Abstract
Color and texture are important attributes related to vegetable quality. The objective of this study was to determine the changes in quality characteristics, caused by cooking time, and pH (alkali and acid addition) in some vegetables. Four different vegetables — carrots, spinach, red cabbage and cauliflower — were analyzed for changes in color and texture associated to time of boiling, the effect of alkali (baking soda), and the effect of acid (vinegar and cream of tartar). Samples of each vegetable were prepared and treated for five minutes. Color change was measured visually and using the L*a*b* technique. Color and pH of water was determined visually and using paper strips and pH meter, respectively. Texture was also measured after each treatment. A five minute boil was used as control experiment. Changes in color and texture changes were found between the four vegetables and the different treatments. The greatest variation in color was obtained for the spinach and red cabbage. In conclusion, boiling and pH-modification treatment with acid and alkali both affect the quality of color and texture in vegetables.
Introduction
Vegetables are valuable as food because of their nutritional components. Vegetables are rich in vitamins, mineral and fiber content and they are low in calories (Dey and others 2014). In addition, they add color, flavor and texture in different menus, which in most cases help in the decision of eating and enjoying the meal.
Changes in vegetable characteristics are associated to the components of vegetables, including their pigments, related with the color and appearance, and others, such as cellulose and hemicellulose that are related to texture changes (Vaclavik and Christian 2014). Texture is an important quality attribute in vegetables (Hong and others 2014). In many cases sensory properties, especially texture, are used to decide when a vegetable is cooked (Bongoni and others 2015).
Different pigments are present in vegetable and fruit, including chlorophyll, carotenoids, anthocyanins, betalains and anthoxanthins, giving the color to vegetables and fruits. Changes in pH can affect those pigments changing the color in some vegetables. These modifications can be accelerated in the presence of heat.
Many different techniques have been used to determine food color, such as visual observations and L*a*b* color space method. This last technique is designed to simulate human vision so the perception of color is very similar to the human eye and has been proved useful in determining the quality control in fruits and vegetable (Markovic and others 2013; Demiray and Tulek 2015).
The objective of the experiment was to study some vegetables — carrot, spinach, red cabbage and cauliflower — for quality characteristics changes caused by time of cooking, and pH-modification treatment through the addition of either alkali or acid. The null hypothesis was that there is no difference between the control and treatment in changes of quality characteristics in vegetables.
Materials and Methods
Vegetable preparation.
Carrots, spinach, red cabbage and cauliflower were prepared for different treatments. Carrots were peeled and cut into strings of medium thinness. Spinach was chopped into large strips. Red cabbage was cut in wedges. Cauliflower was separated into flowerets.
Sample treatments.
Five different treatments were applied to each vegetable: two different cooking times —five minutes, used as a control, and 20 minutes —; one alkali effect, using baking soda; and the effect of acid, using vinegar and cream of tartar.
General procedure.
For treatment control, 350 mL of de-ionized water were placed in a 1-quart saucepan with a lid on. Water was boiled and then the vegetables were added. After water started boiling again, the vegetables were left for five minutes. In the case of 20 minute cooking, 1050 ml of de-ionized water and a 2-quart saucepan were used instead.
For different treatments, 350 mL of deionized water containing each of the substances (baking soda 0.5 g; vinegar 30 mL and cream of tartar 1.0 g) was placed in a 1-quart pan with a lid on. When water was finally boiling, the prepared vegetables were added and allowed to boil for five minutes. Water was drained, placing part of it in a custard cup. The particular vegetable was placed in a plate with the cup, containing the water, on the side and labeled for the vegetable and treatment in order to identify it. After water cooled to 50 ºC, pH was measured using a pH meter and paper test.
Characteristics such as juice color, vegetable color, and vegetable texture were evaluated by visual observation and the L*a*b* color space technique, meaning that their changes were measured at both a qualitative and quantitative level.
Results and Discussion
Four different vegetables, carrots, spinach, red cabbage and cauliflower were study to determine changes in quality characteristics after heat, acid and alkali treatment. In vegetables, keeping natural color is a challenge when they are cooking because the heat can change pigments, making the food look dull and less attractive.
Vegetable, juice color, pH, and texture were measured after each treatment. These characteristics are important when determining the quality of food. A five minute boil was determined as control.
In all treatments, carrot color remained orange. Measurements of L*a*b* showed positive values in all treatments (Figure 1a), with some small differences with respect to the control (5 minute boil). This may be explained by their molecular components, particular to this type of vegetable.
Carrots contain carotenoid pigments. These pigments are stable during cooking, and they are not affected by pH, especially by alkali, explaining the stability of the color in this vegetable. Carotenoids and chlorophyll are slightly soluble in water, so juice color did not have big changes.
Texture also changed between control (firm) and other treatments, where it ended up mushy and rubbery. Presence of positive values for L*a* b* are related to the brightness of this vegetable and to the presence of red and yellow color which is associated to a* and b*, respectively.
L*a*b* provides useful information for color, especially as a quality control in vegetables (Demiray and Tulek 2015). Presence of both colors, yellow and red, define the orange color in carrots.
Figure 1. L*a* b* values for color and pH in carrot (a), spinach (b), red cabbage (c), and cauliflower (d) using different treatments.
Miglio and others (2009) also demonstrated a decrease of firmness in boiled carrot associated to its components. Hardness of carrots is related to the content of some compounds such as lignin and cellulose. Changes in carrot’s texture can be associated to the solubilization of the pectin related to the cleavage of these pectinases (Borowska and others 2004). Reduction of hardness has been demonstrated in carrots with a long cooking time at high temperature (MdShahidul and others 2003; Hong and others 2014). On the other hand, Demiray and Tulek (2015) reported a negative effect of high temperature in raw carrots, especially in their nutritional and chemical composition, which are an important component of the contemporary American diet.
exture is an important quality attribute of processed fruits
and vegetables.
Texture is an important quality attribute of processed fruits
and vegetables.
In the case of spinach, color changed from a dark green, in control, to different types of green in all treatments. L* a* b* values are reported in Figure 1b. Only a* showed negative values, in all but the vinegar treatment. This value is associated to green when it is negative and red when it is positive (León and others 2006). Color change in spinach can be associated to changes in chlorophyll.
An intensified green color in the control is related to the initial heating, which causes the subtraction of air from the tissues causing a better visibility of the chlorophyll and giving a bright green color in this vegetable. When green leaves are overcooked or are in contact with acids, chlorophyll can change to pheophytin (Bartira and others 2016) that causes an olive green color, as was demonstrated in this experiment.
This change in pigments is influenced by pH which is low, between three and four, on acid treatments (vinegar and cream of tartar) (Figure 1). On the other hand, contact with alkali, such as baking soda (pH around 9.6), induces the production of chlorophyllin, which causes the bright-green color.
Texture also changed in all but the vegetables that were boiled for 20 minutes, where spinach was kept moist. Textures, such as soft and rubbery, were present for alkali and acid treatments. Changes in texture are related to the destruction of the hemicelluloses with either heat or alkali.
Red cabbage color was different between treatments and the control. L*a* b* values varied in the different treatments (Figure 1c). For the 20 minute boil and baking soda treatment (alkali), a* and b* values were negative, indicating more green and blue color. Negative values were also found for b* in vinegar and cream of tartar but they were near to cero, so they were not significant.
Red cabbage contains anthocyanins pigments in the group of flavonoids. These pigments are red when the pH is acid and blue to purple in alkaline pH, which explains the change in color when this vegetable is processed.
Texture also changed from hard to tender, soggy and rubber-like. Changes in pH in juice are associated with the treatment, being acid in vinegar and cream of tartar treatment and alkali in baking soda treatment (Figure 1c).
On the other hand, cauliflower did not modify substantially during the process because it contains different pigments. This vegetable maintained its same color, between white and a little yellow, throughout the whole treatment. Stem green color varied a little beat with treatment. L* values were high in all treatments. In contrast to a* value was negative and b* values were positive in all treatments.
As was explained before, a* negative values are associated with green color and b* positive values with yellow color. This vegetable contains anthoxanthin pigments which are white or without color.
Changes in pH to acid or alkaline produce variations in color as was evident in the experiment for baking soda and vinegar. In contrast, texture changed from smooth and spongy, to squishy, slimy, rubbery and squishy.
pH control is important in order to preserve color and texture in vegetables (Andres-Bello and others 2013). The little differences noticed in pH measured with paper test and pH meter are related to the sensibility of the method, being the pH meter method more objective, sensible and accurate than the paper test, which can be subjective.
Differences were found between the control, 5 minute boil, and most of the treatments, rejecting the null hypothesis and concluding that boiling and pH (acid and alkali) treatments caused changes in color and texture of some vegetables.
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