Fruit Properties
Maturity, Harvesting, and Handling of Grapes for Processing
Physical (size, shape, color, and the nature of the waxy cuti- cle) and chemical properties (moisture content, sugar content, and acidity) of grapes at harvest have effect on the quality of raisins produced. These properties are influenced by several factors, some of which cannot be manipulated by the grower (variety and root stock, age of the vine, soil and climatic conditions), and others such as soil quality, irrigation man- agement, nitrogen and potassium nutrition, growth regulator, pruning, and crop load, which can be altered by the grower.
Some factors, like exposure to pathogens and diseases also affect the maturation and processing quality of grapes.
Gibberellins (GA3) are commonly used during the bloom to loosen clusters and improve the quality of raisins. Peacock and Beede (2004) evaluated the effects of GA3 application at different stages of berry development on dry down ratio, yield, and quality of raisins. They showed that bloom-time application of GA3 improved raisin quality; the raisin and fresh weight yield were greatest when GA3was applied at a rate of 20 g/ha 7 days after the bloom (berry set stage).
Grape ripening is normally associated with an increase in sugars (◦Brix), a decrease in acidity, and the development of characteristic color, texture, and flavor. These changes con- tinue as long as the grapes remain on the vine, and stop after they are harvested. Such changes result in a gradual im- provement until the optimum stage is reached, followed by a steady deterioration. Fresh grapes, with relatively high wa- ter contents, are very sensitive to microbial spoilage during storage; even in refrigerated storage. Therefore, they must be either consumed or processed within a few weeks fol- lowing harvest. The optimum harvest maturity of grapes for
processing represents a compromise of a variety of factors, although consumer satisfaction should usually be the decid- ing factor (Winkler et al. 1974). Firm-ripe grapes ship and store better than either underripe or overripe ones.
The appearance of the berries, associated with the bril- liance of their color and taste, help in the selection of clusters to be picked. Red and black grapes become intensely colored as ripening advances. The green color of white cultivars be- comes more nearly white or yellow. The cluster stems also mature along with the berries obtaining a wood, straw, or yellow color. Owing to the absence of reserve material such as starch, which might be hydrolyzed to sugars after picking, grapes do not ripen after harvest. Thus, if the grape is not at its best in respect of maturity and quality at the time of harvest, it never will be, because all changes after harvest are deteriorative. However, appropriate postharvest handling and storage techniques can slow the rate of deterioration. While stem browning and excessive water loss can decrease quality, trimming, and careful handling improve the fruit quality.
Criteria for harvesting grapes include sugar (◦Brix), acid- ity, pH, and sugar:acid ratio, the most important criterion being the sugar content of the grape. It is often determined as the total soluble solids using Brix refractometer or a hy- drometer. Abbe’s refractometer or saccharometer may also be used. While the hand refractometer is an efficient tool for judging the harvest maturity of individual clusters of table grapes, it is not recommended for use in determining the har- vest maturity of wine grapes. Portable near-infrared (NIR) spectroscopy can be used to take nondestructive, accurate measurements of several chemical compounds in grapes, in- cluding sugars and acidity (Temma et al. 2002, Saranwong et al. 2003, Chauchard et al. 2004). The sugar:acid ratio is a better criterion than either sugar or acid alone for deciding the correct time of harvesting. Depending upon the cultivar and climate, acceptable sugar:acid ratio may fluctuate from 20:1 to 35:1 (Winkler et al. 1974). With an advancement in grape maturity, the acid content may drop significantly, especially in warm tropical regions, as a result of prolonged hot periods during ripening, thus, reducing fruit quality. Winkler (1954) reported a similar effect resulting from over cropping, which may delay maturation.
Weather conditions and date of harvest significantly affect drying time, with later harvests and shorter days slowing down drying by a few days (Peacock and Christensen 1998).
In the United States, grade- and standard-based maturity and berry characteristics, such as freedom from decay, visual mold, immature berries, sunburn, freezing, insect injury, and foreign material, are available for grapes used in processing and freezing. According to these standards, to be graded as US No. 1, grapes should be mature (i.e., have soluble solids of not less than 15.5% as determined by an approved refractometer), have similar varietal characteristics (skin and pulp color), and be free from defects due to decay, visible mold, immature berries, sunburn, freezing, attached insects or insect injury, and foreign materials, or any cause within the
established tolerance limits (USDA 1997). However, grapes with lower maturity or fairly well maturity (soluble solids of not less than 14.5%) with defects within the tolerance limits are graded as US No. 2. However, no tolerance is provided for grapes in either standard that do not meet the maturity requirement.
Table grapes are harvested manually, while mechanical harvesters are used for grapes destined for raisin and juice production. Mechanical grape harvesters are large machines that pass over the vines and shake the grapes from their stems into collecting troughs, which are then emptied into boxes to be carried by trucks to a processing plant.
Drying Characteristics Drying and Dehydration
Drying is probably the oldest and one of the most cost- effective methods for preserving fruits. It is a major industry in several parts of the world where grapes are grown. Drying of grapes on the vine or by open-air sun drying, shade drying, or mechanical drying produces raisins.
The grape, with an outer waxy cuticle and a pulpy mate- rial inside, is a complex product for dehydration. The outer waxy cuticle controls the moisture diffusion rates during dry- ing. A chemical or physical treatment is generally applied to decrease skin resistance and, hence, improves moisture dif- fusion through the waxy cuticle (Ponting and McBean 1970).
Loss of moisture from berries during air-drying is accompa- nied by changes in fruit structure and texture, such as fruit softening or loss of firmness, which are related to the fruit’s microstructure (Bolin and Huxsoll 1987; Aguilera and Stan- ley 1999). Texture is an important quality criterion for raisins that are consumed after rehydration in breakfast cereals, and dairy and bakery products.
Sorption Equilibrium
The quality of dehydrated products like raisins largely de- pends on the water activity of the product, which in turn depends on the moisture content and temperature (Singh and Singh 1996). The adsorption and desorption isotherms demonstrate a concurrent increase in the equilibrium mois- ture content with increasing equilibrium relative humidity.
This relationship is manifested in the form of sigmoid-shaped curves. The desorption curve identifies the type of water present in the product, and thus, provides preliminary in- formation on the conditions driving the mass transport. The state of equilibrium resulting from multiple interactions on a microscopic scale is described by the relationship between the equilibrium water content of the product to be dried and the relative humidity of the atmosphere which surrounds it at a constant temperature. Desorption isotherms for grapes can be determined by placing the product in an atmosphere in which the relative humidity is controlled by solutions of
sulfuric acid (Azzouz et al. 2002). Environments in which the humidity is controlled can also be created using saturated solutions of various inorganic salts (LiCl, MgCl, MgNO3, NaCl2, and KCl), and then the equilibrium moisture contents for desorption isotherms can be determined using gravimetric method (Greenspan 1977; Suthar and Das 1997).
A number of models have been suggested to describe the relationship between equilibrium moisture content and equi- librium relative humidity (Mujumdar 1995). These have been adopted as standard equations by the American Society of Agricultural Engineers (ASAE) for the description of the moisture sorption of biological materials. Of these, the mod- ified Henderson, modified Oswin, modified Chung-Pfost, and Guggenheim, Anderson, and de Boer (GAB) equations take into account the effects of temperature. According to the Hen- derson equation, the relationship can be written as follows:
aw=Experimental (−BXCeq), (27.1) whereawis the water activity,Xeqis the equilibrium mois- ture content on a dry basis (kg/kg), and B and C are the empirical constants. For Sultana (Turkish variety) grapes, Azzouz et al. (2002) estimated the following values for theB andCcoefficients:
B = −0.892T −314, (27.1a) C = −0.086T +31, (27.1b) whereTis the absolute temperature (K).
This relationship can be fitted to experimental curves.
Product Shrinkage
Shrinkage of tissues is a major physical change that occurs during drying of grape berries. It is recognized as an impor- tant consequence of fruit drying that has to be accounted for, since it modifies the dimension of the products, which in turn affects the mass transport phenomena (Wang and Brennan 1995; Ramos et al. 2003). Cellular shrinkage causes modifi- cations in the global structure of grape berries and is directly related to the loss of water from cells. According to Hills and Remigereau (1997), drying results in loss of water from vac- uolar compartment, with minor changes in the water content of cytoplasm or the cell wall compartments in parenchyma apple tissue. Loss of water during drying leads to loss of turgor pressure and cellular integrity (Jewell 1979).
Shrinkage of fruits also results in case hardening, a phe- nomenon that occurs when the drying rate is rapid. As the fruit surface dries faster than its pulp, internal stresses develop and fruit interior becomes cracked and porous. “Non-volatile compounds migrate with diffusing water, precipitate on the product’s surface, and form a crust that keeps fruit dimen- sions thereafter.” Consequently, the overall shrinkage is less when the drying velocity is higher.
Shrinkage and other changes in geometric features of cells can be quantified by image analysis (Bolin and Huxsoll 1987) using stereomicroscope (Ramos et al. 2003). These changes
follow a smooth, exponential decrease over time, and a first- order kinetic model easily fits the data (Ramos et al. 2003).
Cellular shrinkage, which follows an Arrhenius type of be- havior increases with temperature from 20◦C to 60◦C. Drying conditions play an important role in determining the textural properties of raisins. “Slow drying achieved by low temper- ature, low air velocity, and high relative humidity produces uniform and dense products (Brennan 1994)”. On the other hand, fast drying rates result in less dense, tougher products with a crust on the surface, a higher dehydration rate, and products with soft texture upon rehydration.
Azzouz et al. (2002) developed the following model to estimate shrinkage in Chasselas and Sultana varieties:
Shrinkage=0.79×Average local moisture content Initial moisture content +0.22.
(27.2a)
Chemical Changes During Drying of Grapes
Drying either on the vine or on the ground provokes changes in the physical, chemical, and biological properties and mod- ifies characteristics of grape berries. The moisture loss in DOV raisins occurs in a graduated, stepwise manner, with a rapid decline from an initial 86±2% to 60±5%, 108 days after first bloom, followed by a slower loss, and a final accel- erated loss to 25±4% 151 days after first flowering (Aung et al. 2004). Although the pattern of dry-matter accumula- tion was the same in large, medium, and small berries, the large berries contained higher levels of dry matter than the medium and small berries. Sucrose exhibited two maxima, on the 96th day and 123rd day from first bloom. A rise in su- crose levels preceded rises in sucrose, fructose, and sorbitol (Aung et al. 2004). Since sorbitol was not detected in mature berries, but was present and increased during the drying pro- cess, the authors proposed sorbitol or its biosynthetic enzyme as a useful indicator for determining raisin harvest dates. The moisture loss in untreated and pretreated grape berries is far more rapid with solar- or mechanical-drying processes than with the DOV method, but the pattern of moisture loss is the same (Di Matteo et al. 2000).
Air-Drying Kinetics
In the dehydration of grape berries by means of warm air, si- multaneous heat and water (liquid and vapor) transport takes place in the pulp, in the peel (if present), and in the gaseous film surrounding the grapes. Since the duration of the thermal transient is generally far less than the duration of the drying process, mass transport occurs under isothermal conditions.
In other words, the whole drying process is controlled by mass transport only (Bird et al. 1960; Peri and Riva 1984).
Under the assumptions that pulp and peels (if present) are uniform and isotropic, and the grape berries are spherical, the mathematical model of grape dehydration can be reduced
to that of mass diffusion from a spherical body (Bird et al.
1960; Carslaw and Jaeger 1980). Any mathematical model that describes the diffusion of water through whole grape berries must account for its diffusion both in the pulp and in the peel (Di Matteo et al. 2000). Therefore, both processes are described by the model:
∂Ci
∂t =Di
∂2Ci
∂r2 +2∂Ci
r∂r
, (27.2b)
where the indexi=1 refers to the pulp [i.e., forr∈(0, R1)]
andi=2 to the peel [i.e.,r∈(0, R1, R2)],D1 is the water diffusivity in the grape pulp, which is much higher than that in the grape peel (D2),C1 the water concentration in grape pulp,C2 the water concentration in the peel,rthe distance from the grape center, and␦is the peel’s thickness.
The drying curves for various values of drying conditions obtained for a single layer of grapes do not clearly indicate the existence of a first phase of drying. From the beginning of the process, the diffusion of water from the interior to the surface, where it evaporates, is limited. Diffusion becomes more difficult over time because of the shrinkage of the grapes and because of the formation of dry layers at the surface of the fruit (Diamante and Munro 1991).
Knowledge of the drying rate constant of grapes and its dependence on the conditions of the drying air is necessary to design optimal grape dryers. Several studies have been con- ducted to determine the drying constants for grapes under different drying conditions: at a single air temperature with two airflow rates for berries of different weights with various pretreatments (Martin and Stott 1957), taking into consider- ation velocity, ambient air temperature, and pretreatment of grapes on their drying time (Eissen et al. 1985), open-air sun drying and forced convective drying (Riva and Peri 1986), and microwave drying of grapes at different temperatures but one air velocity (Tulasidas et al. 1993).
Pangavhane et al. (2000) determined the drying rate con- stant for Thompson seedless grapes using a commercial pre- treatment (cold dip) under a wide range of controlled drying air conditions (temperature, velocity, and relative humidity) normally used for commercial drying, and obtained the val- ues of constants for empirical relations of the Arrhenius type.
They described the drying behavior of grapes over time for the given conditions of the drying air with the following equation:
MR= M−Me
M0−Me =cexp (−kt), (27.2c)
where MR is the moisture ratio,Mis the moisture content on a percentage dry basis at timet,Methe equilibrium moisture content on a percentage dry basis,M0 is the initial mois- ture content on a percentage dry basis,cis the constant of the equation,kis the drying rate constant (per hour), andtis the time (h).
The equilibrium moisture content,Me, for the raisin at dif- ferent air temperatures and humidities can be computed us-
ing the well-known GAB equation (Singh and Singh 1996).
However, this model was found to be inadequate for cal- culating the loss of moisture from composite materials. To describe the drying kinetics of such products, Page’s equa- tion has been shown to be better (Diamante and Munro 1991;
Tulasidas et al. 1993; Doymaz 2006)
MR=exp (−ktN), (27.2d)
whereNis Page’s number.
The dependence of the drying constant,korNfrom Page’s equation, on the drying-air variables are modeled in the form of and Arrhenius-type model:
korN=␣0V␣1H␣2Experimental−␣3
Tabs, (27.2e)
where␣0,␣1,␣2, and␣3 are the empirical constants of an Arrhenius-type equation,V is the airflow velocity,H is the absolute humidity,Tis the air temperature,absis the absolute temperature (K), and exp is the experimental value.
According to the authors listed earlier, Page’s equation adequately describes the drying kinetics of a single, thin layer of Thompson seedless grapes, and the drying constant is influenced more by temperature than by the velocity or relative humidity of drying air. The experimental data fitted properly into the Arrhenius model.
In Monukka seedless grapes, Xiao et al. (2010) showed that both the drying temperature and air velocity affect the drying kinetics and quality of raisins produced under a thin-layer, air-impingement drying system. Xiao et al. (2010) found that compared to air velocity, the effects of drying temperature on drying time were more significant. They used Fick’s sec- ond law to describe the drying characteristics of Monukka grapes. Using the slope of a linear plot [ln (MR) vs. time], they found that the moisture effective diffusivity (MR) changed from 1.82 ×10−10 to 5.84×10−10 m2/s for temperatures between 50◦C and 65◦C. The drying temperature had a sig- nificant influence on Deffand E␣was found to be 67 kj/mol.
While the air hardness of dried raisins increased as the drying temperature increased, there was no relationship between air velocity and raisin hardness. Drying temperature also had a significant effect on the retention of vitamin C.
Ca˘glar et al. (2009) determined the thermal and moisture diffusivity of seedless grapes at a wide range of temperatures (50◦C and 80◦C) under infrared drying using the following equation:
␣DeffdM
dt =a+bãM+CãT +dãT ãM+eãM2+T2. (27.3) The earlier equation can be used to predict drying rates for seedless grapes without pretreatment. However, in case of pretreated seedless grapes, the equation given later was more suitable for prediction of drying rates (Ca˘glar et al. 2009):
dM
dt =␣exp(bãMc)ãdTe. (27.4)
Ramos et al. (2010) developed a computer program for estimating water diffusibility parameters in a dynamic dry- ing process with grapes. The program numerically solves Fick’s second law for a sphere, by explicit finite differences in shrinking systems with anisotropic properties and chang- ing boundary conditions. These authors used their program in a pilot-scale convective dryer with simulated air conditions observed in solar dryers. This method was found to yield very good predictions of the dynamic drying curves.
Pretreatment of Grapes Before Drying
In grape drying, the rate of moisture diffusion through the berries is controlled by the waxy cuticle of the grapes. A number of authors have reported the effects of pretreat- ments on drying rates and quality parameters (Bolin et al.
1975; Guadagni et al. 1975; Raouzeos and Saravacos 1986;
Aguilera et al. 1987; Saravacos et al. 1988; Pala et al. 1993;
Mahmutoglu et al. 1996; Tulasidas et al. 1996). Dipping in hot water, and the use of chemicals such as sulfur, caustic, and ethyl oleate (EO) or methyl oleate emulsions are some of the pretreatments that are widely used in grape drying.
These chemicals facilitate the drying process by altering the structure of the waxy layer, thus reducing its resistance to water diffusion (Ponting and McBean 1970; Petrucci et al.
1973; Riva and Peri 1986; Doymaz 1998; Di Matteo et al.
2000). EO acts on grape skin by dissolving the waxy compo- nents, which offer high resistance to the transfer of moisture;
however, high alkali concentrations and long dipping times can cause adverse changes in dried grapes (Saravacos et al.
1988).
The golden-bleach process is commonly used to produce light-colored (bleached) raisins in California. In this process, SO2 is employed as a bleaching agent, which is also neces- sary for drying and storage to preserve the yellow color of the raisins. In the Greek process, Thompson seedless grapes are immersed in an aqueous solution containing 4.5% K2CO3, 0.5% Na2CO3, and 1% completely emulsified 0.4% olive oil for about 5 minutes. The dipped grapes are drained, and then spread on trays under direct sunlight for drying. In the soda-dip process, grapes are dipped for 2–3 seconds in a solution of 0.2–0.3% NaOH (caustic soda) at 93.3–100◦C.
Faint checks develop on the skins after the grapes are cooled by rinsing with cold water. Weaker sodas (Na2CO3 and NaHCO3) are preferred to avoid the danger of over dip- ping. A small quantity of olive oil is often used in soda-dip process.
Raisins obtained from pretreatment with 2% dipping in oil were lighter in color, had better skin integrity, and scored the highest points for quality (41.1 of a total of 50 points) compared to those pretreated with 0.5% hot NaOH (35.7), 2.0% EO+2.5% K2CO3(39.9) and 0.4% olive oil+7.0%
K2CO3(38.7), and the control (36.4). Pretreatment with 0.5%
NaOH at 93.0±1.0◦C for 5 seconds produced raisins with a gummy or sticky surface, caused by oozing of syrup through
microcracks in the skin; and pretreatment with 0.4% olive oil + 7.0% K2CO3 produced raisins with an oily surface (Pangavhane et al. 1999b). NaOH pretreatment adversely affected the uniformity of color, whereas 0.4% olive oil+ 7.0% K2CO3 pretreatment imparted a reddish color to the raisins. Untreated berries produced brownish raisins.
Doymaz and Pala (2002) dipped grapes in an alkaline emulsion of EO (AEEO) and potassium carbonate (PC or K2CO3) solutions for 1 minute prior to drying and evalu- ated the effects on the drying rate and color of the raisins.
Dipping time in the AEEO and PC solutions was around 1 minute under ambient conditions. The grapes were then dried in single layers in batches at temperatures ranging from 50◦C to 70◦C with an air velocity of 1.2 m/s. Grapes dipped in the AEEO and PC solutions exhibited shorter drying times than untreated grapes. Dipping in AEEO enhanced the drying rate to a greater extent than dipping in PC, and resulted in raisins with higher color values. The highest raisin quality was ob- tained with grapes pretreated with AEEO and dried at 60◦C.
Mixed fatty esters were prepared in the laboratory from five vegetable oils (groundnut, safflower, sunflower, cotton seed, and rice bran oil) using the Central Food Technological Re- search Institute (CFTRI) process, and used as dipping oil for grapes before drying. Drying rates for grapes treated with mixed fatty acid esters were longer than those obtained with the commercial dipping oil (Giridhar et al. 2000).
Doreyappa Gowda et al. (1997) evaluated 17 new grape hybrids consisting of 7 yellow seedless, 5 black seedless or soft seeded, and 5 seeded grapes, and compared them with Thompson seedless and Arkavati grapes to test their suitability for dehydration. Sensory evaluation of the dry product revealed that hybrids E-29/5, E-12/3, and E-12/7 produced raisins that were superior to Thompson seedless and were comparable to Arkavati, with distinctly lower acidity.
Vazquez et al. (1997) described a pilot-scale drying plant comprising a closed-circuit, hot-air convection chamber with a heat pump for drying grapes that were pretreated in various ways.
A novel physical treatment process to enhance the dry- ing rate of seedless grapes has been used as an alternative to the conventional EO-dipping method in the production of raisins. This new process consists of preliminary abrasion of the grape peel using an inert abrasive material such as grapes subjected to either abrasion drying or EO-dipping pretreat- ments were dried in a convection oven at 50◦C (with an air speed of 0.5 m/s) until the average moisture content was re- duced to about 20% (w/w). Assessment of the drying rate, drying time, and microstructure of the pretreated grapes and color of the dried samples was used to compare the effec- tiveness of the two processes. The physical abrasion method was found to be as effective as the conventional dipping method for removing the waxy outer layer of the grapes prior to drying. Although a darker product was obtained with the physical method, it made no use of chemical additives, thus allowing production of safer raisins (Di Matteo et al. 2000).