Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.
2008
Volume 11
Issue 1
Topic:
Agricultural Engineering
ELECTRONIC
JOURNAL OF
POLISH
AGRICULTURAL
UNIVERSITIES
Góral D. 2008. THE EFFECT OF DIFFERENT TECHNIQUES OF FREEZING AND DEFROSTING ON THE QUALITY OF SPICE VEGETABLES AFTER A LONG-TERM FREEZER STORAGE, EJPAU 11(1), #01.
Available Online: http://www.ejpau.media.pl/volume11/issue1/art-01.html

THE EFFECT OF DIFFERENT TECHNIQUES OF FREEZING AND DEFROSTING ON THE QUALITY OF SPICE VEGETABLES AFTER A LONG-TERM FREEZER STORAGE

Dariusz Góral
Department of Refrigeration and Food Industry Energetics, University of Life Sciences in Lublin, Poland

 

ABSTRACT

The purpose of the paper was to study the effect of the conditions of freezing and defrosting of spice vegetables on selected determinants of their quality considering the freezer storage. The studies were conducted on carrot, parsley and chive. The studied raw materials were frozen (sample weight 0.5 kg): using the method of impingement (air temperature -20°C), in a freezer (temperature -30°C) and in an ultrakryostat (temperature -70°C). Freezing in a freezer and an ultrakryostat was continued until the temperature of -20°C was reached in the thermal centre of samples. After freezing, the raw materials were stored at the temperature of -20°C for the period of three months. Defrosting was continued until the temperature of 10°C was reached in the thermal centre of the sample: in the conditions of free convention in the air, by the method of revered fluidization, and in a water bath, keeping constant temperature of the environment of 15°C. The analysis was made of the kinetics of the processes, losses of the weight of samples, and an organoleptic evaluation was made of the samples after freezer storage and defrosting. During the storage, on average every 18 days the weight and the general state of the samples were examined comparing them with the samples of fresh raw materials. The lowest weight losses of carrot in freezing (2.35%) were caused by the air chilling method, while in case of parsley it was reversed fluidization. In case of chive no change of the weight after freezing and after defrosting was observed. Carrot weight losses after defrosting were the lowest (1.59%) among the samples frozen and defrosted by impingement method. The lowest losses of the weight of parsley samples (0.04%) were caused by freezing with air chilling method and defrosting in a water bath. Vegetables frozen by the method of reversed fluidization, stored and defrosted in the air in the conditions of free convention received the highest scores in the organoleptic test and hence their quality was only slightly changed in comparison with that of the fresh state. The condition of obtaining such quality of the examined vegetables is the period of the frozen storage which should not exceed 2.5 months at the temperature of -20°C.

Key words: frozen vegetables, spices, methods of freezing and defrosting, quality.

INTRODUCTION

With the aim of keeping the aromatic and nutritious valours, spice plants are subject to conservation. The choice of the manner depends on the anticipated time of storage. Sprinkling with water, placing in foil bags or wrapping in aluminium foil are used for the spices meant for a short-term storage. The spices prepared so are most frequently placed in a refrigerator in order to check negative processes. In case of spice plants the choice of the proper method of preserving them is very significant. Badly performed treatment can disqualify the product intended for a consumer [1,2,3,15].

Freezing plant products gives a possibility of long-term storage keeping their quality on the level close to that of the fresh state. In order that the final quality will be the best possible the optimally fast performance of this process is necessary which gives as a result a fine-crystalline ice structure in the product. Such performance of the process also minimizes the unfavourable effects of microbiological and biochemical factors with simultaneous limitation of the quantity losses, favouring the lowering of the costs of the process. Each plant raw material has the physical properties typical of its group and hence for each of them the proper technology of freezing and defrosting is chosen [15,17,21,23].

The purpose of applying differentiated methods of freezing and defrosting was to show which of the schemes of freezing – defrosting caused smaller changes of the physical and organoleptic properties of the examined raw materials after their long-term freezer storage.

MATERIAL AND METHODS

The studies referred to carrot (Daucus carota L.), cv. Ambrozja, common parsley (Petroselinum sativum Hoffm.), cv. Bardowicka, chive (Allium schoenoprasum L.) of the common cultivar of onion. The studies were conducted on samples of the weight of 0.5 kg [13]. All vegetables were characterized by high quality and firmness, and they were deprived of any discoloration or disease symptoms.

Raw materials of similar shape, size, colour and intensity of smell were chosen for the studies. The initial treatment of carrot and parsley consisted of removing the outer cover by means of a knife, and washing under the water current. Next the raw materials were dried by means of a tissue paper and they were chopped: carrot into cubes with the dimensions of 11×11×11 mm, while parsley into cubes with the dimensions of about 10×10×10 mm.

The initial treatment of chive consisted of washing it under a current of lukewarm water, drying in a tissue paper and next cutting into pieces about 10 mm in length.

Next the raw materials were weighed (scales WPE 2000, accuracy ± 0.05 g) and the characteristic dimensions were marked (slide caliper, accuracy ± 0.05 mm). The samples were also weighed after each stage of treatment. The freezing processes were carried out: in a special laboratory equipment used for freezing by means of impingement, in a freezer (Whirlpool type AFG 543-C/H), in a kryostat (Ultra-kriostat, N180, LN2). The freezing of vegetables by means of the method of reversed fluidization was conducted at the temperature of -20°C, in a freezer at -30°C, while in a kryostst at -70°C. Freezing was continued until the temperature of -20°C was reached in the thermal centre of the sample [5]. Recording the temperature of the product during the freezing and defrosting processes was made by means of a measuring kit (type TIK) equipped in thermo-steam NiCrNi (accuracy of measurement ±0.1 K) working with a computer. Thermo-steams were placed in the thermal centre of samples. Before the air-chilling the samples were placed in tightly closed foil bags. The frequency of reading the temperature was 1 per second. Full processes of freezing the samples were conducted in two repetitions by each method. The temperatures of the samples recorded during the realized processes were converted onto the Microsoft Office Excel. The mean speed of freezing as well as the initial and final kryoscopic temperature of the samples were determined on the basis of the marked curves. The samples were weighed after freezing to a given value of temperature. Defrosting of carrot and parsley took place in a water bath, in the air in the conditions of free convention and by means of impingement. On the other hand, chive was defrosted only in the air in the conditions of free convention, because of the worse quality resulting from the use of water bath for defrosting. During the defrosting the temperature of the environment was maintained on the level of 15°C, and the process proceeded until 10°C was reached in the thermal centre of the sample. Analogously to freezing, the mean speed of defrosting was established on the basis of the temperature characteristics. The analysis of the sample weight was made considering the defrosting leak as a consequence of the effect of the speed of defrosting and the conditions of the freezer maintenance. The leak was established from the difference of the weights of samples before and after defrosting.

Samples constituting 10% of all the sample weight were isolated from the frozen samples. All samples and tests were placed on a net shelf in the central part of the freezer (Danfrost model GM 20) [8]. Samples were isolated for the analysis of the examined determinants with the aim of making the process of water steam condensation impossible in the whole test during the studies. The frozen material was kept at the temperature of -20°C for about three months. During the storage, on average every 18 days the weight was measured and the general state of the raw materials was estimated comparing them with the properties of fresh raw material. The weight loss was calculated for the separated samples and next it was converted in reference to the whole test. The defrosting leak was marked as:

where:
W – percentage value of defrosting leak,
mp – sample weight before freezing, g,
mr – sample weight after defrosting, g.

The weight loss as a result of freezer storage was estimated using the following formula:

where:
U – value of the weight loss,
mp – sample weight before freezing, g,
mpz – sample weight after storage before defrosting, g.

The rate of freezing and defrosting was marked from the following relation:

mm min

where:
v – rate of freezing or defrosting, respectively,
g – sample thickness, mm,
t – time of the process, min.

Results of measurements of the examined samples before placing them in thermosteams were used for calculations. During each singular test, the process of three samples of the raw material was realized. The statistical evaluation of the results of measurements and calculations was performed additionally using Excel 2003. After defrosting the raw materials were evaluated organoleptically using the test in a 5-score preference scale. The objects of the evaluation were taste, colour, flavour, juiciness and texture of the samples after the treatment, compared to the samples of fresh, unprocessed raw materials. The value of 5 was the highest score and it characterized the raw material with the properties close to those of the fresh state. On the other hand, 1 was given to the samples whose quality determinants were fairly considerably changed in relation to the fresh samples. The total quality of the samples was estimated as the result of the sum of points from the evaluation of particular quality determinants. The samples were classified into definite quality groups on this basis [14]. On the basis of the results of the organoleptic evaluation of the examined vegetables the value of Rang Spearman’s correlation coefficient was calculated.

RESULTS AND DISCUSSION

Analyzing the obtained curves it should be stated that the initial values of kryoscopic temperatures TCP are differentiated and, as follows from the measurements, they are dependent in a certain range on the method and conditions of sample freezing. In case of the final values of kryoscopic temperatures TCK, their probable relation to the temperature of the chilling environment was observed (Table 1). The values of kryoscopic temperatures decreased with its decrease [10, 19, 23].

Table 1. Kryoscopic range of carrot and parsley samples

Examined material

Method/Temperature of the chilling environment

Air-chilling -30°C

Impingement -20°C

Kryostat -70°C

Kryostatic temperature

TCP, °C
Initial

TCK, °C
Final

TCP, °C
Initial

TCK, °C
Final

TCP, °C
Initial

TCK, °C
Final

Carrot

-2.3

-8.1

-3.8

-6.2

-2.3

-10.9

Parsley

-4.2

-9.1

-3.5

-6.1

-5.8

-15.0

Differentiated freezing methods resulted in different rate of sample freezing (Fig. 1). The studies found out that in the case of carrot and parsley the highest rate of freezing was obtained by means of impingement. Freezing in this way shortens the freezing time three time in relation to the time of air chilling in comparable other conditions. The most significant parameter in this case is the coefficient of heat penetration, which reaches the mean value of about 190 W·m-2·K-1. For a comparison, the value of this coefficient in cooling furniture is below 10 W·m-2·K-1 [3,6,7,8,19].

Fig. 1. Freezing rate of carrot and parsley in the studied conditions

The mean weight losses of samples frozen by means of different methods (Fig. 2) are on a similar level. In case of chive no change of weight was observed resulting from its freezing. The weight losses of parsley depending on the freezing method are insignificantly differentiated between each other. In case of carrot, air chilling caused the smallest weight losses of the samples. Besides the conditions relating to the process (kind of environment), this can be affected by the structure and functions of the cells of the examined raw materials. Plants with fibrous tissues and with thick-walled cells expose only slight changes after defrosting. The situation is similar with young cells, which are much more resistant to changes than older ones. All changes of weight after defrosting the plant products are connected with the semipermeable properties of the cell membrane. The cells, which are filled with hypertonic liquid, undergo the process of water loss as a result of the formation of ice crystals in the intercell spaces (kryoosmosis, kryoconcentration). This phenomenon is largely dependent on the freezing rate. With slow freezing, the phenomenon of dehydration lasts until the free water is completely lost. The product frozen in this way has ice crystals formed outside the cells, whereas in the flabby cells the unfrozen solution remains. There is far-reaching unanimity in claiming that the slower the freezing is, the stronger the destructive effect of this process on the tissue structure of the product [3,6,7,8,15,20].

Fig. 2. Weight loss of carrot and parsley depending on the freezing method

Mean rates of defrosting the studied samples were established on the basis of the recorded curves of defrosting (Fig. 3). The slowest rate of defrosting of the examined materials was obtained in the air in the conditions of free convention regardless of the method used. In the other cases, the obtained rates were comparable but in case of parsley the highest rate was characteristic of defrosting in a water bath in the conditions of free convention after freezing in a freezer. The highest defrosting rate of carrot was obtained using the method of impingement after freezing it in the same environment. So far it has been difficult to explicitly characterize the relation between the defrosting rate and the conditions outside the process [1,2,3,12].

Fig. 3. Mean rates of defrosting carrot and parsley in the studied conditions

Analyzing the carrot weight after defrosting in the studied conditions it is possible to point out that the manner of defrosting has an effect on its changes (Fig. 4). It was found out that defrosting by means of impingement causes a limited defrosting leak on the level close to that which is connected with defrosting in water. An increase of the weight in case of water defrosting of a product frozen by means of air chilling could have been caused by intensified exchange of weight, which resulted in the worse final quality of the product. It was also observed that an increase of the weight of parsley took place only in case on the raw material defrosted in water after freezing by means of impingement method. On the other hand, the lowest weight losses of the raw material frozen by means of air chilling were noted after defrosting in the water environment. In the case of freezing by means of reversed fluidization the lowest losses were found after defrosting parsley samples in the air. It was found out that the way of freezing affected the size of the defrosting leak while defrosting in the air in the conditions of free convention. The situation is similar to defrosting in water. Big weight losses accompanying defrosting by means of impingement could have been additionally caused by the appearance of water loss of the raw material. Generally, it can be assumed that the main effect on the quantity of the leak is exerted by the rate of freezing. A smaller leak was obtained if freezing lasted from 10 to 12 minutes [9,11,12,20,21,22].

Fig. 4. Defrosting leak of the studied materials

Considering the results of the organoleptic test (Fig. 5), it should be stated that in case of carrot the smallest effect on the change of its quality was exerted by air chilling and defrosting in the air. Freezing by means of impingement and defrosting in a water bath had a similar effect on the quality of carrot. The raw material was classified into the group of good final quality on this basis. In the other cases of the chilling treatment each time a similar result was obtained and on this basis it was established that the raw material was characterized by the quality on the admissible level. The worst evaluation was obtained by the raw material frozen by air chilling and defrosting in a water bath. A good result was achieved using impingement for defrosting, thanks to which the treatment time was made shorter and the quality of the product was not lost. In all the examined samples, the flavour, taste and texture were comparable to those of the fresh product and it was only in case of freezing in a freezer and defrosting in water that considerable changes of these properties took place. The flavour and smell were little felt and the texture was characterized by smaller fragility than in fresh samples.

Fig. 5. Results of the sensoric evaluation of the studied raw materials after defrosting

In case of parsley, its evaluation after defrosting in the studied conditions was related to the method of freezing. It was found out on the basis of the results of the organoleptic test that a good quality product came from defrosting in the air independently of the freezing method, and from impingement defrosting after freezing by means of the same method. In the other cases, the product had the quality similar to that of the admissible level. However, freezing parsley by means of reversed fluidization and defrosting it in the air made it possible to obtain the material with a slightly better evaluation than in case of the air chilling method. The use of the impingement method for defrosting caused that the examined material was characterized by higher quality than the material from the same freezing samples but defrosted in a water bath in the conditions of free convention. The quality determinants evaluated in parsley were identical to those of carrot, i.e. defrosting in a water bath caused changes of its physical properties, thus considerable worsening the final evaluation. It can be stated on the basis of the obtained results that the choice of the proper chilling treatment has a direct effect on the final quality of parsley.

In case of chive defrosted in the air, a differentiated evaluation was obtained from the sensoric test, depending on the freezing method. The final product possessed the properties that remarkably distinguished it from the fresh material. However, analyzing the observed changes and on the basis of the test it was found out that freezing chive by means of impingement affects its worse quality after defrosting in the air in a lesser degree than the use of air chilling. The obtained data made it possible to classify the studied material in both cases of the experiment to the group of admissible quality. The cause of the changes of the examined determinants could have been atmospheric oxygen, which together with oxidization enzymes could have caused considerable changes, disqualifying the studied raw materials or contributing to classifying them in the lower quality class. A big role in the final evaluation was also played by defrosting leak and the nutritious elements dissolved in it, which remarkably lowered the nutritious values of the materials subjected to defrosting [12,15,16,17,18].

The correlation coefficient of Rang Spearman calculated for the results obtained from the organoleptic analysis (rs= 0.72) points to a smaller linear relation between the examined methods of treatment and the final quality of the examined raw materials.

Results of the measurement of weight conducted in the course of 3-months’ freezer storage made it possible to determine the weight losses of the examined materials (Fig. 6). In case of carrot and chive samples it was found out that freezing by means of reversed fluidization caused smaller weight losses of the samples during the freezer storage than in case of air chilling. In case of parsley those differences were only slight. The divergences were probably caused by the dependence on the rate of freezing, whose later sign were the effects of re-crystallization in the form of the exudation of considerable quantities of ice crystals on the surface of the material [4].

Fig. 6. Weight losses of the studied raw materials during their freezer storage

Considering the water losses of products in freezer storage (Fig. 7), it was found out that it was only in case of chive that its higher value characterized the samples frozen by means of impingement. In the other cases, higher values result from air chilling. These differences are significant and they point out that the choice of the proper freezing method has a decisive influence on the quality of the ready product [1,2,4].

Fig. 7. Water loss during frezer storage

The value of standard deviation calculated for the water loss did not exceed ±0.002 and hence it was not placed in Fig. 7. The effect on the water loss during the storage of products can follow from the hypothesis concerning the thermodynamic theory of heat-humidity processes, according to which the heat inflow to the room and its temperature are the cause of water loss [24]. Another significant factor determining the water loss can be the manner of placing the product in a heap, i.e. in the horizontal cross-section the highest losses take place in the products placed from the outside of the chamber wall, while the lowest in the central layer. In the vertical cross-section the mean and smallest losses are shown by products placed in the lower layer of the heap, while the highest – on the upper surface. The size of the water loss can also be explained by the specific properties of the stored raw materials and the packages used [4,12]. On the basis of the obtained results of the organoleptic evaluation of the examined raw materials they were classified into definite quality classes (Fig. 8). The kind of the freezing treatment of carrot significantly affects its final quality after freezer storage. It can be stated on the basis of the analysis of the samples and the data concerning their weight changes that air chilling causes higher water losses than impingement. Air chilling of carrot during the storage probably caused its darkening. Although the carotenoid pigments are more durable and their changes take place more slowly than in the others, after three months of freezer storage visible changes took place in the colour of carrot. All changes that appeared in the material frozen by air chilling caused that its final evaluation was lower than in case of the samples frozen by mean of impingement.

Fig. 8. Results of the sensoric evaluation of the studied raw materials 3 months after their freezer storage

Analyzing the data obtained from the organoleptic test and the changes of chive weight it was found out that the treatment method had a significant influence on its final quality. The final quality of chive frozen by means of air chilling was much lower than in case of the use of impingement. Colour evaluation had a considerable effect on the final evaluation of the samples. Chlorophylls are not durable pigments and they easily undergo different changes, as the result of which chive quality after chilling treatment was considerably different from its quality in the fresh state.

Results of the final evaluation of parsley point out that freezing by means of impingement caused a worse quality of the raw material to a lesser degree than the process in a freezer. Air chilling caused that during the storage following it greater quantities of ice appeared on the outside of the product than in case of the samples frozen by means of reversed fluidization.

The studied raw materials frozen by means of impingement achieved better scores in the organoleptic test than the samples frozen in a freezer. This must have been influenced by the freezing rate first of all.

The main factor that could have had an effect on the final quality of the studied raw materials were the following:

  • considerable weight losses during the whole chilling treatment as the cause of the loss of the properties characterizing the fresh product and the appearance of new atypical changes, i.e. becoming mat on the surface, the appearance of irregular spots, shades, etc. The advancement of these changes depends on the kind of raw materials and the size of the weight loss. The changes are visible already after exceeding 1-1.5% of the initial weight loss;

  • worse flavour of the studied vegetables which resembles the flavour of musty hay, which could have happened as a result of changes of WKT and their peroxides formed from the decomposition of lipids [12].

The value of Rang Spearman’s correlation coefficient (rs = 0.93) calculated on the basis of the results of the organoleptic test carried out after long-term freezer storage points to a big relation between the applied treatment method and the obtained score from the evaluation of the examined determinants of quality.

CONCLUSIONS

  1. The experiments conducted on the studied vegetables pointed out that they can be preserved through freezing and their level of quality is similar to that of the fresh state.

  2. Big freezing rates obtained as a result of the methods of impingement and nitric kryostat caused that the structure of so treated raw materials underwent smaller damage than in case of freezer freezing. It was observed that in case of parsley there is a relation between the weight losses and the freezing rate. The smallest weight losses were observed after freezing by means of impingement, while the highest after freezing in a freezer.

  3. The rate of defrosting the samples frozen by means of impingement was lower than in case of the samples frozen by air chilling. In case of defrosting in a water bath, the weight of the raw materials increases, and hence the process of washing away the valuable substances could have taken place. On the other hand, defrosting by means of impingement makes it possible to achieve the rate of the process similar to the defrosting rate in a water bath with analogous temperatures of the environment, and this method does not have the faults of water treatment.

  4. According to the organoleptic evaluation carried out after defrosting, the highest quality was characteristic of the samples frozen by means of impingement and defrosted in the air.

  5. Water losses during the freezer storage of the studied raw materials occurred in a smaller percentage in the samples frozen by means of impingement than in the samples frozen by air chilling. Chive was an exception since smaller water losses occurred in the samples frozen by means of air chilling. A similar result was obtained in reference to the defrosting leak.

  6. The evaluation of the studied raw materials conducted 3 months after freezer storage showed that all samples defrosted in the air and in the water that were frozen by means of impingement had a higher sensoric value than the samples frozen in a freezer and defrosted in the same environments.

  7. In the majority of the studied raw materials browning started to proceed 3 months after the freezer storage. The recommended optimum period of freezer storage of these raw materials should be 2.5 months at the temperature of –20°C.


ACKNOWLEDGEMENT

The research was sponsored in the years 2004-2007 by the Research Committee as a research project.

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Accepted for print: 02.10.2007


Dariusz Góral
Department of Refrigeration and Food Industry Energetics, University of Life Sciences in Lublin, Poland
44 Do¶wiadczalna
20-280 Lublin
Poland
Phone +48 81 4610061
email: dariusz.goral@up.lublin.pl

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