QUALITY ASSESSMENT OF ITALIAN BROCCOLI PROCESSED IN CONVECTION-STEAM OVEN

Beata Ślaska-Grzywna
Department of Food Engineering and Machinery, Agricultural University of Lublin, Poland

ABSTRACT

Paper presents the results of Italian broccoli thermal processing in a convection-steam oven. Technological treatment was carried out under variable process parameters: at the temperature of 80 and 100°C, time 5, 10, 15 min and the quantity of supplied steam: 0%, 20%, 40%, 60%, 80%, 100%. The values of parameters were determined in preliminary tests. After thermal processing the shearing forces for broccoli samples were measured and their dependence on processing temperature, time and quantity of supplied steam, was determined. The results of shearing force tests were statistically analysed using the Statistica 6.0 software, they were verified by the Tukey’s test. Significant effects of treatment time and quantity of supplied steam on the changes of shearing forces were found for thermally processed vegetable samples. Next, the samples of Italian broccoli were subjected to the sensory evaluation according to five-score scale with following quality factors: colour, consistence, flavour, taste, succulence and a hedonistic scale. The optimum parameters for thermal processing of Italian broccoli in a convection-steam oven were determined.

Key words: Italian broccoli, thermal processing, shearing force, five-score scale, hedonistic scale.

INTRODUCTION

Broccoli is one of the most valuable vegetables. Apart from high contents of vitamins (C, A, B1, B2, B3, PP), nicotinic, pantothenic and folic acids, it contains more mineral compounds (Fe and Ca in particular), dry matter (7.8-14.9%) and protein than the cauliflowers [9]. Relatively few research works showed that the side roses may be of richer chemical composition than the main roses [3, 7]. Broccoli is an excellent raw material to be pasteurized and freezed as it maintains its taste values at higher nutritive and biological values. However, it is completely unfitted for storage [1, 4, 5, 9, 11]. Broccoli contains a number of health supporting antioxidants: beta-carotene, sulforaphan – an anticarcinogenic compound, quercetin, indoles, lutein and glutathione. Chromium contained in broccoli is the basic insulin co-factor. Co-operation of both these compounds stimulates the efficiency of energy conversion in cells and enables to keep up an optimum sugar level in blood. Because of their sensory features, these vegetables are characterized by attractive flavour and pleasant taste. They are easy to cook, being especially recommended to direct consumption or canning as well as a component enriching such products as the bread-stuffs and ready-to-cook products. Owing to their nutritive values and supporting the organism‘s homeostasis [6, 7] broccoli are instrumental in keeping good health.

Broccoli, as the majority of vegetables, needs to be properly prepared before consumption. It has to be thermally treated [7]. Application of the convection – steam ovens in Polish catering business created new possibilities of preparing meals, the vegetables inclusive. Basic advantages of the convection-steam oven application include much shorter duration of the heat treatment and less raw material losses by-weight, as compared to traditional boiling in water [8].

The study aimed at the sensory evaluation of Italian broccoli after thermal treatment in a convection-steam oven at the temperature of 80 and 100°C, as well as finding out its shearing force as affected by heat treatment duration and quantity of supplied steam. Preliminary tests were carried out to determine the parameters for use in true experiments.

MATERIALS AND METHODS

The roses of Italian broccoli, Iron variety, were used as experimental material. Samples of broccoli roses, weighing 5 g (±0.5), were subjected to thermal treatment in a convection-steam oven (Unox, XV 303 G model) at the temperature of 80 and 100°C; six measurement series were carried out for each temperature and every treatment time 5, 10, 15 min:

• series I: 0% steam added in relation to initial relative humidity of the air in oven chamber,

• series II: 20% steam added,

• series III: 40% steam added,

• series IV: 60% steam added,

• series V: 80% steam added,

• series VI: 100% steam added,

Shearing force for broccoli samples was measured on an Instron 4302 apparatus at uniaxial shearing and knife head shift velocity 50 mm·min^-1. Bilaterally bevelled knife of the edge thickness 2 mm and angle 15°, was used to cut the samples. 30 representative broccoli samples were selected from every measurement series, for each thermal treatment duration.

The results of shearing force tests were statistically analysed using the Statistica 6.0 software. The significance of differences among the average shearing forces was verified by the Tukey’s test.

After thermal treatment the sensory evaluation was done on the basis of five-score scale with following quality factors: colour, consistence, flavour, taste, succulence, as well as a hedonistic scale describing the desirability of broccoli samples. The sensory evaluation was realized by a team of fifteen persons 25-45 years old [2, 10].

RESULTS AND DISCUSSION

Analysis of measured shearing force for Italian broccoli samples after heat treatment at the temperature of 80°C shows that the shearing force was affected by the treatment duration and quantity of added steam (fig. 1, tab. 1). The lowest shearing force (26.05 N) was obtained after 15 min treatment with steam addition 80% and (29.33 N) with 100% steam addition. In second case the value of shearing force was higher than the value for control sample (28.07 N), without any thermal treating. The highest shearing force was noted for broccoli samples after 5 min treatment with 60% steam addition (44.23 N). It is worth to mention that the value of shearing force increased or remained on the same level at steam addition ranging from 0 to 60% for three treatment durations, next it decreased at 80 and 100% steam additions.

There is significant difference in the values of the shearing force between samples of 80 and 100% steam addition and control sample (without steam addition). It is supposed that just 80 and 100% steam addition exceed loss of water caused and in consequence it causes decrease of the shearing force.

Figure 2 presents the shearing force for Italian broccoli after thermal treatment at 100°C depending on the treatment duration and quantity of supplied steam.

Table 2 presents the significance of differences among the average shearing forces for Italian broccoli after heat treatment at 100°C and significance of these differences verified by the Tukey’s test. After heat treatment at such temperature the lowest values of shearing force were obtained at 60-100% steam supply after 15 min treatment duration (6.92, 7.19 N and 5.47 N, respectively). The highest shearing forces were observed at 0-20% steam addition for three treatment durations; next, the value of shearing force dropped down along with increasing quantity of the steam added. Values of shearing force obtained after heat treatment at the temperature of 100°C were much lower than values of shearing force obtained after heat treatment at the temperature of 80°C. Higher temperature needs the shorter time to wetting the samples of broccoli, supposed.

Almost all the quality factors after three heat treatment durations without steam addition showed the highest sensory values (tab. 3).

The colour of broccoli for all three heat treatment durations – likewise the cauliflower, after 10 and 15 min thermal treating at 80°C, obtained highest assessment from valuating persons – for 100% steam addition. The consistence – after 5 min thermal treating – achieved highest score (4.0) at 80% steam addition and increased (to 4.2) after 10 and 15 min thermal treatment at 100% steam addition. Extended duration of the heating and increased steam supply affected not only the consistence (that appeared to be compact and obtained highest score number on five-score scale), but also the taste and pleasant scent. Broccoli roses were strongly desirable by valuating persons and their aroma was perceptible. After 5 min thermal treatment the taste and flavour reached the highest grades at 60% steam addition; however, after 15 min heating at 80% steam addition, were more desirable by valuating persons. The taste of broccoli roses after 10 min heat treatment appeared to be unsatisfactory reaching the highest grade at 20% steam addition, whereas the scent reached the same score after 15 min thermal treating, also at 20% steam addition. The succulence was valuated on maximum level (4.2 score) for all three heating durations and 80% steam addition. From among five persons valuating in hedonistic scale all opinions on the succulence were on level of “like very much”.

Evaluation of thermally treated broccoli samples in respect of their colour, for all three treatment durations, showed the best results (4.2 score) achieved at 100% steam addition, while the worst (3.0 score) at 20% steam addition (tab. 4). After 10 and 15 min treatment the highest grade was obtained by the consistence and taste, thus the best desirability among assessing persons. In hedonistic scale the majority of judge opinions were “l like very much”. After 5 min heat treatment the opinions on broccoli consistence and taste were the best for variant of 60% steam addition. The flavour was intensive after 5 and 10 min heat treatment with 100% steam addition and became sensible after 15 min heat treatment with 60% steam added.

The succulence of broccoli was highly estimated after 15 min heat treatment with 100% steam addition, achieving 4.0 score, whereas after 5 and 10 min treatment the broccoli succulence was accounted the best at 80% steam addition (fairly succulent). The poorest evaluation touched almost all quality factors, with except of colour, after three heat treatment durations without any steam added. Opinion of the judging team was “almost do not like”.

CONCLUSIONS

The most suitable colour of Italian broccoli was achieved in case of 10 and 15 min thermal treatment duration, with 100% steam addition at both temperatures, 80 and 100°C.

Best consistence of Italian broccoli was obtained for 100% steam addition, 10 and 15 min thermal treatment duration at the temperature of 100°C.

The pleasant flavour of Italian broccoli after thermal treatment was the stronger, the higher was steam addition (from 60 to 100%), for all treatment durations.

The succulence of Italian broccoli was highly evaluated after 15 min heat treatment with 100% steam addition at the temperature of 100°C.

Highest desirability among the judging persons achieved Italian broccoli thermally treated in convection-steam oven for 10 and 15 min, with 80% steam addition.

The longer was duration of Italian broccoli heat treatment at the temperature of 100°C and the higher was quantity of supplied steam, the less was the value of shearing force.

REFERENCES

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Accepted for print: 12.03.2007

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.