Valentina V; Pratiwi AR; Hsiao PY; Tseng HT; Hsieh JF; Chen CC

Research Article

Austin Food Sci. 2016; 1(6): 1027.

Sensorial Characterization of Foods Before and After Freeze-drying

Valentina V¹, Pratiwi AR¹, Hsiao PY², Tseng HT², Hsieh JF^2,3 and Chen CC³*

¹Department of Food Technology, Soegijapranata Catholic University, Indonesia

²Department of Food Science, Fu Jen Catholic University, Taiwan

³Ph.D. Program in Nutrition & Food Science, Fu Jen Catholic University, Taiwan

*Corresponding author: Chen CC, Ph.D. Program in Nutrition & Food Science, Fu Jen Catholic University, Taiwan

Received: October 11, 2016; Accepted: November 10, 2016; Published: November 16, 2016

Abstract

Freeze-dried foods can be easily transported at normal temperatures, stored for a long period of time and consumed with a minimum of preparation. The purpose of the study was to investigate the study the freeze-dry processing, to study the sensory characteristic of the initial and final dried product. The food sample used banana, apple, kiwi, baked sweet potato, tofu, pudding, plain yogurt drink and brown rice milk. Physico-chemical quality parameters such as color values, texture, shape and taste profiles also moisture content were determined for the fresh and freeze dried samples. The moisture content value decreased after freeze dry processing whereas color brightness values increased, the original color and the shape of the sample is maintained, the texture become crisp, spongy, soft, easily to destroy and crumbly when exposed with tongue.

Keywords: Freeze-dried foods; Freeze-dry processing; Moisture content

Introduction

The food processing and preserving may partially or totally affect the quality of a food product. Various changes may occur in physical, chemical or biological characteristics of foodstuff during processing, storage and distribution [1]. The physical and biochemical changes of the foodstuff can reduce the product quality and the efficiency of the process, whereas the quality standard of food is importance to increasing the consumer’s choice [2]. There are some main physical and biochemical changes as below:

Physical Changes
Glass transition

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 [1]. The glass transition temperature of dry solid would be an important optimization parameter can be used as useful tool for the choice of the most appropriate materials to be freezing-dried [3].

Texture

Texture is one of the most important properties connected to product quality. Factors that affect texture include moisture content, composition, variety, pH, product history (maturity) and sample dimensions. The chemical changes associated with textural changes in fruits and vegetables include crystallization of cellulose, degradation of pectin and starch gelatinization. High air temperatures (particularly with fruits, fish and meats) cause complex chemical and physical changes to the surface and the formation of hard impermeable skin. This is termed “case hardening”.

Color

The conservation of color is considered indications of quality in dried fruits given that non-enzymatic browning processes develop during the drying process [4]. Freeze-dried fruits better maintain red and yellow colors than fruits dried using traditional methods [5]. The loss color in freeze-dried product is compared to the color loss in airdried products. The color loss is noted, due to the decomposition of pigment [6]. Rapid freezing can produced a more intense white value.

Biochemical changes

Browning reactions: Browning reactions are important in terms of the alteration of appearance, flavor and nutritive value. Browning is undesirable for fruits, vegetables, frozen and dehydrated foods as it results in off-flavors and colors. The effect of browning is the lowering of the nutritive value of the food article. Rate of browning reactions depends on temperature of drying, pH and moisture content of the product, time of heat treatment and the concentration and nature of the reactants. Browning reactions change color, decrease nutritional value and solubility, create off flavors and induce textural changes. In freeze-dried, browning occurs because an enzymatic reactions.

Lipid oxidation: Lipid oxidation is responsible for rancidity, development of off-flavors and the loss of fat soluble vitamins and pigments in many foods, especially in dehydrated foods. Factors that affect oxidation rate include moisture content, type of substrate (fatty acid), extent of reaction, oxygen content, temperature, presence of metals, presence of natural antioxidants, enzyme activity, ultraviolet light, protein content, free amino acid content, other chemical reactions. The effect of oxygen on lipid oxidation is also closely related to the product porosity. Freeze-dried foods are more susceptible to oxygen because of their high porosity.

One of the oldest methods of food preservation is dehydration. Dehydration is a means of preserving foods in a stable and safe condition as it reduces water activity and extends self-life much longer than the food in fresh condition [7]. There are so many conventional thermal methods, including airflow drying, vacuum drying and freeze-drying, result in low drying rates in the falling rate period of drying [8]. Freeze-drying is not really a new process. Primarily it was developed for the pharmaceutical industry and drugs drying for nearly forty years before applied to the food industry. Freeze-dried foods have some high quality characteristic compared to products of

alternative drying process such as aroma and shape retention, high porosity, good rehydration, superior taste and low bulk density. Freeze-drying is based on the dehydration by sublimation of a frozen product where the water or solvents are removed as a vapor from the frozen material in a vacuum chamber [9]. Very good physical and chemicals properties of food and biotechnological products make this method the best for drying exclusive products.

Freeze-drying is an expensive processing for food industry of dehydrated product. Equipment innovation and pre-treatment of raw material can reduce the time and energy that needed for this process. Nowadays, freeze-drying process is used to produce high quality value products for example coffee, crispy fruits and vegetables, ingredients to ready to eat foods (corn, flakes, cereal bars, ice cream or pastry making) and some aromatic herbs. Therefore, the general purpose of this research was to study the sensory characteristic of the initial and final dried product.

Materials and Methods

Sample preparation

Eight food samples, namely banana, apple, kiwi, baked sweet potato, tofu, pudding, plain yogurt drink and brown rice milk were purchased from the supermarket in Taipei, Taiwan. For all the fruits were washed under tap water and peeled. Banana and kiwi were peeled first and cut into 0.3 cm thick slices, apple were peeled and cut into six slices of every half a loop, sweet potato were peeled and cut into (1x1x1) cm³, tofu were cut into (1.5x3x1) cm³, pudding were cut into three parts, all of samples were using a stainless steel knife before drying, then, plain yogurt drink and brown rice milk were poured into the OPP plastics (± 200 ml).

Sensory evaluation

The sensory evaluation of the food samples was carried out by a taste panel of five untrained judges. The panelists were asked to indicate their preference for each sample, based on the quality attributes of color, shape, texture and taste before and after freeze-dry processing.

Freeze-dry processing

The food samples used freeze dryer (FD24-3S-12P, Taipei, and Taiwan). For the freeze-drying experiment, the food samples were spread out on the flat metal trays and frozen at -24±1°C for 3 days in the freezer. Then, the frozen samples were put in the freeze dryer at -40°C, 60 millitorr for three days until they were completely dried and measurement.

Determination of color

After the freeze-dry processing, the evaluation of color of three pieces each kind of food samples were carried out using a tristimulus colorimeter (Spectrophotometer SP60, X-Rite, Inx. Grandville, MI, USA). The system provides the values of three color components. L* (black-white component, luminosity) and the chromatins coordinates. a* (+ red to-green component) and b* (+yellow to -blue component) [10].

Determination of moisture content

Based on the [11] method, all food samples were weighed by Analytical Balance and determined the moisture content. The food samples were under specified conditions such as freeze-dries processing and the loss of weight is used to calculate the moisture content of the sample. The method to count the moisture content was shown in below:

MC= ((W-d)÷W)× 100%

W= Wet weight, D= Weight after drying

Statistical analysis

Data were expressed as means ± standard deviations. The data were analyzed using Statistical Package for the Social Science Software (SPSS for Windows, version 10.0.7C, SPSS Inc, IL, and USA). Statistical significance among the treatments was determined by a one way ANOVA followed by Duncan’s multiple range tests. Three determinations for each treatment were made and the significance level was set at P<0.05.

Results and Discussion

Raw materials and pretreatment

All of the food that will be freeze-dried must be checked first for the contamination and the purity [12]. Fruits, vegetables and some other edible foods are tested for bacterial counts and spoilage. Some drinks are purchased as a pre-brewed concentrated liquid. Unlike the water, the oil is not removed during the drying process [13]. Almost all fruits and vegetables can be freeze-dried. Liquids, thin portion of meat and small fruits and vegetables can be freeze-dried easily. Fruits can be cut in half or sliced. Thin, uniform, peeled slices dry the fastest. Apples can be cored and sliced in rings, wedges or chips. Bananas can be sliced in coins, chips or sticks. Spray drying is a common method for producing powder from liquid; it is more simple and cheap, because actually freeze drying is a method of preservation of foods and biological materials [14].

Freeze-drying principle

The main principle in freeze drying is a phenomenon called sublimation. Sublimation is the transformation of ice directly into a gas without passing through a liquid phase [15]. Sublimation occurs when the vapor pressure and the temperature of the ice surface are below those of the triple point (4.58 mm Hg, 0°C), as shown in the pressure-temperature phase diagram of pure water [15].

Freeze-drying separation method involves the following four main stages:

Sample preparation
Freezing stage
Primary drying stage and
Secondary drying stage

Annealing

In the majority of lyophilized functional properties and stability of the lyophilization product, annealing is an optional step that used to crystallize the formulation component. If the solute separates out in crystalline form, it is known as the eutectic temperature. In contrast, if an amorphous form is formed, the temperature is referred to as the glass transition temperature, which leads to “melt back” or “collapse” phenomenon [15].

Freezing

In which the liquid sample is cooled until pure crystalline ice forms part of the liquid and the reminder of the sample is freezeconcentrated into a glassy state where the viscosity is too high to allow further crystallization. The objective of the freezing stage is to freeze most of the water originally present in the product for its posterior sublimation, generally in a blast freezer at a temperature of about minus 40°F [15].

Primary drying

This step is carried out at pressures of 10-4 to 10-5 atmospheres and a product temperature of -45 to -20°C. Sublimation during primary drying is the result of coupled heat and mass transfer process. When the water molecules sublime and enter the vapor phase, they also keep with them a significant amount of the latent heat of sublimation (2840 Kj/kg ice) and thus the temperature of the frozen product is again reduced. The time at which there is no more frozen layer is taken to represent the end of the primary drying stage [15].

Secondary drying

The desorption of the remaining water from the solids is called secondary drying while maintaining low pressures. The bound water is removed by heating the product under vacuum. The following product temperatures are usually employed: (a) between 10 and 35°C for heat sensitive products and (b) 50°C or more for less-heatsensitive products [15].

Structural changes

The sought after freeze-drying product are porous that maintain their volume, can have fast and nearly complete rehydration when water is added and do not shrink [14]. 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 [16]. Some phenomenon’s that can occur during the freeze-drying process are:

Collapse = loss of structure, reduced pore size and volumetric contraction [17].
Rehydration = the restoration of raw material properties when dried material is contacted with water, then the volume changes [16].
Shrinkage = Most of the shrinkage occurs in the early drying stages [16].
Porous = Porous sponge-like structures are excellent insulating bodies and generally will slow down the rate of heat transfer into the food [16].

As shown in (Table 1), all of the fruit after freeze drying become to more white, spongy, smooth, firm, the original taste doesn’t change and the shape is maintained. For the bake sweet potato and tofu the color become whiter, the texture more strong, crisp and the shape is maintained, but the taste become sticky and starchy. For the liquid sample, the shape become solid like crystal grains, the texture become smooth, very easy melt in the tongue and has a strong original taste of each sample. Also, interestingly, some fruits are perceived as very sour after freeze-drying and some others not. Thus, wherein the ice formed during the freezing is removed by sublimation under vacuum at low temperatures, leaving a highly porous structure in the remaining amorphous solute that is typically 30% water. The reducing sugar content of the dried fruits in net weight was found higher than the content of fresh fruits due to the moisture loss; we can conclude from these results that the sugar content of dried fruits was a?ected by both pre-treatments and freeze drying conditions [18].

Table 1: Consensus attribute lists before and after freeze-drying.

    
    
    Table 1:  Consensus attribute lists before and after freeze-drying.

As shown in (Table 2), the results of color measurements of fresh and dried samples. The L* color parameter indicates whiteness of the product. L* is a gradation of the skin brightness between the brightest white (high value) and midnight black (low value), with a stretch value of 0-100. Parameter a* shows the gradations of color measurement spectrum of colors between green (-) to red (+), while the parameter b* show gradations of color spectrum between blue (-) to yellow (+). Each has its own stretch of values between -60 to +60. In general, the drying treatment resulted in significantly improved whiteness increased L* value from 52.09 to 92.86, a* value from 0.42 to 11.12 and b* value from 9.02 to 36.54. Therefore, this research results were similar with [19] and [20], in L* value, there were clearly significant difference for apple, kiwi, tofu, sweet potato, yogurt and brown rice milk. During freeze dry processing, at low temperature, freeze drying removes water by sublimation of ice and prevents enzymatic browning reactions. The browning was not promoted due to less oxygen and lower temperature in freeze drying, resulting in a greater increase of L* values. In a* value, there were clearly significant difference for banana, apple, tofu and yogurt. In b* value, there were clearly significant difference for banana, apple, tofu and pudding.

Table 2: The color measurement between before and after freeze-drying.




  
    
    Food    samples 
  
  
    Banana 
    Apple 
    Kiwi 
    Tofu 
    Sweet    Potato 
    Pudding 
    Yogurt 
    Brown    Rice Milk 
  
  
    L* values
    Before
    61.62±9.40^a
    73.37±1.99^b
    62.01±9.58^b
    57.18±1.41^b 
    52.09±6.16^b
    73.84±3.97^a
    74.17±6.09^b 
    58.54±4.40^b
  
  
    After
    75.69±0.85^a
    85.43±1.15^a
    86.60±1.27^a
    88.65±3.30^a 
    70.17±2.70^a 
    80.30±2.86^a 
    92.86±2.16^a
    68.63±2.95^a
  
  
    a* values
    Before
    10.37±0.78^a
    05.15±0.02^b
    -00.42±0.76^a
    02.24±0.18^b
    03.97±1.86^a
    07.65±0.70^a
    11.12±0.80^a
    08.97±3.59^a
  
  
    After
    06.92±0.59^b
    07.37±0.47^a
    00.51±0.38^a
    04.19±0.71^a
    03.95±0.76^a
    06.47±2.01^a
    00.69±0.14^b
    09.13±0.50^a
  
  
    b* values
    Before
    34.10±5.32^a
    24.27±1.15^b
    18.80±4.33^a
    13.46±0.65^b
    26.18±8.10^a 
    36.54±1.53^a
    09.02±1.93^a 
    19.23±6.62^a 
  
  
    After
    20.39±1.28^b 
    31.13±0.80^a
    21.26±1.90^a
    19.19±0.96^a
    27.45±1.74^a
    26.99±2.64^b
    09.89±1.51^a
    18.40±0.82^a
  
  
    The data depicts the mean values of three replications. In    the columns means values chased by dissimilar letters are statistically    different based the on Duncan’s range test at P < 0.05. Results are mean ±    SD (n= 3). L* (black to white component, luminosity), and the chromatin’s    coordinates, a* (+ red to - green component) and b* (+ yellow to-blue    component).



Table 2:  The color measurement between before and after freeze-drying.

Drying removes the moisture from the food so bacteria, yeast and mold cannot grow and spoil the food, so can extend the shelf life. Drying also slows down the action of enzymes, but does not inactivate them. Because of drying removes moisture, the food becomes lighter in weight. Low humidity allows moisture to move quickly from the food to the air. The result in (Table 3) depict the moisture content of the final samples depend on wet basis. Tofu was found to have the highest moisture content (0.905%) whilst sweet potato had the lowest (0.635%). The application of freeze-drying process to foods is most important appears to be for meats such as beef, pork, chicken and fish. The second most interesting commodity group is fruits and vegetables. The functional properties of the freeze-dried milk are not as desirable as those of spray-dried milk [21]. Found that overall flavor and storage characteristics of freeze-dried whole milk are essentially the same as spray-dried whole milk. Both milks have essentially the same flavor characteristics when fresh and tallow in storage. Also, freeze-drying was found to be the preservation method of choice because complete pasteurization would cause denaturation of valuable antibodies and loss of some nutrients. Obviously the cost of production is very high and distribution is quite restricted. Freezedrying yoghurt may help maintain a sufficient quantity of viable probiotics [22]. Previous research has found that certain strains of probiotics are better able to survive the freeze-drying process [22]. The freeze-drying yoghurt also preserves the yoghurt in a highquality shelf-stable powder form [23]. It would be beneficial if the yoghurt were concentrated before drying to increase its total solids, which improves the efficiency of the drying process [23]. There are no previous studies have conducted on the preservation of tofu, pudding and baked sweet potato by freeze-drying.

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Citation: Valentina V, Pratiwi AR, Hsiao PY, Tseng HT, Hsieh JF and Chen CC. Sensorial Characterization of Foods Before and After Freeze-drying. Austin Food Sci. 2016; 1(6): 1027.

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Food samples
Banana	Apple	Kiwi	Tofu		Sweet Potato		Pudding	Yogurt	Brown Rice Milk
Moisture Content (%)	0.756±0.006	0.835±0.001	0.853±0.006	0.905±0.004		0.635±0.071	0.754±0.036	0.851±0.003	0.855±0.002
The data depicts the mean values of three replications. Results are mean ± SD (n=3).