Jan. 19, 2022
Dec. 21, 2021
Measuring internal color of produce can provide stakeholders with vital consumer acceptance and quality information. Internal color can attract consumers and improve the value of a product or, just as easily, have the opposite effect. Spoiled fresh produce can also be quickly identified by changes in internal color. It is easily visible to the human eye and is one of the first quality parameters that was and is still used by food producers and consumers to judge quality. These days, however, there is more to food color than meets the eye.
Internal or pulp color is a fruit quality parameter widely used at harvest and postharvest stages in the food supply chain.
Appearance is the first and most important information customers get from fresh produce. It is the most straightforward means of quality control since internal color is associated with physicochemical properties. Hence, people use color to estimate ripeness, maturity, and indicator of flavor, nutritional value, and defects.
Fruit and vegetables get their color from pigments, whose concentrations change as they mature and ripen. The various colors are a result of the following pigments:
The color of fresh produce can be affected by biochemical, chemical, physical, and microbial changes during maturation, ripening, and postharvest handling.
Chemical changes: Lipid oxidation can cause browning that makes fresh produce unattractive, even if the quality is not affected.
Enzyme Action: Browning through oxidation can also occur due to enzyme activity, such as phenolic compounds by enzymes polyphenol oxidase causing discoloration. Peroxidase also causes enzymatic browning.
Ethylene production: Ethylene, a natural phytohormone produced during ripening, also leads to tissue decay and senescence, which can spoil produce appearance and color.
Figure 1: The increase in the browning index and total color difference (ΔE) of fresh-cut green bean is due to long storage times, Kasim and Kasim, 2015. (Image credits: https://doi.org/10.1590/1678-457x.6523)
Environmental conditions are also crucial. Fresh produce is often stored and transported for weeks or months before sale and the storage conditions will influence the color of the fruit pulp.
Temperature: Higher temperatures will reduce quality in many ways.
While lower temperatures are ideal for storing fresh produce to preserve internal color, very low temperatures can cause chilling injury that can damage tissue, leading to microbial decay, discoloration, and decline in quality.
Light: Ultraviolet and high-intensity light in storage areas can reduce browning and fall in quality by inhibiting the activity of oxidizing enzymes. For example, cabbage stored at 7oC and continuous light of 24 μmol m−2 s−1 inhibited polyphenol oxidase activity by 26% and peroxidase activity by 16%. This resulted in a 33% recuction in browning, compared to dark storage. Light has similar benefits for highly perishable products like lettuce.
Oxygen: Exposure to oxygen, especially with fresh-cut produce, will encourage microbial growth. Moreover, it will speed up chemical processes that cause browning, as in cut apples or potatoes.
However, if oxygen levels become too low, it can cause anaerobic respiration that degrades tissue and appearance.
Carbon dioxide: If carbon dioxide accumulates due to improper ventilation, it can also lead to anaerobic respiration.
Relative humidity: Increase in relative humidity in combination with a rise in temperatures can change the color of fresh produce, such as the de-whitening of mushrooms.
Storage time: There is a limit to how long climacteric fresh produce can be stored. If kept too long, the fruits can suffer negative color change, making them look unattractive. For non-climacteric fresh produce, discoloration starts in a matter of days; see Figure 1.
Microbial attacks: Spoilage happens due to fungus, bacteria, and yeast. Combinations of higher temperature and relative humidity create ideal growing conditions for the microbes that cause rot which changes internal color. Ripening weakens the skin structure, allowing more infection, for example, by Penicillium, Alternaria, Botrytis, Pseudomonas, Bacillus, Clostridium, etc.
Mechanical injury: Bruising can lead to discoloration, which can result from chemical and physiological changes.
Like other parameters, such as dry matter and soluble sugars, internal color is essential for the quantitative estimation of quality. However, internal color can also indicate fresh produce health and the nutritional benefits people can get from fruits and vegetables.
Figure 2: "2a-Butternut Squash External Color Values at 3 harvest stages; 2b Butternut Squash Internal Color Values at 3 harvest stages," Cantwell and Suslow. (Image credits: Photo Credit: Marita Cantwell, UC Davis).
Usually, external fruit color is used to judge maturity or ripeness to fix harvest time. However, in some cases, external color cannot be used as a harvest index since fruit skin color does not reflect ongoing internal changes in maturity, but the internal flesh color does. This occurs, for example, in winter squash, where the skin color changes during maturity are very slight; see Figure 2a. For butter squash, internal color is used to judge maturity; see Figure 2b.
Many varieties of citrus, which are non-climacteric, including Valencia’s sweet orange [Citrus sinensius (L.) Osbeck], do not change skin color with ripening.
Climacteric fruits are harvested when they are mature but not ripe, so skin color is not essential. Internal pulp is used as the maturity index, along with dry matter, soluble sugars, and firmness. For example, in mangos, where color development starts at the center from the seed; see Figure 3.
Figure 3: Changes in Internal color and other quality parameters during maturity and ripeness; the University of Florida and the University of California-Davis. (Image credits: https://www.mango.org/wp-content/uploads/2017/10/Mango_Maturity_And_Ripeness_Guide.pdf)
Fruits and vegetables that are peeled and cut are still living and respiring tissue. Fresh-cut, prepared produce is one of the fastest-growing segments of the food market due to its sheer convenience.
Contrary to other processing, fresh-cut produce commodities have a reduced shelf life since peeling removes the natural protective fruit skin. Cutting and exposing the tissue leads to a loss in phytochemicals, breakdown in cells, and release in oxidizing enzymes that cause decay and browning. Peeling and cutting also expose tissue to microbes and other forms of contamination during handling.
For fresh-cut vegetables and fruits, consumers use color to judge the freshness and flavor of products. The colors must be bright, and there should be no visual defects. Less intense colors indicate produce that is not ripe. The appearance of dryness or dullness in produce has a negative influence on consumer choice.
Green vegetables, like spinach or broccoli become undesirable as they yellow from loss of chlorophyll over time.
After peeling, oxidation through atmospheric exposure can cause browning in fresh-cut products.
Marketers capitalize on vibrant internal color by using transparent packaging to attract consumers. To maintain the color and prevent browning, people will also use many physical and chemical treatments.
Bruising is the most common form of mechanical damage that occurs due to poor postharvest handling. It can result in discoloration or pitting, as in apples and cherries.
It can lead to loss of quality, firmness, and weight. The damage modifies the physiological and metabolic processes in the fresh produce, leading to internal browning and faster ripening due to ethylene production and attracting microbial infection.
Internal color changes identify the depth and extent of bruising. Using a near-infrared spectrometer, like those offered by Felix Instruments – Applied Food Science, these changes can be seen well before the damage becomes visible to the human eye; see Figure 4.
Figure 4. "Bruise measurements (A) bruise diameter and (B) bruise depth, "Pathare et al. 2021. (Image credits: Front. Sustain. Food Syst., 04 August 2021 | https://doi.org/10.3389/fsufs.2021.658132)
Fresh produce is processed to extend its storage time or make products like juices, jams, alcoholic and non-alcoholic beverages, chips, and other ready-to-eat food.
Hence, the choice of processing to extend storage will depend on the fresh produce. Moreover, the importance of internal pulp color as a quality parameter to choose raw materials will increase in the case of products that are peeled before use.
Studies have shown that people judge the flavor of processed products with a heavy emphasis on color. It has been shown that juices and other processed products must have the expected associated colors, like red with tomatoes and strawberry or orange for tangerine products.
Hence, it is crucial to measure and standardize the internal color of raw materials and processed products.
Figure 5: Color wheel showing health benefits of different colored food, courtesy of N.C. Department of Agriculture & Consumer Services. (Image credits: https://plantsforhumanhealth.ncsu.edu/2013/07/04/the-colors-of-patriotic-produce-reds-whites-and-blues/)
The pigments that produce the brilliant colors in the fresh produce are phytochemicals that have valuable health benefits. These can be vitamins, carotenoids, phenols, or chlorophyll.
N.C. State University's Plants for Human Health Institute (PHHI) has listed the various health benefits, which briefly are as follows:
To get more details, check the synergy color wheel, which covers health benefits from both external and internal colors of fresh produce, in Figure 5.
The most critical group of nutraceuticals, including carotenoids and phenols, are antioxidants. Using color as an indicator, it is possible to judge the amount of antioxidants people are consuming. Foods with hue values above 180° and below 20° have the highest antioxidant content; see Figure 6. Therefore, red, blue, purple, and orange-colored foods are antioxidant-rich.
Figure 6: "Relationship between hue and (a) total antioxidant capacity (TAC) of fruits and vegetables measured by the ABTS assay, and (b) potential contributions to meet the recommended daily TAC requirement calculated per serving size of fruits and vegetables," Comert et al. 2020. (Image credits: https://www.sciencedirect.com/science/article/pii/S2665927119300097)
Based on the importance and applications, the following stakeholders use internal color:
Traditionally, the color of fresh produce was measured by color charts and color indexes. Color is, however, subjective and depends on the observer and conditions in which food is viewed.
Nowadays, there are colorimeters based on various technologies that can objectively measure color. This is done by measuring the following aspects:
These three aspects are used in estimating color in terms of chroma and hue.
NIR spectrometers can accurately, rapidly, and non-destructively predict internal color, using chemometrics, as pigments react strongly with light. Commercial tools like the Felix Quality Meters, manufactured by Felix Instruments, Applied Food Science, are simple to use and understand and are used throughout the fresh produce supply chain.
A wide range of methods and color indices are used to measure color, making it difficult to compare results. Most of the traditional techniques are also destructive and objective, even if they seem easy to use. Hence, it is vital to have non-destructive and objective measurements of internal color, such as colorimeters or NIR spectrometers.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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