Structural , physical , functional and nutraceutical changes of freeze-dried fruit

This review examines the structural, physical, functional and nutraceutical changes of lyophilized fruits. Collapse, porosity, color, glass transition temperature, rehydration capacity, ability to retain water, volatile compounds, phenolic compounds, ascorbic acid, and beta-carotene, were defined, and the causes of changes in these parameters, during freeze-drying, were analyzed. Advantages and limitations of the freeze-drying, were shown, and strategies to reduce the costs associated with its use were proposed. It was concluded that lyophilized fruit retained to a greater proportion characteristics of fresh fruits, compared with other methods of dehydration. The effects of freeze-drying on physical and chemical properties vary in accordance with factors intrinsic to the fruit, and with extrinsic properties inherent to process. Most fruits maintain their color using freeze-drying. The porosity of freeze-dried fruit depends on the freezing speed. The glass transition temperature of dry solid would be an important optimization parameter for the freezing-drying process. The majority of phenolic acids and volatile compounds were conserved in freeze-drying. Freeze-drying increases the rehydration capacity of dried fruits, to a greater extent the hydrophilic groups which are responsible for interaction with water. However, dried fruits by freeze-drying can show structural collapse. The long processing time and energy costs are limiting the application of technology. The researchers recommend using combined to potentiate the benefits of freeze-drying and to lessen their limiting technologies.


INTRODUCTION
Fruits are necessary in the human diet because of their vitamin, mineral and antioxidant content; they are remarkable for their exuberant flavors, colors and smells.These properties ought to be preserved in agro-industrial processing (Kirmaci et al., 2008).The World Health Organization (WHO) promotes the consumption of 400 g of fruit per day (Montenegro et al., 2009;Orrego et al., 2009;Shofian et al., 2011).However, the perishability and seasonal availability of many fruits means that they are not always available to consumers.To combat this problem, the industry often offers processed fruits that have partially or completely lost their physical, nutraceutical *Corresponding author.E-mail: lserna@unal.edu.co.
Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License and nutritional properties (Sijtsema et al., 2012).
Dehydrated fruits are excellent alternative, making year-round availability more feasible and extending the shelf life of fruits (Marques et al., 2007).The methods of fruit dehydration include drying in hot air (Giraldo et al., 2010), refraction window (Ocoro and Ayala, 2012), osmotic dehydration (Ayala et al., 2010), frying (Villamizar and Giraldo, 2010), drying by microwave (Duan et al., 2010;Jiang et al., 2010) and freeze-drying (Ayala et al., 2010;Ceballos et al., 2012).These methods diminish water activity and therefore reduce the number of enzymatic, chemical and microbiological reactions that take place (Hincapié et al., 2010).Dehydration methods should minimize the loss of nutrient and antioxidant contents (Santos and Silva, 2008); however, because some anti0oxidant compounds are weakly bonded to water in the fruit, some nutritional content will be lost in the dehydration process.Generally, fruits with the greatest initial water content lose a greater portion of soluble solids during dehydration (Ceballos et al., 2012).
Freeze-drying is the removal of water from a product through sublimation (Rothmayr, 1975).Sublimation is the conversion from ice directly to vapor without passing through a liquid state.In water, sublimation occurs when the vapor pressure and the temperature of the ice's surface are below the triple point [4.58 mm Hg (610 Pascal), 0°C] (Jennings, 2002).The process of freezedrying has three essential steps: freezing, primary drying and secondary drying.Approximately 90% of the water is removed from fruits in the first drying phase (Welti et al., 2005).This article reviews the structural, physical, nutraceutical and functional changes of freeze-dried fruit.Advantages and limitations of the freeze-drying process as well as strategies for improvement are discussed.

STRUCTURAL CHANGES THAT CAN OCCUR IN THE FREEZE-DRYING PROCESS
The sought-after freeze-drying products are porous fruits that maintain their volume, can have fast and nearly complete rehydration when water is added and do not shrink (Duan et al., 2010).However, some freeze-dried products undergo undesirable structural changes.Microscopy can be used to study structural changes in freeze-dried fruits and to find a relationship to some physical properties (Yeom and Song, 2010).Table 1 presents a variety of fruits, processing variables, physical changes and the structural and nutraceutical effects of freeze-drying.

Collapse
Collapse and contraction are terms used to describe the loss of structure.A collapsed product may show reduced pore size and volumetric contraction (Khalloufi et al., 2010;Madiouli et al., 2012).According to Harnkarnsujarit and Charoenrein (2011), collapse is the viscous flow that occurs when the viscosity diminishes beyond the glass transition temperature (Tg).Loss of structure occurs when the material is incapable of supporting its own weight.If the temperature of a porous product is above the glass transition temperature, the viscosity of the solid material may not be able to support the structure, causing collapse and contraction (de Oliveira et al., 2010).Collapse is alternatively defined as a decrease in volume or an increase in apparent specific density (Cui et al., 2008).
Collapse is noted by the shrinkage of the dried product.These structures are sensitive to physical, chemical and microbiological changes, which reduce the shelf life and stability of the product (Queiroz et al., 2008).Several studies have shown that collapse slows some chemical reactions, such as oxidation (Prado et al., 2006) as well as the liberation of trapped volatile compounds (Levi and Karel, 1995).
The primary drying temperature can be manipulated to control collapse.A high freeze-drying temperature negatively affects the humidity of the final product, which can lead to structural loss or collapse.Collapsed products are tougher, are have less aroma, and have less rehydration capacity (Harnkarnsujarit and Charoenrein, 2011).
The structural collapse of fruit can be physically controlled, by modifying the product structures before freeze-drying, and can be chemically controlled, through the addition of compounds that modify the collapse temperature.Products with high sugar content, such as fruit juice, have low collapse temperatures (de Oliveira et al., 2010).Collapse can be decreased, by controlling variables in the process such as the freezing speed and temperature (Ceballos et al., 2012).Ayala et al. (2010) describe collapse (19.15% volume loss) during the osmotic dehydration of yellow pitahaya.Evidence of this collapse can be observed in the deformation of cell walls and diminished cell turgor.Freeze-dried pitahaya, even without pre-treatment, did not collapse, losing only 2.6% of the original volume.Similarly, freeze-dried papaya did not present collapse (Marques et al., 2009).
Collapse is related to product porosity and rehydration.When collapse is not visually obvious, collapse can be evaluated through rehydration because structural collapse is known to diminish rehydration capacity (Marques et al., 2009).
Freeze-drying tropical fruits such as pineapple, guayaba, mango and Barbados cherries does not produce signs of collapse (Marques et al., 2006).The same is true for acerola pulp, which does not show visible signs of collapse (Marques et al., 2007).However, acerola and guayaba have a low rehydration capacity, which can be described as collapse.Mango, papaya and pineapple are readily rehydrated and are not thought to collapse in the freezedrying process (Marques et al., 2009).de Oliveira et al. (2010) describe the freeze-drying of peki, a Brazilian fruit.The pulp was pretreated with sucrose and ethanol, which accelerates the freeze-drying process.This pretreatment protected the structure from collapse.
There are reports that freeze-dried apple and carrot also undergo collapse; however, compared with those dried by hot air or microwave, these freeze-dried products lose substantially less volume than their counterparts (Cui et al., 2008).

Variation in porosity
Using scanning electron microscopy (SEM), changes in a model system's microstructure can be studied, and the pore size of freeze-dried fruit can be measured.Similarly, SEM can be used to determine the size and shape of the particulates in pulverized fruits (Yeom and Song, 2010).
Porosity is defined as the relationship between fractional pore volume and the total food volume (Rahman et al., 2005).Porosity affects the texture and quality of dry foods and foods with moderate moisture levels.This characteristic is more important in freeze-dried foods than in microwaved or air-dried foods (Purnama et al., 2010).
The pores of dried products are composed of the small or large spaces formed by ice crystals during the freezing process.These spaces facilitate the diffusion of water vapor during the drying process.The porosity of freezedried fruit depends on the freezing speed, which, when slowed or augmented, can cause the ice to form smaller or larger pores, respectively (Voda et al., 2012).Slow freezing leads to the formation of voluminous crystals and pores that facilitate dehydration and subsequent rehydration (Jennings, 2002).Pore size has a small effect on vapor flow (Pardo and Niranjan, 2006).
Information about the characteristics of individual pores and structural characteristics of dried fruit can be used to design processes, determine product quality and estimate other properties such as thermic conductivity, density, water diffusivity and characteristics related to the extraction of bioactive components (Rahman and Sablani, 2003;Madiouli et al., 2012).
Porosity in fresh and freeze-dried fruit can be measured by comparing the apparent (ρu) and true density (ρr).Pore thickness can be measured using a method proposed by Madiouli et al. (2012) that includes open and closed pores.Direct experimental methods do not permit measurement of the formation of closed pores.
Freeze-dried guanabana has medium-sized pores on the dry outer layer that are associated with rapid freezing (2.4 and 3.1°C/min), which causes the formation of smaller ice crystals.Medium-sized pores on the dry layer make sublimation more difficult and therefore increase the moisture level of the freeze-dried product (Ceballos et al., 2012).

PHYSICAL CHANGES TO FRUIT DURING THE FREEZE-DRYING PROCESS Color
The conservation of color is considered an indication of quality in dried fruits given that non-enzymatic browning processes develop during the drying process (Ceballos et al., 2012).Freeze-dried fruits better maintain red and yellow colors than fruits dried using traditional methods (Shishehgarha et al., 2002).However, kinetic color analyses of freeze-dried fruit show some deterioration of reds and yellows (Guiné and Barroca, 2012).
Strawberries that are freeze-dried at temperatures below -50°C retain their color better than strawberries freezedried at temperatures above -50°C.This effect is due to a higher concentration of solutes (reduction of water) and the effects of pH on anthocyanin (Shishehgarha et al., 2002).Fresh alpine strawberries maintained their color after freeze-drying; color values for freeze-dried strawberries were L* = 58, a* = 23, and b* = 26, while these values were and L* = 57, a* = 23 and b* = 26 for fresh strawberries (Yurdugül, 2008).
Color variation in freeze-dried Granny Smith apples was lower (3,537 ± 1,717) than the variation after convective drying [5,279 ± 0.989], convective drying in a vacuum (11,308 ± 1,729) and microwave drying (5,958 ± 0,580) (Valencia et al., 2011).Moßhammer et al. (2006) found that freeze-drying drastically affected the color of nopal, a change that can be attributed to the formation of soluble melanoidins.Pulverized nopal does not exhibit the same color change.
Sliced carrots maintain their color throughout the freeze-drying process.Sliced apples exhibit minor color variation (Cui et al., 2008).The loss of color in freezedried green peppers is minor compared to the color loss in air-dried peppers.However, comparing the color values L*, a* and b* of fresh peppers (37.22 -14.11 and 22.52, respectively) and freeze-dried peppers (44.12 -12.11 and 19.59, respectively), green color loss is noted, due to the decomposition of chlorophyll (Guiné and Barroca, 2012).Guiné and Barroca (2012) reported that freeze-dried squash had L*, a* and b* values of 77.70, 15.25 and 41.43, respectively, exhibiting color losses in red and yellow compared to fresh squash (68.97, 18.21 and 49.82).
Rapid freezing of guanabana (from 1.1 to 3.1°C/min) produced a more intense white value (L* = 85.10 to 88.92).This higher color intensity can be explained by small pores, (an effect of rapid freezing), due to, the small pores dispersing more light than large pores (an effect of slow freezing) (Ceballos et al., 2012).Freeze-dried papaya was more luminescent, and freeze-dried guayaba maintained better color than the same fruits dehydrated in a vacuum (Hawlader et al., 2006).

Glass transition
The glass transition temperature (Tg) is a key physical parameter that explains the chemical and physical behavior of a food system.Freeze-dried foods must have stable moisture levels and good packaging during storage.Absorption of additional moisture can lead to a state of amorphous disequilibrium, which brings with it a transformation from a glass solid state to a plastic fluid state when the glass transition temperature is reached (Duan et al., 2013).The glass transition temperature of dry solid would be an important optimization parameter for the freezing-drying process.This parameter can be used as useful tool for the choice of the most appropiate materials to be freezing-dried (Ratti, 2001).
Freeze-dried cubed apples were analyzed by differential scanning calorimetry (DSC).The Tg was below -30°C (Duan et al., 2013).The Tg of freeze-dried tomatoes was determined using DSC.The thermogram revealed the existence of two similar but distinct Tg values, which generated a water plasticization effect in hygroscopic regions of the product (Tg= -50°C) (Telis and Sobral, 2002).Guizani et al. (2010) studied freeze-dried Deglet Nour palm dates using DSC.The thermograms generated showed a diminishing glass transition temperature (-13.8 to -48.7°C).However, the Tg rose with an increase of the solid content of the fruit.The Tg of the common mushroom (Agaricus bisporus) was as low as -77.9°C after freezedrying (Shi et al., 2012).Freeze-dried kiwi [Grosella espinosa China] had a Tg of -57.2°C for optimal freezingconcentration conditions (Wang et al., 2008).

Rehydration capacity and water retention
Rehydration capacity (RC) is the ability to reabsorb water relative to the water lost during dehydration, while water retention capacity (WRC) is defined as the ability to absorb and retain water relative to mechanical force.These indicators are related to the structure, the tissue and the capacity to retain absorbed water.Increases or decreases in these indicators can be attributed to the denaturation and/or aggregation of proteins due to heat, salt concentration, desorption of water and destruction of pectins in the cell membrane (Sanjuán et al., 2001).As mentioned above, porosity also influences the rehydration capacity of freeze-dried fruits.The formation of large water crystals facilitates large pore formation, which in turn facilitates rehydration (Jennings, 2002).
The quality of a freeze-dried product is marked by the speed and ease of reconstitution or rehydration.Rehydration capacity is a key characteristic of freeze-dried fruit and is dependent on the size, geometry, composition, water content and porosity of the fruit (Sanjuán et al., 2001).Rehydratability is also affected by the method of rehydration utilized and the temperature, time and conditions of agitation (Arriola et al., 2006), as well as pretreatment factors.Sliced yellow pitahaya, for example, has 24% less rehydration capacity when osmotically dehydrated prior to freeze-drying (Ayala et al., 2010).In avocado, the speed and capacity of rehydration are independent of immersion temperature (Arriola et al., 2006).
The rehydration capacities of freeze-dried carrots and apple chips (3.94 and 5.61, respectively) were similar to the values obtained by combined methods (microwavefreeze-drying) (3.95 and 5.22, respectively).Porosity was similar in both of these foods (Cui et al., 2008).Freezedried Chilean guava has an external appearance similar to that of the fresh fruit, although the freeze-dried product is gelatinous inside (Reyes et al., 2010).
The rehydration capacity of freeze-dried acerola is high (10.1 kg/kg) when it is frozen with liquid nitrogen at 73.6°C/min, which does not cause cell rupture in the samples (Marques et al., 2007).The rehydration capacity of freeze-dried yellow pitahaya is high, reaching a moisture content nearly equal to that of the fresh fruit (3.393 kg water/kg m.s) (Ayala et al., 2010).

CHANGES IN VOLATILE COMPOUND CONTENT IN FRUIT DURING THE PROCESS OF FREEZE-DRYING
Fruit aromas and flavors are transmitted by volatile compounds, such as esters, alcohols, terpenes (derived from the metabolism of mevalonic acid), aldehydes, carbonyl compounds, acids, brass, phenols, hydrocarbons, arylpropanoids (derived from the shikimic acid metabolic pathway), sulfur compounds and glucosinolates (derived from the amino acid metabolic pathway), among others (Wang et al., 2007;De Torres et al., 2010).Volatile compound content changes as fruit ripens.Ripe cocona, for example, has an increased concentration of esters and alcohols, decreased concentration of carbonyl compounds and high concentrations of methyl salicylate and -terpineol, as well as (Z)-3-hexenol, though in lesser proportion.The concentrations of the aldehydes (Z)-3hexenal, (Z)-2-hexenal and (E)-2-hexenal diminish as the fruit ripens (Quijano and Pino, 2006).
The volatile compounds in freeze-dried fruits are retained by entrapment in microregions of dry material.Loss of volatile compounds is due to adsorption of water by the dry product, which increases the permeability of these dry regions and therefore, the loss of these compounds.Carbohydrate-carbohydrate bonds are replaced by carbohydrate-water bonds.However, for the critical water content, Xc, microregions remain sealed and cease to lose volatile compounds.Xc is defined as the water content at the transition point between the primary and secondary drying phases (Jennings, 2002).Wang et al. (2007) demonstrated that freeze-dried plantains maintain their ester content.Esters are the most important volatile compound in plantains, though unlike enols, which are lost, esters have little influence on the aromatic profile of the plantains.Coffee that is freezedried under conditions with slow freezing and rapid drying retains aromatic compounds (Sagara et al., 2005).Grape peels in freeze-dried grapes retain terpenes, sesquiterpenes norisoprenoids, aldehydes and esters, among other compounds, which ensure wine quality (De Torres et al., 2010).
Analysis of apples by mass spectrometry and gas chromatography [GC/MS] showed that the volatile compounds most common in apples-ethyl acetate, ethyl butyrate and methyl anthranilate-were retained during the freeze-drying process (Krokida and Philippopoulos, 2006).
The chalarina (white zapote), a Peruvian fruit, retained a larger proportion of volatile and thermosensitive compounds, such as vitamin C, when dehydrated via freeze-drying (Castañeda et al., 2010).The majority of phenolic acids and volatile compounds were conserved in freeze-dried strawberries (Yurdugül, 2008).

CHANGES IN THE NUTRACEUTICAL CONTENT OF FREEZE-DRIED FRUIT Total phenolic compounds (TPC)
Phenolic compounds are a large group of antioxidants that are present in nearly all plant-based foods.The measured total phenolic compound content depends on the type of fruit, ripeness, agricultural and climactic factors, post-harvest storage conditions and method of anti-oxidant extraction (Shofian et al., 2011).
Freeze-drying fruit can diminish the phenolic compound content because ice crystals formed during freezing can cause cell wall rupture and the subsequent release of oxidative and hydrolytic enzymes that degrade phenolic compounds (Chang et al., 2006).However, there are fruits, such as tomatoes and blueberries that have significantly higher levels of phenol compounds after freeze-drying.This effect is due to the liberation of phenolic compounds that had been trapped in the cell wall (Reyes et al., 2011).
The antioxidant content of freeze-dried açaí was 255.1 mg/100 g of freeze-dried fruit for cyanidin-3-glucoside and 25 mg/100 g of freeze-dried fruit for ferulic acid (Rojano et al., 2011).The polyphenol content in fresh Chilean guava is 1460 mg/100 g (dry mass), a value that drops at least 40% after freeze-drying.Freezing speed affects polyphenol content; rapid freezing led to a polyphenol content of 793 mg/100 g (dry mass), and slow freezing produced 815 mg/100 g (dry mass) (Reyes et al., 2010).
In watermelon, papaya, carambola, mango and melon, the levels of phenolic compounds decreased by 48.23, 39.73, 24.08, 23.19 and 10.41%, respectively, during the freeze-drying process (Shofian et al., 2011).The phenolic compounds found in strawberries, esters, terpenoids, carbonyl compounds and various alcohols and acids, do not change during the freeze-drying process (Yurdugül, 2008).Reyes et al. (2011) demonstrated that freeze-drying diminished the phenolic compound content of blueberries from 832.9 mg/100 g to 769.2 mg/100 g (dry mass); however, this loss was insignificant compared to other dehydration methods.Freeze-dried Chilean guava lost 33.3 and 67.5% of the original total antioxidants after rapid and slow freezing, respectively (Reyes et al., 2010).

Ascorbic acid
Ascorbic acid content can decrease in high-temperature treatments, with this compound degrading when oxidized to dehydroascorbic acid and hydrolyzed to 2,3diketogulonic acid.The latter can further polymerize to form other nutritionally inactive products (Chang et al., 2006).Some studies have indicated that freeze-drying allows the retention of ascorbic acid when the freezedrying is conducted at low temperatures, minimizing deterioration of this water-soluble vitamin (Shofian et al., 2011).Chang et al. (2006) compared the ascorbic acid content of fresh, freeze-dried and hot-air dried tomatoes and found that freeze-drying reduces the loss (8.2%) compared to hot-air drying (56-61%).The losses of vitamin C in the pulp of cherries, papaya, pineapple, mango and guava due to freeze-drying were 3.59, 24.2, 26.92 and 37%, respectively (Marques et al., 2006).Vitamin C loss was 10% in freeze-dried nopal and 50-55% in spray-dried nopal (Moßhammer et al., 2006).
The original ascorbic acid content was preserved in freeze-dried carambolo, mango, papaya, melon and watermelon.This study concluded that freeze-drying maximizes ascorbic acid retention and that freezing had a minimal degradation effect on vitamin C (Shofian et al., 2011).The ascorbic acid content of freeze-dried guanabana declined from 81 mg/100 g to 40.57mg/100 g under rapid freezing conditions (3.1°C/min), and from 81 mg/100 g to 53.76 mg/100 g under slow freezing conditions (1.1°C/min) (Ceballos et al., 2012).
The acerola is a Brazilian fruit with a high vitamin C content that varies according to maturity.The green fruit has 179.7 mg/100 g, the red-yellow fruit has 176.4 mg/100 g, and the red fruit has 149 mg/100 g.The vitamin C content declines significantly during the freeze-drying process, to values as low as 55.1 and 72.1 mg/100 g in green and red fruits, respectively.In the yellow-red fruit, vitamin C loss was 13% (Marques et al., 2007).Freezedried blueberries lose 60.43% of their initial vitamin C content (151.9 mg/100 g dry mass) during drying (Reyes et al., 2011).

β-Carotene content
β-Carotenes are compounds found in the lipid membranes or vacuoles of leafy green vegetables and yellow to orange fruits and vegetables.The degradation of phenolic compounds can lead to the degradation of β-carotenes (Shofian et al., 2011).Freeze-dried carrots retain 95.4% of their initial β-carotene; this value is a larger proportion than that obtained through other dehydration methods, including microwave and hot air dehydration (Cui et al., 2008).In the process of freeze-drying watermelon, mango, carambola and papaya, 43.1, 26.19, 15.75 and 8.16% of their initial β-carotene was lost, respectively (Shofian et al., 2011).

ADVANTAGES AND LIMITATIONS OF FREEZE-DRYING FRUIT
As has been demonstrated in the studies described, the effects of freeze-drying on physical and chemical properties vary in accordance with factors intrinsic to the fruit and extrinsic properties inherent to the process.Regardless, general advantages and limitations can be identified.
Among the advantages of freeze-drying are enhanced stability, rapid and easy solubility, prolonged shelf life, and increased availability and permanent access to fruit.Freeze-drying guarantees high-quality products with superior sensory and nutritional properties.
As with traditional dehydration methods, freeze-drying is not completely reversible by rehydration; however, damage to the hydrophilic groups responsible for the interaction with water is lower than that of other methods.Freeze-dried products rapidly rehydrate and regain water content and organoleptic properties similar to those of the fresh product (Jiang et al., 2010).
In comparison to products obtained through other dehydration methods, freeze-dried fruits have a more porous structure as well as superior flavor and aroma retention (Ceballos et al., 2012).High porosity facilitates rehydration with water or other adequate solvents, and greater porosity is directly related to a greater rehydration capacity and is an indicator of product quality (Ayala et al., 2010).The reduction of volume during freeze-drying is minimal (Shishehgarha et al., 2002).The temperatures reached in the freeze-drying process are outside of the range of temperatures that induce chemical changes in fruit.There is minimal loss of volatile compounds at freezedrying temperatures (Yurdugül, 2008).
Freeze-dried fruits have a very low moisture content, which allows them to be stored for long periods; freezedried goods are highly stable products (Shishehgarha et al., 2002).Freeze-dried fruits can be used as antioxidants and colorants in natural foods (Yurdugül, 2008).Freezedrying allows the production of powdered fruit that can be used as base materials in the food and pharmaceutical industries.Freeze-drying is an ideal technology for consumers without access to fresh produce, such as astronauts (Aguilera and Stanley, 1999).
Among the limitations of freeze-drying is the cost.Freeze-drying requires long processing times and consumes large amounts of energy to freeze the fruit, sublimate the ice, dry the product and condense the vapor while maintaining vacuum conditions (Sadikoglu et al., 2006;Benlloch et al., 2012).The vacuum pump and sublimation account for 26 to 45% of the total energy consumed in the process (Menlik et al., 2010).Equally important to energy costs are general operating conditions, such as the chamber pressure and heating and freezing speeds (Arriola et al., 2006).Structural collapse is a limiting factor for some fruits for which freeze-drying causes a wrinkled and compromised structure that cannot be reversed (Lee et al., 2006).
A final limitation, closely linked to freeze-dried product stability, is hygroscopicity.Freeze-dried products are highly hygroscopic, and further extension of the shelf life can be gained with the addition of solutes such as Arabic gum, tricalcium phosphate and maltodextrin that can increase product stability and act as a barrier to water absorption (Kaushik and Roos, 2007;de Oliveira et al., 2010;Fabra et al., 2011;Benlloch et al., 2012;Ceballos et al., 2012;Mosquera et al., 2012).
In an effort to reduce energy costs when maintaining a high-quality product, investigators have sought to combine technologies such as microwave (Cui et al., 2008), hot air (Giraldo et al., 2010) and osmotic dehydration (Ayala et al., 2010) to eliminate a portion of the water before freeze-drying (Zhang et al., 2006).Table 2 outlines the investigations of combined dehydration technologies as a cost-saving measure.

CONCLUSIONS
The structural, physical, functional and nutraceutical effects of freeze-drying produce are dependent on intrinsic factors that are inherent to the fruit and to extrinsic factors that are inherent to the process.Freeze-drying  Cui et al. (2008) technology offers advantages and has limitations.
Freeze-drying is an ideal method for heat-sensitive fruits that require special care during processing.In many fruits, properties such as shape, dimension, appearance, flavor, color, texture and nutraceutical ingredients are retained after freeze-drying, adding value of approximately 120%.The technology is equally valuable when a high rehydration capacity is required, as is the case for powdered freezedried fruit.Limitations to the application of this technology include the prolonged processing time, high-energy costs, high product hygroscopicity and undesirable physical changes such as collapse.
Investigators have sought to lower freeze-drying costs through the use of pretreatments and the combination of freeze-drying with other dehydration technologies, such as hot air, microwave and osmotic dehydration.

Table 1 .
Process variables and physical, structural and nutraceutical changes in freeze-dried fruit.

Table 2 .
Combining methods to reduce freeze-drying costs.