The color, flavor, texture, and the nutritional value of fresh-cut fruit and vegetable products are factors critical to consumer acceptance and the success of these products. In this chapter, desirable and undesirable quality attributes of fresh-cut fruit and vegetable products are reviewed. Both instrumental and sensory measurements for determining these critical quality attributes are discussed. The advantages and disadvantages of sensory and instrumental quality measurements are described. A review of typical unit operations involved in the production of fresh-cut products is presented. The effects of fresh-cut processing techniques and treatments on sensory quality, including the appearance, texture, flavor (taste and aroma) of vegetables, and fruits are detailed.
COMPONENTS OF FRUIT AND VEGETABLE QUALITY
Quality is a term which denotes a degree of excellence, a high standard or value. Kramer (1965) stated that: Quality of foods may be defined as the composite of those characteristics that differentiate individual units of a product, and have significance in determining the degree of acceptability of that unit to the user.
Attributes of Fruit and Vegetable Quality
In reference to fruits and vegetables, the characteristics that impart distinctive quality may be described by four different attributes—1) color and appearance, 2) flavor (taste and aroma),
3) texture and 4) nutritional value. As consumers, these four attributes typically affect us in the order specified above, for example we evaluate the visual appearance and color first, followed by the taste, aroma, and texture. Kramer (1965) stated that the appearance of the product usually determines whether a product is accepted or rejected; therefore, this is one of the most critical quality attributes. Nutritional value is a hidden characteristic that affects our bodies in ways that we cannot perceive, but this quality attribute is becoming increasingly valued by consumers, scientists, and the medical profession.
We eat with our eyes. The shape, size, gloss, and vibrant color of a fruit or vegetable attract us and entice us into picking it up by hand or fork. Once we are attracted by the appearance and color of a product, we put it into our mouths, where the aroma and taste take over. Freshness, spiciness, sweetness, and other flavor attributes are critical to our eating pleasure. Aroma refers to the smell of a fruit or vegetable product, whereas flavor includes both aroma and taste. Once the product is placed in the mouth, one can perceive the smoothness, thickness, firmness, hardness, or crispness of the fruit or vegetable material. As chewing proceeds, the perception of textural quality changes and products generally become softer. Nutritional value is an extremely important quality component that is impossible to see, taste, or feel. Nutrients are critical for the growth and long-term development of our bodies, and include both “micro” nutrients and “macro” nutrients. There are some associations between textural attributes, especially juiciness and flavor and between the color and nutritional composition of fruits and vegetables.
CHEMICAL AND PHYSICAL BASIS FOR FRUIT AND VEGETABLE QUALITY
Color is derived from the natural pigments in fruits and vegetables, many of which change as the plant proceeds through maturation and ripening. The primary pigments imparting color quality are the fat soluble chlorophylls (green) and carotenoids (yellow, orange, and red) and the water soluble anthocyanin (red, blue), flavonoids (yellow), and betalains (red). In addition, enzymatic and non-enzymatic browning reactions may result in the formation of water soluble brown, gray, and black colored pigments. The enzymes involved in browning reactions include polyphenol oxidase, which catalyzes the oxidation of polyphenolic compounds, and phenylalanine ammonia lyase, which catalyzes the synthesis of precursors to phenolic substrates.
The chlorophylls are sensitive to heat and acid, but stable to alkali whereas their counterpart carotenoids are sensitive to light and oxidation but relatively stable to heat. Carotenoids may be bleached by an enzyme called lipoxygenase, which catalyzes the oxidation of lipid compounds. Anthocyanin are sensitive to both pH and heat, while the flavonoids are sensitive to oxidation but relatively stable to heat. Betalains are heat sensitive as well (Clydesdale and Francis, 1976).
Appearance is determined by physical factors including the size, the shape, the wholeness, the presence of defects (blemishes, bruises, spots, etc.), finish or gloss, and consistency. Size and shape may be influenced by cultivar, maturity, production inputs, and the growing environment. It is important for fruits and vegetables to be of uniform size and characteristic shape (Mitcham et al., 1996). Some consumers associate larger size with higher quality. The wholeness and absence of defects will be affected by exposure to disease and insects during the growing period and the harvest and postharvest handling operations. Mechanical harvesting, for example, may incur more bruises and cracks in fruits and vegetables than hand harvesting. Fruit and vegetable gloss are related to the ability of a surface to reflect light and freshly harvested products are often more glossy (Mitcham et al., 1996). Gloss is affected by moisture content, wax deposition on the surface, and handling practices postharvest. Consistency or smoothness may be used as an appearance term, but is typically applied to semi-solid products, where it indicates the product thickness.
Flavor—Aroma and Taste
Flavor has been defined (Anon, 1959) as: A mingled but unitary experience which includes sensations of taste, smell, and pressure, and often cutaneous sensations such as warmth, color, or mild pain. Flavor is typically described by aroma (odor) and taste. Aroma compounds are volatile—they are perceived primarily with the nose, while taste receptors exist in the mouth and are impacted when the food is chewed. While color and appearance may be the initial quality attributes that attract us to a fruit or vegetable product, the flavor may have the largest impact on acceptability and desire to consume it again. Taste has been divided into five primary tastes—sweet, sour, salty, bitter, and umami. Umami can be described as a taste associated with salts of amino acids and nucleotides (Yamaguchi and Ninomiya, 2000). Odors are much more diverse and difficult to classify, but an attempt by Henning (Gould, 1983) includes the following— spicy, flowery, fruity, resinous or balsamic, burnt, and foul.
Stevens (1985) stated that it is possible to classify vegetables into two major groups, depending on their flavor characteristics. The first group of fruits and vegetables has a strong flavor that can be attributed to a single compound or group of related com-pounds. Bananas with isoamylacetate, onions with characteristic sulfide compounds, and celery, with distinctive phthalides are examples of this group. The second group of fruits and vegetables includes those whose flavor is determined by a number of volatiles, none of which conveys the specific characteristic aroma. Examples in this group include snap beans, muskmelons, and tomatoes.
In the evaluation of fruit and vegetable flavor, it is important to consider “off-flavors” as well as desirable ones. These off-flavors may be produced through the action of enzymes such as lipoxygenase or peroxidase, which form reactive free radicals and hydro peroxides that may catalyze the oxidation of lipid compounds. When these reactions occur, the result may be the development of undesirable flavors described as rancid, cardboard, oxidized, or wet dog. However, there are instances of enzyme-catalyzed reactions that result in desirable flavors. For example, hydro peroxide lyase catalyzes the production of typical tomato flavors (Anthon and Barrett, 2003).
Textural parameters of fruits and vegetables are perceived with the sense of touch, either when the product is picked up by hand or placed in the mouth and chewed. In contrast to flavor attributes, these characteristics are fairly easily measured using instrumental methods. Most plant materials contain a significant amount of water and other liquid-soluble materials surrounded by a semi-permeable membrane and cell wall. The texture of fruits and vegetables is derived from their turgor pressure, and the composition of individual plant cell walls and the middle lamella “glue” that holds individual cells together. Cell walls are composed of cellulose, hemicellulose, pectic substances, proteins, and in the case of vegetables, lignin. Tomatoes are an example of a fruit vegetable that is approximately 93–95% water and 5–7% total solids, the latter comprised of roughly 80–90% soluble and 10–20% insoluble solids. The greatest contributor to the texture of tomato products are the insoluble solids, which are derived from cell walls. The three-dimensional network of plant cell walls is still unresolved, but is a topic of great interest to scientists in that to a large degree it dictates the perception of consistency, smoothness, juiciness etc. in fruit and vegetable tissues (Waldron et al., 2003).
According to Bourne (1982) the textural properties of a food are the “group of physical characteristics that arise from the structural elements of the food, are sensed by the feeling of touch, are related to the deformation, disintegration and flow of the food under a force, and are measured objectively by functions of mass, time, and distance.” The terms texture, rheology, consistency, and viscosity are often used interchangeably, de-spite the fact that they describe properties that are somewhat different. In practice the term texture is used primarily with reference to solid or semi-solid foods; however, most fruits and vegetables are viscoelastic, implying that they exhibit combined properties of ideal liquids, which demonstrate only viscosity (flow), and ideal solids, which exhibit only elasticity (deformation).
Fruits and vegetables are a major source of both “macro” nutrients such as fiber and carbohydrates, and “micro” nutrients such as Vitamin C, B complex (thiamin, riboflavin, B6, niacin, folate), A, E, minerals, and the lesser-studied polyphenolics, carotenoids, and glucosinolates. Nutrients may be classified as either water or lipid soluble—meaning they dissolve in water or a lipid medium. Water soluble nutrients include Vitamin C, B complex, polyphenolics, and glucosinolates. Fat soluble nutrients include Vitamin A, E, and other carotenoids such as lycopene and β-carotene. Vitamin C is one of the most sensitive vitamins, being degraded relatively quickly by exposure to heat, light, and oxygen. For this reason, it is often used as an index of nutrient Department of Health and Human Services and the degradation.
DESIRABLE AND UNDESIRABLE QUALITY ATTRIBUTES IN FRESH-CUT FRUITS AND VEGETABLES
Fresh-cut fruits and vegetables must have an attractive appearance, acceptable flavor, appropriate texture, and a positive nutritional image to attract initial and continued purchases by consumers. Consumers may try a new product if attracted by its appearance, but they are unlikely to repurchase an item if it fails to deliver on the promise of that appearance. Quality can be viewed from either a product or a consumer orientation (Shewfelt, 1999). A consumer orientation views the product through the sensory perspective of the consumer at the points of purchase and consumption (Shewfelt and Prussia, 1993). Consumers often buy the first time based on appearance, but repeat purchases are driven by expected quality factors determined by flavor compounds and texture (Beaulieu, 2006a; Waldron et al., 2003).
Color and Appearance
Color and appearance attract the consumer to a product and can help in impulse purchases. At the point of purchase the consumer uses appearance factors to provide an indication of freshness and flavor quality. External appearance of a whole fruit is used as an indicator of ripeness, although it can be a misleading one (Shewfelt, 2000a). Consumers have a preferred color for a specific item (Crisosto et al., 2003). Bananas are supposed to be yellow with no brown spots, tomatoes red not orange, cherries red not yellow, and kiwifruit green-fleshed not yellow. With the exception of the outside of a few fruits like Bosc pears and kiwifruit, fresh fruits and vegetables should not be brown. Gloss on the outside of whole fruits tends to be a desirable attribute for whole fruits. Fresh-cut fruits and vegetables must appear to be fresh, generally indicated by the brightness of color and the absence of visual defects or drip. Sheen on the out-side of most cut fruits is preferred to a dried appearance. Color and appearance of the package can also influence the purchase decision.
Just as an attractive product can stimulate impulse purchases, an unattractive appearance can repel a consumer away from an intended purchase. Colors that are not appropriate for the item, indicative of loss of freshness or suggestive of a lack of ripeness, can turn away willing consumers. Some consumers tend to reject sweet (yellow with brown spots) bananas, nutritious, high β-carotene (yellow and orange) tomatoes, and flavorful Ranier (yellow with red blush) cherries due to unexpected coloration. Wilting, browning, dull colors, and drip are all indicators of loss of freshness in fresh-cut vegetables (Shewfelt, 1993).
White blush in cut carrots is a quality defect (Emmambux and Minnaar, 2003). Russet (brown) spotting and brown stain (Kader and Saltveit, 2003) (two separate disorders) are undesirable visual defects in lettuce. Visible wilting in lettuce and celery and shriveling in fruits reduce consumer acceptability. Yellowing in green vegetables due to loss of chlorophyll is un-acceptable (Shewfelt, 2003). Less intensity of color indicates lack of ripeness in fresh-cut fruits. Browning is a serious quality defect in fresh-cut fruits. Many purchasers of organic fruits and vegetables may actually favor items with visual defects as evidence of authenticity.
Flavor of fresh-cut fruits is more important than for fresh-cut vegetables due to the way the products are consumed. Fresh-cut vegetables tend to be consumed as components of salads or sandwiches. Since fresh-cut fruits are more likely to be consumed without other ingredients, they must be sweet without the presence of off-flavors. Since sweetness increases with ripening and ripe fruits deteriorate more rapidly, most fruits are harvested before full sweetness has been achieved. Sweetness does not increase in coated, cut cantaloupe during storage (Eswaranan-dam et al., 2007), and it is unlikely that significant increases in sweetness will occur in other fresh-cut fruits after packaging. Development of more intense aroma has been achieved by feeding precursors into the atmosphere of strawberry tissue cultures and fruit (Zabetakis and Holden, 1997), but this technique is not being used commercially.
Bitterness is an undesirable taste found in some fresh-cut vegetables such as salad greens (Dinehart et al., 2006). When Cruciferae cells are ruptured, glucosinolates undergo enzymatic hydrolysis with the endogenous myrosinase enzymes, releasing thiocyanates, isothiocyanates (Wattenberg, 1978), sulphate, and glucose (Ju et al., 1982). Processing and packaging precautions must be taken to ensure that off-odors and off-flavors do not jeopardize the marketability of shredded Crucifer products. Sourness is an indication of the use of immature fresh-cut fruits such as may occur in the case of apples (Harker et al., 2003).
Consumers have clear expectations for the texture of fresh-cut vegetables and fruits. Salad vegetables like lettuce, carrot, celery, and radish should be crisp. Soft fruits such as cantaloupe and peach should yield to chewing without being mushy. Other fruits like apples should be crisp and crunchy. While consumers generally cite flavor as the most important quality attribute for fruits and vegetables, textural defects and the interaction of fla-vor and texture are more likely to cause rejection of a fresh product (Harker et al., 2003). Consumer and panel testing indi-cates that they are actually more sensitive to small differences in texture than flavor (Beaulieu et al., 2004; Shewfelt, 1999). Undesirable textural attributes are the opposite of the desirable ones. Wilted lettuce, limp carrots or celery, and flaccid radish are unacceptable as are crunchy or mushy cantaloupes and peaches. Soggy or mealy apples are also likely to be rejected.
Consumers expect fresh fruits and vegetables to be good sources of dietary fiber and many vitamins and minerals. Unfortunately, they have no way of distinguishing between individual products that have high versus low concentrations of phytonutrients. Many factors contribute to the nutrient content of a fruit or vegetable available for sale including genetics, growing conditions (light, temperature, etc.) and production practices (fertilization, irrigation, etc.), maturity at harvest, and postharvest handling conditions. During storage little change occurs in dietary fiber and mineral content, but the vitamins are lost. Cutting stimulates ethylene production which in turn increases respiration and senescence leading to even more rapid loss of certain vitamins. Vitamin C is the vitamin that usually degrades most rapidly and can be used as an index of freshness. Vitamin C is un-stable in many vegetables such as asparagus (Saito et al., 2000) and jalapeno pepper (Howard and Hernandez-Brenes, 1997). Slight vitamin C losses in stored fresh-cut cantaloupe were also reported recently (Beaulieu and Lea, 2007; Gil et al., 2006).
The attribute that drives fresh-cut products is convenience (Raegert et al., 2004). Consumers purchase cut fruits and vegetables for consumption right out of the package. The former International Fresh-Cut Produce Association defined fresh-cut produce as trimmed, peeled, washed, and cut into 100% usable product that is subsequently bagged or prepackaged to offer consumers high nutrition, convenience, and value while still maintaining freshness (Beaulieu and Gorny, 2004). These products should be clean with no evidence of soil or odor of chlorine or other sanitizers. All pieces in the package should be edible and require no further preparation steps other than transfer from package to plate.
ADVANTAGES AND DISADVANTAGES OF SENSORY AND INSTRUMENTAL QUALITY MEASUREMENTS
The quality of fresh-cut fruits and vegetables can be measured by sensory and instrumental methods. In general, sensory methods are more useful in developing new products and determining product standards while instrumental methods are superior in measuring quality on a routine basis (Shewfelt, 1993).
Sensory evaluation of food products is divided into two components—analytical and affective measurements. Analytical measurements can be used to detect differences (difference tests) or to describe the product (descriptive analysis). Analytical sensory tests are usually conducted by small panels with some training of the panelists. Affective measurements deter-mine preference (which samples are preferred over others) and usually require large numbers of na¨ıve panelists (Institute of Food Technologists, 1981).
There are many advantages to sensory methods of quality measurement. Since human perception is involved in sensory testing, quality attributes are clearly defined in terms that are relevant to consumer acceptability. Affective consumer tests are the only way to determine what consumers like and what they do not like. Sensory descriptive panels can be used to identify small differences in quality between similar samples. Well-trained descriptive panels are able to screen out competing attributes to focus on an attribute of specific interest. Among the disadvantages of sensory methods, analytical and affective (consumer) sensory panels are the complex logistics. Descriptive panels re-quire extensive training and can produce highly variable results if training is inadequate. The results from consumer panels tend to be highly variable. In addition, it is difficult to relate sensory data to chemical composition in an effort to determine mechanistic reasons for differences in samples.
Instrumental measurements encompass a wide range of techniques used to determine color, appearance, flavor, texture, and nutritional quality. Instrumental techniques are advantageous in that they tend to provide accurate and precise results. The results of instrumental tests can generally be related directly to chemical and physical properties allowing the investigator to gain a mechanistic understanding of observed differences. Instruments tend to be more sensitive to small differences between samples and may be able to detect trends in quality loss before they can be detected by humans (Thai et al., 1990; Brosnan and Sun, 2004). Instruments do not object to working at nights and week-ends and can produce large amounts of data without complaint, making them excellent monitors in Quality Control operations. A primary disadvantage of instrumental testing is that many instrumental measurements have little relevance to consumer acceptability and thus should never be used to define quality attributes for a specific product. In other words, “it is better to measure what is really important than to believe something is important because you measure it really well.” (Shewfelt and Phillips, 1996).
Sensory and instrumental tests are best used in conjunction with each other using the most appropriate test to meet the desired objective. Affective testing helps determine which at-tributes are important to the consumer. Difference tests can determine if individual units are noticeably different, and sensory descriptive analysis can identify the attributes that cause the differences. When carefully coordinated, the sensory tests can be very effective in developing new products and establishing quality standards. Instrumental tests are more useful in measuring standards in a quality control setting and in determining the mechanistic reason for differences.
SENSORY METHODS OF QUALITY MEASUREMENT
In-depth descriptions of sensory techniques are available for measuring food quality in general (Institute of Food Technologists, 1981; Meilgaard et al., 1999; Lawless and Heymann, 1998) and fruit and vegetable quality specifically (Shewfelt, 1993). As described above, sensory evaluation is divided into two components—analytical and affective measurements. Two types of analytical tests are difference tests and descriptive analysis.
Difference tests are conducted to determine if there is a detectable difference between two samples, for example two different varieties of cut cantaloupe. The most common difference test is the triangle test. Each panelist is given three samples, two are similar and one is different. The panelist is asked to identify the different sample. Panelists in this type of test are not trained. At least 30 panelists are required for difference tests, and 50 or more panelists are preferred. Samples should be presented as the item is normally consumed and presented in a controlled environment. The order of sample presentation must be randomized. Precautions in running a difference test include making sure that there are no unintentional clues to signal the difference. For ex-ample, if the test is to determine possible differences in flavor, steps must be taken to make sure that differences in color, size, shape, and texture are not providing clues to the panelists. Also, investigators must ensure that all samples are at a similar stage of ripeness.
Descriptive analysis involves the development of a lexicon and panel training (Meilgaard et al., 1999). A lexicon is the list of terms used as descriptors and a precise definition of these terms. If the testing material is an item like tomatoes that has been well characterized (Krumbein et al., 2004), terms and definitions are selected from previous studies. For other items such as eggplant (Sesena et al., 2002) the descriptive panel evaluates an extensive range of varieties for that item to develop terms that describe the range of products to be tested. Lexicons can be extensive with up to 50 descriptors of minimalist with as few as 5 descriptors. Upon selection or determination of a lexicon, the panel must go through training and calibration to ensure that the panel results are accurate and precise. Training can be as extensive as two hours a week for six months to less intensive sessions over a one-to-two-week period. Panelists who are outliers undergo additional training or are dismissed. Upon the completion of training, the evaluation of the samples is conducted in partitioned booths using one of several evaluation methods including Spectrum (Meilgaard et al., 1999)