A COMPARISON OF PROCESSABILITY OF SELECTED VARIETIES OF CELERIAC FOR THE PRODUCTION OF MINIMALLY PROCESSED SHREDDED CELERIAC

Elżbieta Radziejewska-Kubzdela, Janusz Czapski

ABSTRACT

The comparison of some properties of 4 celeriac varieties and evaluation of quality of minimally processed product obtained from them were studied in this work. Among the investigated varieties the flesh of cv. Mentor exhibited the slowest browning rate. The value of color brightness L* for the flesh of that variety was the highest. Air-packed celeriac flakes obtained from cultivars Mentor, Luna F1 and Makar after 6 days of storage at the temperature of 4°C exhibited typical sensory attributes. After 12 days of storage of minimally processed products a deterioration of color, smell and taste was observed. The color of flakes obtained from root celery variety Mentor during storage was evaluated considerably higher than that of flakes obtained from the other varieties.

Key words: root celery, enzymatic browning, minimal processing..

INTRODUCTION

In many countries numerous studies concerning the minimal processing of fruits and vegetables are conducted. Products of this type have the attributes of convenience and fresh-like quality. An example of a minimally processed product is shredded celeriac. Celery root is a rich source of mineral salts and dietary fiber. It also contains microelements, small amounts of vitamins and phenolic compounds. The main part of soluble solids content is mannitol. The consumption of root celery is recommended because of its dietary and therapeutic properties. Shredded celeriac may be a product used both in home consumption and the catering services.

Minimally processed vegetables and fruits are obtained as a result of the cleaning of the raw material, the removal of inedible parts and subsequent slicing or shredding. The product is most frequently packed on trays wrapped in foil or in plastic bags.

As a result of raw material damage during preliminary processing and increased surface due to comminution, physiological, enzymatic and microbiological changes may occur in the tissue of minimally processed vegetables and fruits.

The level of physiological and biochemical activity of the tissue is dependent on the species, variety, the degree of ripeness, the structure of the plant, as well as storage duration and regime of the raw material after harvest [20]. Variety traits may also determine the degree of microbiological contamination of the raw material, e.g. through different contents of toxic compounds, i.e. polyphenols [15].

One of the main problems in minimal processing of celery roots is enzymatic browning of the flesh. The mechanism of the browning reaction is connected with the activity of enzymes from the group of polyphenol oxidases (PPO, E.C. 1.14.18.1), catalyzing phenolic oxidation with oxygen. In this reaction, quinones are formed. Then they polymerized to colorful high molecular weight compounds. The darkening of the shredded raw material is observed as a result of this reaction. The susceptibility of fruits and vegetables to the process of enzymatic browning is dependent e.g. on the variety of the plant [18], polyphenol oxidase activity [22, 23], the enzyme site [11], the content and types of substrates [10, 13, 14], and the degree of ripeness and storage time of the product [6]. The disadvantageous change in the product color lowers its commercial value and shortens its shelf-life. For this reason varieties with limited susceptibility to browning are recommended for processing [19].

Genetic traits of the raw material also affect its shape and texture. In case of celery, a round or oval root with scarce lateral roots concentrated in the lower part of the protuberance facilitates cleaning, peeling and cutting. Moreover, the color and surface of the celery root skin are also of importance. Varieties with a light skin and smooth surface (with no bulges or cracks and with delicate marks of fallen leaves) are found preferable. The flesh of raw material destined for minimal processing material should be compact and firm, with not tendency to form empty spaces.

Thus, a proper selection of the raw material plays an essential role in determining the quality and shelf life of minimally processed vegetables and fruits. For this reason it seems advisable to determine the processability of selected root celery varieties for the purpose of the production of shredded celeriac.

MATERIALS AND METHODS

In the conducted experiments the experimental material consisted of four varieties of root celery: Mentor NL, Makar, Luna F1 NL and Feniks PL. The raw material was tested after harvest and after 6 months of storage.

The suitability of these varieties as a raw material for the production of shredded celeriac was determined on the basis of:

• morphological evaluation,

• susceptibility to enzymatic browning, measured on the basis of flesh color changes 2, 60 and 420 min after homogenization. The measurement of color parameters in the CIE L* a* b* system was conducted using a HITACHI U-300 spectrophotometer at the light source C using an integrating sphere for measurements in reflected light. The measuring device was graduated on the basis of the white standards,

• total polyphenol content with the Folin-Ciocalteu reagent [23] converted into chlorogenic acid. Polyphenol extraction from celery was conducted according to Mapson [12],

• polyphenol oxidase activity using the acetone powders [14]. The measurement was taken at the substrate concentration (chlorogenic acid) of 0.01 M using the spectrophotometric method at the wave length of 325 nm. The activity unit was defined as a change in absorbance by 0.001/min ˇ cm³.

In the investigated varieties the dry matter content was also determined using the gravimetric method [16], the total soluble solids content was determined refractometrically and the total sugars - according to Lane-Eynon [17].

The quality of the minimally processed product obtained from the above mentioned celery varieties was determined in the next stage of the investigations. The raw material was washed, hand-peeled and washed again. Next it was shredded in the blender into flakes and air-packed in portions of 50 g each into bags made from laminate (oriented polyamide/polyethylene). Samples were stored at the temperature of 4°C for the period of 12 days. The product was evaluated after 1, 6 and 12 days of storage on the basis of color parameter measurements in the CIE L* a* b* system, sensory examination in a 5-point hedonic scale, pH and total acids. Color parameters were measured in homogenized flakes.

RESULTS AND DISCUSSION

A comparison of selected properties of root celery varieties

On the basis of morphological evaluation it was found that roots of varieties Mentor, Makar and Feniks were oval-shaped, whereas variety Luna F1 had conoidal roots. Most of the investigated varieties had a medium degree of rooting, with Makar constituting an exception here, with its numerous lateral roots growing from the bottom part of the protuberance. A very good quality flesh was observed in varieties Luna F1 and Mentor. The tendency to form empty spaces was found in case of roots of varieties Makar and Feniks. The flesh of varieties Makar, Luna F1 and Mentor was white-creamy, whereas in Feniks it was creamy-yellow with numerous discolorations.

Among the 4 root celery cultivars, examined after harvest and after 6 months of storage, Mentor turned out to be the variety most resistant to darkening. The flesh of this variety exhibited the highest brightness value L*, measured 2 min after homogenization. After a 60-min storage of the flesh this value was higher than those of the other varieties. Feniks was a fast darkening variety with a relatively low value of parameter L* already 2 minutes after the homogenization of the flesh (Fig. 1 a, b).

After 6 months of raw material storage decrease was found in the flesh brightness L* in varieties Mentor and Feniks. Subsequently the flesh of cv. Mentor was lighter in color than the flesh of the other varieties. In varieties Makar and Luna the color brightness L* during raw material storage did not change (Fig. 1 a, b).

The measurement of changes in brightness L* of homogenized celery flesh (irrespective of the variety) showed that it gets darker to the highest degree within the first 60 min. Later the process occurs very slowly and does not have a significant effect on color (Fig. 1 a, b).

In varieties Mentor, Makar and Luna F1 low values of parameter a* were observed, both after harvest and after 6 months of storage. High values of parameter a* were found for cv. Feniks (Fig. 2a, b).

Values of parameter b* in the investigated celery varieties from the after harvest batches were similar. After a 6-month storage a lower yellow color content was observed in the flesh of varieties Mentor and Makar (Fig. 2 a, b).

Individual varieties of root celery exhibited varied polyphenol contents (Table 1). The biggest amounts of these compounds were found in cv. Feniks. During the storage of celery roots the content of polyphenols decreased. In case of most varieties the decrease was several percent, except for cv. Feniks, in which it was 33% (Table 1).

Polyphenol oxidase activity was dependent on the root celery variety. The highest polyphenol oxidase activity was found in cv. Feniks, whereas the lowest – in varieties Makar and Mentor. Polyphenol oxidase activity decreased in the raw material during storage in case of varieties Mentor, Luna F1 and Feniks, whereas it increased in cv. Makar (Table 1).

The correlation between the flesh browning rate and the polyphenol content and polyphenol oxidase activity was also analyzed in this study. Due to the browning of celery flesh, visible within the first 2 minutes after celery flesh homogenization, the value of color brightness L*, measured 60 minutes after homogenization, was assumed as an indicator of the browning rate, instead of the value defining the difference in flesh color 2 and 60 min after homogenization. A significant effect was found of the phenolic compound contents on the value of the L* flesh brightness parameter only for the raw material after 6 months of storage and a lack of correlation between brightness L* and the polyphenol oxidase (PPO) activity, both in case of celery after harvest and after storage (Table 2).

There are no literature data concerning the effect of a celery variety and the contents of polyphenols and the polyphenol oxidase activity on flesh browning in root celery. Studies determining the dependence between substrate contents, PPO activity and the rate of enzymatic browning have been conducted e.g. on apples, pineapples, peaches, pears and sweet potatoes [1, 4, 7, 9, 10]. However, on the basis of results obtained by various authors it is difficult to define unambiguously a correlation between the above mentioned factors and the process of enzymatic browning.

In the investigated varieties varying dry matter and soluble solids contents were found, whereas the total carbohydrate content was similar. After 6 months of storage the contents of dry matter and soluble solids decreased, whereas the total sugars increased. The highest drop in the above mentioned indexes were noted for variety Mentor (Table 3).

The dry matter content determined in the raw material is consistent with literature data [2, 8], whereas the total sugars was lower than the figures reported by Polish researchers (4.0-8.0 g/100 g raw material) [2, 3], but it was consistent with the data reported by Crosby and Anderson [5] and Hermann [8] (0.9 – 3.3 g/100g). The high difference between the dry matter and soluble solids contents in root celery, and the determined total sugars is probably caused by the content of mannitol, which is a storage substance and constitutes a major part of the soluble solids content [8]. The observed increase in the total sugars after 6 months of storage may be the effect of the mannitol dehydrogenase (MTD) activity, which at low sugar contents may increase, oxidizing mannitol to mannose [21].

The quality of minimally processed products obtained from various root celery varieties

The highest value of color brightness L* and the lowest of parameter a* were found for shredded celeriac obtained from cv. Mentor (Fig. 3a, Table 4). After a 12-day storage these shredded celeriac received in the sensory color examination the 3.5 note and were defined as white-creamy with scarce brown discolorations (Fig. 3b). The value of brightness L* in shredded celeriac of variety Feniks was considerably lower than those for the flakes of the other cultivars.

In the 12-day storage decrease in the brightness L* value was observed in shredded celeriac produced from varieties Mentor and Makar (Fig. 3a). A statistically significant correlation (R²=0.92) was found between the brightness L* value and the sensory examination results for shredded celeriac color.

The highest value of color parameter a* was found for shredded celeriac produced from variety Feniks. An increase in the value of parameter a* during storage was observed in shredded celeriac obtained from varieties Luna and Makar, whereas the value of parameter b* remained similar in all the examined samples (Table 4).

The taste and smell of shredded celeriac obtained from varieties Mentor, Luna and Makar after 6 days of storage were evaluated as typical. In case of shredded celeriac produced from cv. Feniks already after 1 day of storage the above mentioned notes of sensory examination were considered to be medium typical. The consistency of shredded celeriac throughout the storage period was considered desirable. After 12 days of storage of minimally processed celeriac the notes of overall quality were lowered, due to a definite deterioration in the color, smell and taste in all the examined samples (Table 5).

During storage of shredded celeriac an increase in the pH value was observed (from 5.7-5.9 to 5.9-6.2) and a decrease in total acids (from 0.30-0.40 to 0.21-0.27 g citric acid/100 g product) in all the investigated samples. The increase in the pH value may result from the development of microorganisms in the product.

CONCLUSIONS

Among the 4 varieties of root celery the highest brightness L* of flesh measured 2, 60 and 240 min after homogenization, both for the raw material after harvest and after a 6-month storage, was found for root celery variety Mentor. The lowest value of color brightness (L*) in celery flesh was observed in cv. Feniks.

Individual varieties of root celery showed varied polyphenol contents. The biggest amounts of these compounds were found in cv. Feniks. During the 6-month storage of celery roots the contents of phenolic compounds decreased. Polyphenol contents had a significant effect on the value of parameter L* only in the raw material subjected to storage. However, no effect of an increase in the PPO activity was found on the values of color brightness L* both in celery after harvest and after 6 months of storage.

Shredded celeriac produced from celery variety Mentor exhibited the highest brightness values L* and notes in color sensory examination after 1, 6 and 12 days of storage in comparison to samples obtained from the other investigated varieties. The taste and aroma of shredded celeriac produced from varieties Mentor, Luna F1 and Makar after 6 days of storage were typical. After 12 days of storage, irrespective of the cultivar, the above mentioned indexes in the sensory examination of shredded celeriac considerably deteriorated.

On the basis of the conducted investigations it was found that among the 4 examined root celery varieties cv. Mentor is most suitable for minimal processing, as it showed advantageous properties of morphological structure, a white-creamy color of the flesh and low susceptibility to enzymatic browning. Shredded celeriac produced from this celery variety received also higher notes in the overall sensory examination during the 12 days of storage than those obtained from the other investigated celery cultivars.

Due to the considerable deterioration of the notes in overall sensory examination of shredded celeriac after 12 days of storage and an increase in the pH values, further studies are necessary on the extension of shelf-life of root celery flakes obtained from cv. Mentor.

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This work was carried out within the project 3 P06T 00824 supported by State Committee for Scientific Research.

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