKNOWLEDGMENTS

The authors would like to thank the Regulating Council for the Specific Quality Appellation Asparagus of HuCtor-Thjar” for providing asparagus samples. The NIRS analyses were done using NIRS hardware and software from the Centralized NIRS Unit of the University of Cordoba, Spain.

REFERENCES

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CANALEJO, J.E., SANCHEZ, M.T. and CANO, G. 1998. Influencia de las condiciones de almacenamiento posrecolecci6n en la textura de espkrragos verdes cultivados en HuCtor-Tajar (Granada). Alimentacion, Equipos y Tecnologia XVZZ(5), 149-152.

DARDENNE, P., AGNEESSENS, R. and SINNAEVE, G. 1996. Fresh forages analysis by near infrared spectroscopy. In Near-Infared Spectroscopic: The Future Waves. The Proceedings of Th International Conference on Near- Infared Spectroscopic, (A.M.C. Davies and P. Williams, eds.) pp. 531-536, Montreal.

HERNANDEZ, M.T., BERNALTE, M.J. and CARBALLO, B.M. 1993. Evoluci6n de parhnetros fisicoquimicos de espirrago blanco y verde a lo largo de la campaiia. Alimentaria 247, 43-45.

KENNEDY, C.A., SHELFORD, J.A. and WILLIAMS, P.C. 1996. Near infrared spectroscopic analysis of intact grass silage and fresh grass for dry matter, crude protein and acid detergent fiber. In Near-Infrared Spectro- scopic: The Future Waves. The Proceedings of Th International Conference on Near-Znfrared Spectroscopic, (A.M.C. Davies and P. Williams, eds.) pp. 524-530, Montreal.

LILL, R.E. 1980. Storage of fresh asparagus. New Zealand J. Exp. Agric. 8,

MARTiN, N.P., MATTESON, M., UNDERSANDER, D. and PETERS, J. 1996. Near infrared spectroscopy forage and feed testing network: a model of co-operation. In Near-Infared Spectroscopic: The Future Waves. Z7ze Proceedings of Thlnternational Conference on Near-Infrared Spectroscopic, (A.M.C. Davies and P. Williams, eds.) pp. 553-558, Montreal.

MOURE, J., AYUSO, M.C. and LOPEZ, J.L. 1995. Efecto del tratamiento postcosecha en la calidad del espirrago. Alimentaria 268, 31-35.

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550 A. GARRDO, M.T. SANCHEZ, G . CANO, D. PEREZ and C. LOPEZ

ROBERTSON, J.B. and VAN SOEST, P.J. 1981. 22e Detergent System of Analysis and Its Application to Human Foods, (W.P.T. James and 0. meander, eds.) pp. 123-158, Marcel Dekker, New York.

SHENK, J.S. and WESTERHAUS, M.O. 1995. Routine operation, calibration development and network system management manual. pp. 247-277, NIR Systems, Inc., 12101 Tech. Road, Silver Spring, MD.

SHENK, J.S. and WESTERHAUS, M.O. 1996. Calibration of IS1 way. In Near-Infrared Spectroscopic: The Future Waves. The Proceedings of Th International Conference on Near-Znfared Spectroscopic, (A.M. C. Davies and P. Williams, eds.) pp. 198-202, Montreal.

WILLIAMS, P.C. and SOBERING, D.C. 1996. How do we do it: a brief summary of the methods we use in developing near infrared calibrations. In Near Infrared Spectroscopy: the Future Waves. The Proceedings of the Th International Conference on Near Infrared Spectroscopy, (A. M.C. Davies and P. Williams, eds.) pp. 185-188, Montreal.

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