How Is Non-Destructive Quality Assessment of Fresh Produce Changing the Supply Chain?

• The standard quality parameters monitored are color, texture, sugar, titratable acidity, and dry matter content.
• Several destructive methods of estimation exist for the parameters.
• Near-infrared spectroscopy is currently the only non-destructive method for quality control of sugars, titrable acidity, internal color, and dry matter content.
• Near-infrared spectroscopy can precisely and rapidly estimate all quality parameters, except for texture.

Monitoring the quality of fresh produce is now the norm in the supply chain. These developments bring into focus the technology that is used to estimate quality. The trend is to move away from slower, laborious, destructive methods that require skilled people for the job. The new technology used is increasingly non-destructive, rapid, objective, and precise. The standard measurement technique for quality parameters is visible-near-infrared (vis-NIR) spectroscopy. This article compares some standard technologies for quality estimation and highlights the importance of vis-NIR spectroscopy.

Importance of Fresh Produce Quality

The standard parameters that are measured as product and process attributes throughout the fresh produce supply chain are color, texture, sweetness, sourness, and dry matter content.

The reasons for monitoring fresh produce quality can vary and can be one or more of the following:

1. Meet consumer preferences and expectations of quality, taste, safe, and healthy food.
2. Assess the maturity of fresh produce at harvest to determine optimal picking times and post-harvest quality management conditions.
3. Optimize sorting, grading, storage, ripening, authentication, and retailing in the post-harvest supply chain.
4. Enhance nutraceutical content in fresh produce to make it healthier.
5. Meet quality compliance requirements, especially in international trade.
6. Reduce food loss by optimizing harvest time, meeting consumer demand, and extending shelf life.

NIR Spectroscopy Measurements

Near-infrared (NIR) wavelengths of light detect compounds that make up fresh produce like sugars, starch, acids, etc., since NIR light interacts with the C-H, O-H, and N-H bonds that hold the chemicals together. Varying wavelengths are absorbed, transmitted, or reflected depending on the structure of the compounds and the quantities in which they are present. Spectrometers measure the complex spectra generated when NIR light is turned on them. The spectral data are analyzed using chemometric models, the choice of which depends on the analysis aim. The result is simple, actionable numbers, such as Brix or dry matter percentage, which users can easily read and use.

Extensive research and technological development have enabled the miniaturization of the technique, and various visible and NIR spectroscopy-based instruments on the market can measure quality parameters such as color, sugar, acidity, dry matter, and water content.

Internal and External Color

Internal pulp and external skin color are the most commonly checked quality parameters in the supply chain, as these attributes are closely linked to quality and used to track the progression of maturity.

The skin color checked is the base color and/or blush, for example, in apples that are bi-colored. Internal pulp color becomes relevant when external color doesn’t provide information on maturity, as in citrus or climacteric fruits, which are harvested fully mature but still with green skin.

Destructive methods

Manual inspection requires destructive sampling to check internal color.

 Manual inspection: The simplest form of color checks is visual manual inspection for both skin color and pulp color. Manual checks rely on people’s expertise and experience to judge the quality and maturity of fresh produce, and they are very subjective.

Figure 1: Color charts used for fruit skin color grading in apples and tomatoes, WUR. (Image credits:https://www.freshknowledge.eu/en/increase-your-knowledge/how-to-deal-with-fresh- produce/doing-quality-measurements.htm)

Non-Destructive Quality Assessment Methods

Non-destructive means of external color checks are visual inspection, the use of charts, computer and machine images, colorimeter, densitometer, and NIR spectroscopy. For internal color, NIR spectroscopy is the only non-destructive method, as the other techniques require destructive sampling.

Color charts: Color charts are more objective than manual inspection and are easy and cheap to use, see Figure 1. The charts must be cultivar-specific for several fruits, such as apples, tomatoes, mangoes, melons, or plums, that come in varying colors. The color charts are commonly used in the field or packing houses to determine the maturity and ripeness of fruits and vegetables.

Colorimeter: Colorimetry offers an objective and quantifiable method for measuring color. For food, the RGB and CIELAB models are used because they are the most closely aligned with human visual perception.

Figure 2.: “The CIE chromaticity diagram,” Dutta & Nath (2024). (Image credits: doi: 10.5772/intechopen.112099)

The CIELAB (or CIE L* a* b*) is a well-known system for describing color by the Commission Internationale de l’Eclairage (CIE). The system is based on the primary colors red, green, and blue (RGB).

• L* is the axis depicting lightness measured from black (0) to white (100).
• a* represents the red and green colors
• b* stands for blue and yellow colors.

The colorimeter measures the source light emitted and the secondary light reflected by the sample. The measurements are comparative and must be calibrated against known color standards.

Densitometer: The device measures the external color of food based on its density. The principle is that color intensity varies with how dense the substance is. The densitometer measures the reflection of red, green, and blue, as well as the transmission of yellow, cyan, and magenta colors, by the sample.

Machine or computer vision: The technology’s development began in the 1970s, and by 2010, it was used to mimic human vision in machine learning, face recognition, and image processing, among other applications. A light source, camera, and a computer/processor capture and store images of fresh produce, which are analyzed by algorithms based on color, shape, and homogeneity. The device helps sort, grade, and estimate fruit maturity and freshness based on external color.

NIR Devices: NIR spectroscopy can non-destructively estimate both external and internal color, as NIR light can penetrate up to 7 mm into the pulp. Pigments give the strongest spectral signature, as they absorb the most light of all compounds. Therefore, the technique is well-suited to estimate color.

Firmness

Texture, which is a good indicator of maturity and ripeness, is measured by firmness. Pressure and deformation of fruits are used to test firmness.

Destructive method

The destructive methods to measure firmness are penetrometers.

Penetrometers: These are the most common means of estimating firmness for fresh produce, and they can be handheld or automatic. The instruments measure the maximum force necessary to puncture the fruit flesh to a standard depth. The skin is peeled, and a plunger of 8 mm or 11 mm is used for harder fruits. Drill penetrometers use plungers, while those with gauges measure pressure and compression. Magness-Taylor’s Pressure Tester is a popular measure of firmness at ripeness for research and the market.

Non-destructive measurements

The non-destructive methods for testing firmness are a durometer, a texture analyzer, probe deceleration, and aweta. The measurement techniques are not completely non-destructive, but cause less damage. They are also not precise when the fruits are very dehydrated.

Durometers: These devices are handheld and measure the resistance force to a 3mm probe. It can bruise the fruit and break the skin, so it is not always non-destructive. The durometer is the standard non-destructive method, as it is easy to use.

Texture analyzer: These devices measure compression in terms of millimeters or as a percentage of the fruit diameter affected. Some methods use a defined amount of force, and others aim for a targeted compression.

Probe deceleration: These are impact-based methods that measure the returning force of a probe exerting a known force. The probe head can leave a mark depending on the force, the species, and the ripeness of the fruit.

Aweta: The device is acoustic-based and measures the sound of tapping the fruit with a probe. It is best suited for items with a uniform shape.

Soluble Solid Contents

Sweetness is measured by the soluble sugar content (SSC) and total soluble solids (TSS) in fruits and reported as °Brix. SSC and TSS are the primary quality parameters that researchers use to evaluate quality in the supply chain.

Destructive methods

The refractometer, hydrometer, density meter, or sensory evaluation are destructive methods of estimating sugars.

Sensory evaluation: Traditionally, sweetness was judged through tasting by trained and consumer panels using a scoring system, where one was the least desirable and 5 was the most favorable. The method has several disadvantages, including its lack of standardization, instability, and unreproducibility, aside from being expensive and causing taste saturation among panelists.

Hydrometer: It is a simple instrument, but not as accurate as a refractometer. The device measures the specific gravity of juices and provides readings in degrees Brix (°Brix) at 20 °C. Therefore, the readings must be adjusted to actual room temperatures. Manual operation and reading errors make this method less useful.

Density meter: The method measures sugar by estimating the density to indicate the percentage of liquids with sugars and gives the result in °Brix. Digital variations can control temperatures and are more precise than manual devices.

High-performance chromatography is also used to measure sugars in juices extracted from fresh produce. A more recent method is the e-tongue, which is used to measure sugars in several food types, including fruits and vegetables.

Non-destructive methods

NIR spectroscopy is the standard non-destructive method used to estimate sugars. Other emerging non-destructive techniques are hyperspectral imaging and nuclear magnetic resonance.

NIR spectroscopy: Visible and NIR spectroscopy, which utilizes light in the 400–2500 nm range, helps detect sugars. The aim is to detect the O-H bonds in water and the C-H bonds in carbohydrates. NIR spectroscopy-based predictions of SSC have been researched and reported for many horticultural products. It is also possible to detect individual sugars – glucose, fructose, or sucrose. The technique is also helpful for in-line sorting installations in packaging houses.

Titrable Acidity

The sourness from the various organic acids in fresh produce is estimated as titrable acidity (TA).

Destructive methods

The destructive method of TA estimation is through titration and fruit acid meters.

Titration: The TA of fresh produce is estimated through titration, which requires juice extraction. A sample of the juice is mixed with a standard alkaline solution to get a fixed pH endpoint. The alkali commonly used is sodium hydroxide (NaOH). The end pH is detected by color changes in the phenolphthalein indicator from colorless to pink. A pH meter can also be used to judge the endpoint in the potentiometric method.

The method requires basic laboratory equipment, such as beakers, pipettes, flasks, chemicals, and pH meters, as well as personnel skilled in the procedure. The percentage acidity is calculated using the following formula:

% acid (wt/vol) = N × V1 × Eq wt

V2 × 10

where:

N = normality of titrant, usually NaOH (mEq/ml)

V1 = volume of titrant (ml)

Eq. wt. = Equivalent weight of predominant acid (mg/mEq)

V2 = volume of sample (ml)

1000 = factor relating mg to grams (mg/g)

(1/10 = 100/1000)

Fruit acid meters: Small portable fruit acidity meters use electroconductivity to measure acidity. It doesn’t require reagents or calibration and is suitable for a wide range of temperatures. Unlike the wet method, this method is precise, fast, and optimal for the field, making it easy to use.

Non-destructive method

The only non-destructive method of TA estimation is NIR spectroscopy.

NIR devices: NIR spectroscopy is a non-destructive method for measuring titrable acidity as it can quantify the organic acids present in the intact fruits using wavelengths of 700-1100 nm.

Dry Matter Content

Dry matter content is the latest parameter developed to measure quality and is the total of all solids in fresh produce minus water content.

Destructive method

The traditional and destructive technique for dry matter estimation is the oven method.

Oven method: The estimation is performed indoors and requires specific apparatus, including ovens, an electronic balance, a desiccator, and dishes. The fresh/wet weight of the sample is taken, and the sample is dried at 105 + 3 °C in an air drier for three hours. The dried sample is measured, and the dry matter is calculated using the following formula:

(dry weight / wet weight) x 100 = % Dry Matter

Non-destructive method

The only current non-destructive method of dry matter estimation is NIR spectroscopy-based.

Vis-NIR spectroscopy: Dry matter includes solids like sugars, starch, lignin, pigments, phenols, etc., which can be detected and estimated by absorbance or reflected spectra of Vis-NIR light. Several studies of different fresh produce have shown that dry matter can be calculated accurately by NIR spectroscopy. The dry matter at harvest is emerging as a significant quality parameter, replacing other attributes as it is correlated with consumer acceptance in many species.

One Solution

Vis-NIR spectroscopy-based devices, such as those from Felix Instruments Applied Food Science, can offer a single solution for most critical quality parameters, eliminating the need for multiple tools. Moreover, the results of various parameters can be displayed simultaneously on the screen, making quality monitoring more convenient. Quality control meters can save money by reducing the time and costs associated with laborious and expensive procedures. Felix Instruments offers several quality control meters, such as the F-750 Produce Quality Meter and the F-751 series, that can precisely estimate SSC, color, titrable acidity, and dry matter in real-time. The portable tools can be used on-site in fields, laboratories, and the entire fresh produce supply chain for quality control.

Find out more about Felix Instruments Applied Food Science’s quality meters for monitoring fresh produce.

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Dutta, K., & Nath, R. (2024). Application of Colorimetry in Food Industries. IntechOpen. doi: 10.5772/intechopen.112099

 

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WUR. (n.d). Evaluating firmness. https://www.freshknowledge.eu/en/longreads/evaluating-firmness.htm