Pre-harvest screening on-vine of spinach quality and safety using NIRS technology

https://doi.org/10.1016/j.saa.2018.09.035Get rights and content

Highlights

  • NIR spectroscopy was used for prediction of spinach quality and safety attributes.

  • NIRS technology tested to classify spinach as a function of nitrate content.

  • A handheld MEMS-NIR spectrometer, a promising tool for in-situ spinach monitoring

Abstract

The study sought to perform a non-destructive and in-situ quality evaluation of spinach plants using near infrared (NIR) spectroscopy in order to establish its suitability for different uses once harvested. Modified partial least square (MPLS) regression models using NIR spectra of intact spinach leaves were developed for nitrate, ascorbic acid and soluble solid contents. The residual predictive deviation (RPD) values were 1.29, 1.21 and 2.54 for nitrate, ascorbic acid and soluble solid contents, respectively. Later, this predictive capacity increased for nitrate content (RPDcv = 1.63) when new models were developed, taking into account the influence on the robustness of the model exercised by the simultaneity between the NIR and laboratory analyses. Subsequently, using partial least squares discriminant analysis (PLS-DA), the ability of NIRS technology to classify spinach as a function of nitrate content was tested. PLS-DA yielded percentages of correctly classified samples ranging from 73.08–76.92% for the class ‘spinach able to be used fresh’ to 85.71–73.08% for the class ‘preserved, deep-frozen or frozen spinach, both for unbalanced and balanced models respectively, based on Nsingle bondH signal associated with proteins. Overall, the data supports the capability of NIR spectroscopy to establish the final destination of the production of spinach analysed on the plant, as a screening tool for important safety and quality parameters.

Introduction

Spinach (Spinacea oleracea L.) is a green, leafy vegetable, with a shiny, uniform appearance. When sold, either to industry (for preserving, freezing or deep-freezing) or as a fresh product on the market, it is essential for the leaves to look freshly-picked and tender.

Spinach has an extremely high water content (around 92%) and is very low in carbohydrates and fats. It is also a rich source of vegetable protein, with, like other vegetables, high fibre content [1].

Spinach is also noted for its relatively high content of bioactive substances, including vitamin C (ascorbic acid), which is a powerful antioxidant that participates in the scavenging of the reactive oxygen species, regenerating tocopherols from their radical forms [2,3]. However, one drawback is that they accumulate substances which are harmful for the human organism, such as nitrates [4].

Over recent years, consumers have become increasingly aware of the presence of nitrates in foods, among which are vegetables, since nitrates are a serious threat to human health, due to the conversion of nitrate to nitrite, which may produce methemoglobin due to the oxidation of Fe + 2 in haemoglobin [5]. The impaired capacity of methemoglobin to deliver oxygen to tissues may lead to severe toxic effects and may even prove fatal where methemoglobin accounts for over 70% of total haemoglobin, something which affects infants and very young children almost exclusively [6]. Similarly, a number of studies have highlighted a possible link between nitrate exposure and childhood type 1 insulin-dependent diabetes mellitus [7]. Furthermore, nitrite may react with secondary amines (HNR2), which occur in many foods, to form nitrosamines. These substances are highly carcinogenic [8].

However, consumers are also well aware that eating vegetables with a high content of antioxidants, such as ascorbic acid, is beneficial for their health. The World Health Organization [9] showed that iron deficiency anemia is one of the most common nutritional disorders which has a major effect on both health and the economy. Main cause of anemia is not only low iron intake but also poor iron absorption [10]. This global health problem can be addressed by improving the dietary iron bioavailability, which can be altered by various components present in the food which can either enhance or inhibit iron absorption. Thus, when iron is present along with ascorbic acid, the absorption of iron has been shown to increase even in the presence of inhibitors [11].

In the case of spinach and in response to this growing public concern about nitrates, the European Union passed Commission Regulation (EC) No 1258/2011 of 2 December 2011 setting maximum levels for nitrates in this leafy vegetable [12]. Thus, the maximum level for nitrates was set for preserved, deep-frozen or frozen spinach at 2000 mg NO3/kg and for fresh spinach at 3500 mg NO3/kg.

All this has prompted greater attention to spinach safety and quality concerns. Nitrate accumulation, ascorbic and soluble solid contents in spinach depend not only on genotypic characteristics, but also on a number of other factors, including cultural practices, harvesting date and postharvest handling practices [13,14]. As a result, producers are increasingly anxious to provide consumers with assurances regarding the safety, quality and provenance of this product.

The nitrate content in spinach when harvested is in fact key to determining the final destination of the harvested product. The ascorbic acid content is equally important because of the close relationship between this acid and the bioavailability of iron, and also the soluble solid content, which in turn is linked to the vegetable's quality and shelf-life.

NIR spectroscopy in conjunction with the application of multivariate analysis strategies is an appropriate non-destructive technology for the study of chemical constituents of vegetables at field level. This technology represents a marked change from the conventional analytical methods, because a single spectrum allows the simultaneous characterization of different chemical properties, in a matter of seconds and without sample preparation, thus allowing real-time decision making. In the field, it has become easier to use this technology by the development in recent years of compact, portable hand-held instruments which make it possible to measure the quality and safety parameters of vegetables directly on the plant, thus allowing the product to be instantly analysed.

Several authors have shown the feasibility of using NIRS technology for the non-destructive measurement of nitrate content in various fruits and vegetables, including Japanese radishes [15], the leaf stalk of Qing gin cai [16], pineapple [17] and summer squash [18]. In spinach, Xue and Yang [19] and Itoh et al. [20], are the only ones to have carried out this analysis, although these authors used NIRS instruments with optical performance and wavelength ranges different to those used here.

As regards ascorbic acid content, no references have been found where this parameter has been measured in spinach using NIRS. However, some authors [[21], [22], [23], [24], [25]] have shown how NIRS technology can be used to measure ascorbic acid in apples, zucchini, oranges, potatoes and peppers.

In the case of soluble solid content (SSC), no references have been found for measuring this parameter with NIRS in spinach, although several review works indicate that NIRS technology is a viable means of measuring this parameter in fruit and vegetables [[26], [27], [28]].

Taking into account the possibilities of using handheld NIRS sensors to optimize harvesting times and enable the staggered harvesting of spinach for greater quality and safety, thus allowing certain harvested spinach to be used either in the production of baby foods, industrially processed as preserved, deep-frozen or frozen spinach or for fresh consumption, this study sought to assess the feasibility of using NIR spectroscopy to characterize the variations in internal safety and quality—particularly nitrate, ascorbic and soluble solid contents—in intact spinach during on-vine ripening using a low-cost, miniaturized, handheld, near-infrared device based on the MEMS system.

Section snippets

Sampling

A total of 128 samples of spinach plants (Spinacia oleracea L, cv. ‘Solomon’ (62 samples), ‘Novico’ (13 samples), ‘Meerkat’ (10 samples), and ‘Gorilla’ (43 samples), grown in an open-air plantation in the province of Córdoba (Spain), were harvested between January and March 2017.

Reference Data

Nitrate content (mg NO3/kg) was measured using an RQFlex reflectometer (Merck, Darmstadt, Germany) [29]. The reflectometer which measures the colour intensity of Reflectoquant ® test strips (Merck, Darmstadt, Germany)

Spectral Repeatability

The collection of high quality spectra is crucial for the characterization of spinach plants by quality and safety characteristics and to assess its possible industrial use, as well as to construct discriminant classification models for the product depending on its possible use in the processing industry of fresh or processed vegetables (in this case, preserved, deep-frozen or frozen spinach). The RMS cut-off was calculated for the instrument MEMS used as shown in Section 2.4.

For the first

Conclusions

Near infrared spectroscopy is clearly an advantageous technique for the rapid screening of quality and safety according to the SSC and nitrate levels, although further research is needed to make it robust for predicting these parameters. It has also been demonstrated that the NIRS and the laboratory analysis should be performed together.

The results obtained from the classification models of spinach leaves according to their nitrate content, which determines their possible industrial

Acknowledgements

This research was under the Research Project ‘Quality determination of spinach grown in Santaella (Córdoba)’, funded by Gelagri Ibérica, S.L. The authors are grateful to Mrs. María Carmen Fernández of the Animal Production Department for her technical assistance. Furthermore, the authors wish to express their gratitude to the Spanish Ministry of Education, Culture and Sports for the support offered to Irina Torres in the form of the Training Programme for Academic Staff (FPU).

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