Techniques for Quality Estimation of Fruits

GURKIRAT KAUR'*, SWATI KAPOOR2, NEERAJ GANDHI2, and SAVITA SHARMA2

'Election Microscopy and Nanoscience Lab, Punjab Agricultural University, Ludhiana, India

department of Food Science and Technology, Punjab Agricultural University, Ludhiana, India

'Corresponding author. E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

ABSTRACT

Quality determination of fruits and vegetables encompasses varietal characteristics that are mostly visual and tactile in nature. These quality attributes provide an insight into maturity indices of fruits to determine proper maturity conditions and approximate nutritional content. These factors are important to determine end product quality whether fresh or processed. Various methods have been developed to assess the quality parameters in fruits that can be broadly divided into destructive and non-destructive methods. Destructive methods include spectroscopic methods, titration, chromatography, and texture analyzer that include compression forces leading to sample destruction. With advent in technology, non-destructive methods are gaining momentum to efficiently and quickly detect the internal composition of fruits. These methods are based on magnetic, acoustic, and moisture properties of fruits, thereby helping to assess the fruit samples in the field itself without any sample destruction. However, implementation of non-destructive techniques is still in its infancy and experiments are being conducted to develop more techniques that are cost effective and widely available. The present chapter deals with the basic differences between both destructive and non-destructive techniques and the technology behind quality assessment of fruits using both the methods.

INTRODUCTION

The term quality refers to “a distinctive attribute or characteristic possessed by an object.” However, in case of fruits and vegetables, definition of quality varies according to different aspects such as user quality, market quality, biological quality, and so on. From consumers’ point of view, eveiy individual has different preferences and therefore, quality cannot be strictly defined. Various quality indices both internal and external such as appearance, ripeness, uniformity, freshness taste, texture, aroma, and nutritive value have been allocated for describing quality more comprehensively in fruits (Choi et al., 2006). Consumption of fruits and vegetables has increased in the recent years due to its balanced profile of being low in fats and carbohydrates and high in vitamins, minerals, and fiber. Consumers have grown much aware to seek specific qualitative characteristics during purchase of fruits and vegetables. Also, legal requirements have laid down the standards that must be met for successful export of the finished products. All these factors act as a driving force for developing technologies that can conveniently and efficiently detect quality of fruits.

In this regard, recent technologies employing optical, acoustic, and mechanical sensors are coming up for determining different quality indices in fruits. Broadly, quality estimation of fruits and vegetables can be divided into two categories, destructive quality evaluation and non-destructive quality evaluation techniques. Former methods include spectroscopy, texture analyzer, penetrometer, titrimetric assay, gas chromatography (GC), high-pressure liquid chromatography (HPLC), and so on. These methods are time consuming and destructive in nature. Therefore, scientists have come up with novel non-destructive techniques that are based on moisture presence, electrolytic properties, vibrational properties, atomic properties, and so on in fruits. Due to variable properties present in fruits, single non-destructive method cannot be used to assess all the quality components and therefore, different techniques are used in non-destructive methods that measure the quality either directly or indirectly by correlating with other physical properties (Lakshmi et al., 2017). Relationship between destructive and non-destructive tests was studied by Marin (2002) where near infra-red (NIR) was compared with % soluble solids and non-destructive Firmalon for acoustic firmness index (FI) was compared with destructive Magness-Taylor penetrometer (FTA) in apples. A fairly linear and predictable relationship was found between NIR and % soluble solids whereas not much relevant relation was observed between FTA and FI.

Machine vision applications are being extensively used to determine qualitative characteristics and some defects in fruits. Most of these techniques are based on NIR imaging, hyperspectral imaging, thermal imaging, magnetic resonance imaging (MRI), fluorescence imaging, and so on. Bruising is considered one of the major quality defects in fruits that occur during mechanical operations. Opara and Pathare (2014) have discussed various non-destructive techniques to evaluate bruise damage in fruits. NIR systems are based on electromagnetic spectrum using spectral characteristics of the product such as wavelength scattering and absorption processes during radiation penetration. Wavelength in the range of 350-2500 nm is widely used for detection of bruised surfaces (Van Zeebroeck et al., 2007). Hyperspectral imaging is based on image acquisition of product at more than ten wavelengths. Thermal imaging technique is also used for detection of fruit bruises where temperature difference between bruised and unbruised fruit is taken as the basis (Baranowski et al., 2009).

In order to study texture attributes, destructive instrumental tests include shear (Harker et al., 2002), penetration (Rizzolo et al., 2010), compression (Shinya et al., 2013), and tension (Harker et al., 2002). For volatile profiling within a fruit, gas chromatography is widely used technology, but being a destructive method, a newer method, that is, electrochemical technology has been introduced where semiconductor gas detectors are used based on different polymers and metal oxides. The electrical conductivity of these detectors decreases on exposure to volatiles and battery of several detectors can produce a particular characteristic pattern that may indicate maturity or presence of some disorders in fruits (Abott, 1999).

Fruit grading system is based on studying inner tissues of fruits using spectroscopy and narrow beams. As per Swarnalakshmi and Kanclmadevi (2014), quality characteristics like hydration and volume could be determined by using biaxial cameras. Noninvasive tissue inspection of tomatoes was studied by using time-resolved reflectance spectroscopy (Yodli and Chance, 1995). Some of the destructive and non-destructive technique for fruit quality inspection has been enlisted in Table 13.1.

TABLE 13.1 Destructive and Non-destructive Techniques for Fruit Quality Evaluation.

Destructive

Non-destructive

Instrumental

technique

Parameters assessed

Instrumental

technique

Parameters assessed

Spectroscopy

Antioxidant assay: DPPH, FRAP,

Bioactive compounds: Carotenoids, total phenolic compounds, anthocyanins, tannins, Vitamin C

Atomic absorption spectroscopy: Minerals

NMR imaging thermal imaging

Water core in apples (Wanget al„ 1988)

Core breakdown and warm damage in pears (Wang and Wang, 1989; Bellon, 1990)

Fruit bruising (Zhang et al., 2015); bruise detection under the skin of apple

Titration

Total acidity Ascorbic acid Total sugars and reducing sugars

Dielectric sensing

Electromagnetic

field

Firmness Moisture content pH

Soluble solids content (Nelson and Kraszewski, 1990)

Texture

analyzer

Shear

Penetration

Compression

Tension

Image

processing,

Hyperspectral

imaging

Volume and mass of citrus fruits (Omid et al., 2010);

Magness-

Taylor

Penetrometer

Firmness

Acoustic impulse method, Laser Doppler vibrometer

Firmness in watermelons (Mao et al., 2016; Abbaszadeh et al., 2013) Pear texture and freshness (Zhang et al., 2015)

Gas chroma-

tography-Mass

spectra

Volatile compounds

Electronic nose

To assess fruit ripening stage in apricot, banana, and blueberry Aroma profiling during deteriorative shelf life in apple

Maturity stage at harvest in apple, blackberry, durian, grapes, mango, pear, and so on.

Canopy side effect in grapes

(Baietto and Wilson, 2015)

TABLE 13.1 (Continued)

Destructive

Non-destructive

Instrumental

technique

Parameters assessed

Instrumental

technique

Parameters assessed

High performance liquid chromatography

Pro-vitamin A, Vitamin C, Vitamin Bl, B2, and B6

NIR diffuse

reflectance

spectroscopy

Fruit chlorophyll content Vitamin C content (Reddy et al., 2016)

Refractometer

Total soluble solids

Computer vision

Firmness

Soluble solids content Stains detection Chilling injury Starch index Sugar content in apples (Zhao et al., 2009; Menesatti et al., 2009; ElMasry et al., 2008; Qin et al., 2009)

DESTRUCTIVE METHODS

Quality characteristics of fresh fruits are generally based on the physical parameters or chemical composition or combination of these two factors. Quality attributes such as color/appearance, texture/firmness, sensory characteristics, and nutritional quality are estimated using destructive methods which are described as follows;

ANTIOXIDANT ACTIVITY

Determination of the antioxidant activity is one of the ways how to biologically and nutritionally evaluate the quality of the fruit. It has been proved that antioxidant activity depends on the type of phenolic present in the fruit, as some phenolic compounds exhibit higher antioxidant activity than others (Gursoy et al., 2010; Romero et al., 2010). It is assumed that the ability of plant polyphenols to scavenge reactive oxygen radicals participates in the protective mechanism of plants. Due to the chemical diversity of antioxidants present in fruit, their strictly defined content is unavailable. In spite of the fact that total amount of antioxidants in various fruit types need not to represent the total antioxidant capacity (Ling et al., 2010; Gazdik et al.,

2008), almost all phenolic compounds in fruits demonstrate some antioxidant activity (Beklova et al., 2008; Gursoy et al., 2010; Gil et al., 2000). However, detection of therapeutically active components in a biological matrix is a very complex procedure, and their determination differs in individual studies (Adam et al., 2007; Mikelova et al., 2007; Klejdus et al., 2005).

In the field of chemical analyses and biological evaluation of the antioxidant characteristics, several methods enabling determination of the antioxidant activity have been suggested and optimized. These methods are principally different and their modifications are still progressively developing.

(1) Antioxidant activity by the DPPH test

The DPPH test is based on the ability of the stable 2,2-diphenyl-1-pic - rylhydrazyl free radical to react with hydrogen donors. The DPPH radical displays an intense UV-VIS absorption spectrum. In this test, a solution of radical is decolorized after reduction with an antioxidant (AH) or a radical (R*) in accordance with the following scheme:

This method is veiy simple and also quick for manual analysis.

(2) Antioxidant activity by the ABTS test

The ABTS radical method is one of the most used assays for the determination of the concentration of free radicals. It is based on the neutralization of a radical-cation arising from the one-electron oxidation of the synthetic chromophore 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS*):

This reaction is monitored spectrophotometrically by the change of the absorption spectrum. Results obtained using this method are usually recalculated to Trolox* concentration and are described as “Trolox® Equivalent Antioxidant Capacity” (TEAC). For chemically pure compounds, TEAC is defined as the micromolar concentration of Trolox® equivalents demonstrating the same antioxidant activity as a tested compound (at 1 mmolL^concentration) (Re et al., 1999).

(3) Antioxidant activity by the FRAP method

The ferric reducing antioxidant power (FRAP) method is based on the reduction of complexes of 2,4,6-tripyridyl-s-triazine (TPTZ) with ferric chloride hexahydrate (FeCl36H,0), which are almost colorless, and eventually slightly brownish. This chemical forms blue ferrous complexes after its reduction. The method has its limitations, especially in measurements under non-physiological pH values (3.6). In addition, this method is not able to detect slowly reactive polyphenolic compounds and thiols (Ou et al., 2002; Jerkovic and Marijanovic, 2010).

(4) Antioxidant activity by the DMPD method

The compound N,N-dimethyl-1,4-diaminobenzene (DMPD) is converted in solution to a relatively stable and colored radical form by the action of ferric salt. After addition of a sample containing free radicals, these are scavenged and as a result of this scavenging, the colored solution is decolorized (Gulcin et al., 2010; Jagtap et al., 2010).

(5) Antioxidant activity by the free radicals method

This method is based on ability of chlorophyllin (the sodium-copper salt of chlorophyll) to accept and donate electrons with a stable change of maximum absorption. This effect is conditioned by an alkaline environment and the addition of catalyst (Votruba et al., 1999).

(6) Antioxidant activity by the blue Cr05 method

Chromium peroxide (Cr05) is very strong pro-oxidant produced in an acidic environment by ammonium dichromate in the presence of H,0,. It is a deep blue potent oxidant compound, miscible and relatively stable in polar organic solvents that can be easily measured by spectrometry (Charalam- pidis et al., 2009; Grampp et al., 2002).

13.2.1.1 OTHER BIOACTIVE COMPOUNDS

Fruits are the principal source of bioactive compounds such as carotenoids (which play an important role in diet due to vitamin A activity), phenols, anthocyanins, tannins, vitamin C, and so on. Carotenoids have antioxidant action, protecting cells and tissues from damage caused by free radicals, strengthening the immune system, and inhibiting the development of certain types of cancers (Zeb and Mehmood, 2004). Many methods were used for the identification and quantification of carotenoids from food matrix including spectrophotometric, colorimetric, fluorometric, paper, open-column and thin-layer chromatography, HPLC, and capillary electrophoresis. For the quantitative determination carotenoids, the generally used method is the spectrophotometric method, the essential condition being that the pigments are of analytical purity. Of the spectral methods used in detecting carotenoids, there can be enumerated visible spectroscopy, IR NMR, mass spectrometry. HPLC coupled with nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (positive mode atmospheric pressure chemical ionization; APCI+ mode) are often used for characterization and identification studies (Gupta et al., 2015).

Phenolic compounds responsible for bitterness, astringency, flavor, color, and oxidative stability of fruits and vegetables have shown an effect in health protection, with not only antioxidant activity by scavenging free radicals, but also inhibition of hydrolytic and oxidative enzymes and anti-inflammatory functions in human cells (Naczk and Shahidi, 2004). There are many spectrophotometric methods for the quantification of phenolic compounds in plant materials. Based on different principles, these methods are used to determine various structural groups present in the phenolic compounds. Spectrophotometric methods enable either the quantification of all extracted phenolics as a group (Swain and Hillis, 1959; Price and Butler, 1977; Earp et al., 1981), or the quantification of specific phenolic substances such as sinapine (Tzagoloff, 1963) or the sinapic acid (Naczk et al., 1992). Spectrophotometric methods are also used in the quantification of a whole class of phenols such as phenolic acids (Price et al., 1978; Mole and Waterman, 1987; Naczk and Shahidi, 1989; Brune et al., 1991). Some of the most commonly used assay methods for phenolic compounds include the modified vanillin test (Price et al., 1978), the Folin-Denis assay (Swain and Hillis., 1959), the Prussian blue test (Price and Butler, 1977), and the Folin-Ciocalteu assay (Maxson and Rooney, 1972; Hoff and Singleton, 1977; Earp et al., 1981; Deshpande and Cheryan, 1987).

Anthocyanins are responsible for the red, purple and blue hues present in fruits and vegetables. The qualitative and quantitative detennination of anthocyanins can be achieved by a variety of classical (spectrophotometric) or contemporary methods—HPLC coupled with a various types of mass spectrometers or NMR apparatus. The pH differential method has been used extensively by food technologists and horticulturists to assess the quality of fresh and processed fruits and vegetables (Lee et al., 2005). This is a rapid and simple spectrophotometric method and can be used for the determination of total monomeric anthocyanin content, based on the structural change of the anthocyanin chromophore between pH 1.0 (colored) and 4.5

(colorless), the anticipated use of the method is in research and for quality control of anthocyanin-containing fruit juices, wines, natural colorants, and other beverages.

Vitamin C (L-Ascorbic acid) is water-soluble vitamin with strong reducing action and it is an important coenzyme for internal hydroxyl- ation reaction. Vitamin C is found in both reduced form (ascorbic acid) and oxidized form (dehydroascorbic acid). It is widely used food additive with many functional roles, many of those are based upon its oxidation-reduction properties. Functional roles include its use as: a nutrition food additive, antioxidant, reducing agent, stabilizer, modifier, color stabilizer (Eitenmiller et ah, 2008). Many analytical techniques are mentioning in the literature for the determination of vitamin C in different matrices, such as: titrimetric (Verma et ah, 1996), fluorimetric (Xia et ah, 2003), spectrophotometric (Rahman et ah, 2006), HPLC (Nyyssonen et ah, 2000), enzymatic (Casella et ah, 2006), and so on. Spectrophotometric method for determination of total ascorbic acid in fruits and vegetables with 2,4-DNPH (2,4-dinitrophenylhydrazine) is a simple and reliable method. This method is based upon treatment with 85% H,S04 of the chromogen foimed by the coupling of 2,4-dinitrophenyl- hydrazine with oxidized ascorbic acid.

Minerals are of prime importance in determining the fruit nutritional value. Potassium, calcium, and magnesium are the major ones. Atomic absoiption spectroscopy (AAS) is a veiy useful tool for determining the concentration of specific mineral in a fruit samples. Liquefied sample is aspirated, aerolized, and mixed with combustible gases such as acetylene and air or acetylene and nitrous oxide and burned in a flame to release the individual atoms. On absorbing UV light at specific wavelengths the ground state metal atoms in the sample are transitioned to higher state, thus reducing its intensity. The instrument measures the change in intensity and the intensity is converted into an absorbance related to the sample concentration by a computer-based software (Paul et al., 2014).

TITRATION METHODS

13.2.2.1 TITRABLE ACIDITY

It is the sugar/acid ratio which contributes toward giving many fruits their characteristic flavor and so is an indicator of commercial and organoleptic ripeness. At the beginning of the ripening process, the sugar/acid ratio is low, because of low sugar content and high fruit acid content, this makes the fruit taste sour. During the ripening, process the fruit acids are degraded, the sugar content increases and the sugar/acid ratio achieves a higher value. Overripe fruits have very low levels of fruit acid and therefore lack characteristic flavor. Titration is a chemical process used in ascertaining the amount of constituent substance in a sample, for example, acids, by using a standard counter-active reagent, for example, an alkali (NaOH). Once the acid level in a sample has been determined, it can be used to find the ratio of sugar to acid. There are two methods specified for the determination of the titratable acidity of fruits:

  • (1) Method using a colored indicator
  • (2) Potentiometric method, using a pH meter.
  • 13.2.2.2 ASCORBIC ACID

The official method of analysis for vitamin C detennination of juices is the 2, 6-dichloroindophenol titrimetric method (AOAC Method 967.21). Ascorbic acid (vitamin C) is a strong reducing agent. It gets oxidized to dehydro ascorbic acid by 2,6 dichlorophenol indophenol dye. At the same tune, the dye gets reduced to a colorless compound. So the reaction with endpoint can easily be determined.

13.2.2.3 SUGARS

Fehling’s test for reducing sugars has been used since the 1800s to determine the amount of glucose and other reducing sugars (e.g., lactose in milk). It has had many applications including use in agriculture (glucose determination in corn for use in corn syrup) and in medicine (glucose determination in urine for diabetes tests). The test works by taking advantage of the ability of aldehyde-containing sugars to reduce blue Cu2+ ions to Cu+ ions. Methylene blue is a commonly used indicator for oxidation-reduction reactions. It is a deep blue color in its oxidized form but colorless when exposed to reducing agents. After the glucose titrant has completely reduced all of the Cu2+ to Cu+, the methylene blue will be reduced by the glucose, completely removing the blue color from the solution. This total disappearance of color indicates the end point of the titration.

TEXTURE ANALYSIS

Texture is an important component of fruit quality. Texture is related to those attributes of quality associated with the sense of feel, as experienced by the fingers, the hand, or in the mouth. Texture can be measured by determining the force required to compress, penetrate, shear, or deform the produce. Among the methods most widely used the following ones are the most important:

  • (1) Penetrometric (puncture test) methods
  • (2) Texture profile method.

Penetrometric and puncture test methods belong to the simplest methods and are widely used in practice. Mostly the maximal force needed for the penetration of the deforming body to a given depth is measured. A Magness- Taylor or Effigi firmness meter or other similar instruments work on the same principle.

Texture profile Analysis is a popular double compression test for determining the textural properties of foods (Fig. 13.1). It is occasionally used in other industries, such as pharmaceuticals, gels, and personal care. During a TPA test samples are compressed twice using a texture analyzer to provide insight into how samples behave when chewed. The TPA test was often called the “two bite test” because the texture analyzer mimics the moutlus biting action. The texture profile analysis method is widely used in research work and in many cases in industrial laboratories. The data from the time-force curve is used to estimate the degree of crispness, toughness, and hardness. The fruit sample is compressed between parallel plates imitating the chewing and the process is repeated. The viscometric methods generally serve for the evaluation of the consistency of processed fruits and vegetables. Some main terms being measured by texture profile analysis are described as below:

  • (1) Hardness is the maximum force of the first compression.
  • (2) Fracturability is the force at the first peak.

Cohesiveness is the area of work during the second compression divided by the area of work during the first compression.

Springiness is expressed as a ratio of a product’s original height. Springiness is measured by the distance of the detected height during the second compression divided by the original compression distance.

Texture profile analysis

FIGURE 13.1 Texture profile analysis.

GAS CHROMATOGRAPHY/HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY

Fruit flavor is difficult to determine by chemical means, primarily because a complex group of volatile compounds is combined in most to provide the typical flavor. Often the same compounds are present in genetically unrelated fruits, with the proportion of each, or the presence or absence of a few, resulting in vastly different flavors. Techniques in gas chromatography have permitted precise identification of the volatiles contributing to flavor in fruits. Many of the volatiles contributing to strawberry, grape, pear, and citrus fruit flavors are known, but the combinations that produce the unique flavor of one type or cultivar of fruit as compared with another are as yet little understood.

A static headspace gas chromatography coupled to mass spectrometry (SHS-GC-MS) method was validated to determine several major volatile components. The method is simple, fast, linear in the working range, suitably sensitive, repeatable and reproducible, and has a good degree of accuracy for most of the compounds studied. GCHPLC have almost entirely replaced paper and thin-layer chromatography as methods for identifying and quantifying food acids. GC has been used to analyze organic acids in fruit and fruit juice. Analysis involves preparing volatile derivatives such as methyl esters of the organic acids, prior to their injection into the gas chromatograph. Derivatives are chromatographed on a nonpolar stationary phase column and detected by a flame ionization detector.

HPLC is used more extensively than GC to determine organic acids because the technique requires little or no chemical modification to separate these nonvolatile compounds. Separation is usually done on either a reversed-phase C8 or Cl8 column or a cation-exchange resin column operated in the hydrogen mode. Acids are detected by either refractive index (RI) or ultraviolet (UV) detectors. RI detection requires prior removal of any sugars present that potentially can interfere with quantification; sugar removal is not required for UV detection at 220-230 nm.

Chemical methods are also available for detecting physical flaws or injuries to certain fruits. These include immersion in a solution of Methylene Blue dye for open lenticels or minute skin breaks in apples, and soaking in a dilute solution of 2,3,5-triphenyl*2Htetrazolium chloride for peel injury in oranges.

REFRACTOMETER ANALYSIS

Sugars are the major soluble solids in fruit juice. Other soluble materials include organic and amino acids, soluble pectins, and so on. Soluble solids concentration (SSC%, °Brix) can be determined in a small sample of fruit juice using a handheld refractometer (Fig. 13.2). This instrument measures the refractive index, which indicates how much a light beam is ‘bent’ when it passes through the fruit juice. Temperature of the juice is a veiy important factor in the accuracy of reading. All materials expand when heated and become less dense. For a sugar solution, the change is about 0.5% sugar for eveiy 10°F. Good quality refractometers have a temperature compensation capability.

Hand-held refractometer (Erma, Japan)

FIGURE 13.2 Hand-held refractometer (Erma, Japan).

 
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