THE INFLUENCE OF DIFFERENT POT STILL TYPE ON QUALITY OF WILLIAMS PEAR DISTILLATES

Original scientific paper Pear brandy is one of the most desirable alcoholic beverages across Europe. But of all varieties, the most appreciated is Bartlett pear or Williams’s pear as it is better known. The purpose of this work was to create an objective evaluation of the influence of various construction forms of pot still, (alembic, pot and spheric) on final quality of Williams pear brandy. Gas-chromatography and sensory analysis were applied to the determination of quality and aroma profile of Williams pear brandy. Results showed that the type of pot still construction has a significant influence on quality and aroma profile of Williams pear brandy. The samples that are produced by alambic pot still had better quality and more expressive flavour. The best evaluated sample had an amount of ethanol of 59,51 % in total volume, esters 478 mg/l, aldehydes 120 mg/l and higher alcohols of 962 mg/l.


Introduction
Pear is one of the widely grown fruit species across the world and it is considered to be one of the oldest grown species.The earliest written documents on its growing date back to the New Stone Age i.e. over 20,000 years and it is thought that 6000 varieties have been developed to date [1].But of all varieties, the most appreciated is Williams's pear or Bartlett pear.It is the most widespread pear variety of all [2].William's pears are probably best expressed through the famous William's pear brandy.
Production of William's pear brandy starts with alcoholic fermentation of fully ripe fruits which should be conducted slowly at low temperature, most suitably at 18°C, so that valuable aromatics of fruit are preserved as much as possible.Distillation of fermented crushed fruit should be done within 48 h after fermentation is finished, so that harmful components do not increase (acids, methanol, esters, aldehydes, HCN etc.) and accumulated secondary aroma does not become lost.Distillation could be performed in a different type of pot still.William's pear distillate never matures (ages) in a cask because primary distillate components are incompatible with secondary components extracted from a wooden container.After it has been processed (and distillate is diluted to its ultimate strength from 40 ÷ 45 %vol.),Williams pear brandy is consumed colourless and cooled at 12 ÷ 15 °C [2].
Quality of alcoholic beverages is influenced by many factors, mainly by quality of starting raw material and production process [3].Over past years investigations focus was on alcoholic fermentation and distillation procedure, as the most important stages of the technological process of brandy production [2, 4÷8].
Miličević [9] investigated influence of fermentation process with immobilized yeast cells on final quality of tangerine distillates.The obtained results have shown that fermentation with immobilized yeast cells has significant influence on aroma and quality of distillates.Fermentation time was reduced and final distillates had satisfactory sensory and analytical profiles.Nikićević [2] suggests adding suitable pectolitic enzyme and appropriate yeast strain throughout fermentation to disintegrate Bartlett pear fruits and to reduce boiling medium pH.
Volatile aromatic compounds are the most important for the quality of brandies [10].These compounds can be divided into four groups: primary, which derive from fruit varieties; secondary, which develop during the fermentation processes; tertiary substances, which develop during the distillation process; quaternary substances, which develop during the maturing process.The most important volatile compounds present in the distillate come from microbial fermentation and belong to the chemical classes of higher alcohols, esters and aldehydes [11].
The aim of this work was to investigate influence of various types of pot still (alembic, pot and spherical) on quality of Williams pear brandy.

Materials and methods
Each set of samples of fermented Williams pear pulp (100 L) is taken from the production process at "Zvečevo d.d.Food Industry", from the harvest of the year 2011.All samples were protected by SO 2 (30 mg/l).
Fermentation was carried out by selected yeast Feromol-Bouqet 125 in internal loop gas-lift fermenter and controlled thermal regime using outer refrigeration of fermenter with running water and keeping the average temperature in intervals of 18 ÷ 20 °C.The average duration of fermentation process under these conditions was 19 days for each set.
Williams's pear brandy samples were distilled by three different types of pot still provided by Strix d.o.o.Zagreb according to the distillation protocol described by Nikićević [2].Pot still types (Fig. 1): Pot still-alembic (PSA), pot still-pot (PSP) and pot still-spherical (PSS).

Sensory analyses
A sensory analysis of samples was performed according to the method of positive scoring factor according to the German DLG model [12].
This model was based on 4 sensorial experiences, which are marked with grades 0 to 5, including 0, while the average grade is multiplied by the significance factor.Sensory assessment was conducted in two repetition cycles.Each group had ten testers, selected by selection procedure [13].

Chemical analysis of distillates
Standard analytical techniques were applied for the evaluation of the distillates quality.In industrial control laboratories these techniques represent the basis for the determination of the quality parameters.Chemical analysis of the distillates carried out: ethanol, total extract, total acidity, total SO 2 , total aldehydes, total esters, higher alcohols, furfural and methanol analysis [14,15,16].

Analyses of aroma substances
The major volatile components were analyzed on the basis of the European Community Reference Methods for the analysis of spirits using gas chromatography [15,16].
Gas chromatography (GC) analyses were performed on a Chrompack 437A gas chromatograph with a split /splitless injector and a FID detector.For analysis of distillates a Chrompack Poraplot capillary column (25 m × 0,25 µm i.d.0,25 µm) was used.Initial oven temperature was kept at 35 °C for 7 min, then raised at 10 °C/min to 80 °C followed by 25 °C/min to 180 °C, and kept for 4 min at 180 °C.Qualitative analysis was done by comparing the standard retention times (analytical grade from Merck, Germany) with the corresponding peaks of samples.The quantification was carried out by comparing the peak areas to those of the Merck standards.

Sensory analyses
Tab. 1 shows the results of sensor analyses of distillates.Sensory assessment of samples was carried out according to the method of positive scoring with weight factors 3 to 9, according to German DLG model [12].The scoring was performed by a group of ten professional testers with extensive experience in sensory assessment of distillates.Average score of all assessed brandies was relatively high.It was the result of quality fully ripe pears, implemented method of fermentation process and distillation technique.
Total sensory assessment ranged in the interval 70,30 ÷ 98,50, which indicates significant differences among samples.Furthermore, they can be addressed to different construction forms of pot still, since other factors in the production of distillates were the same for all samples.The William pear brandy produced by pot still-alembic had the best sensory properties (98,5 points of 100) characterised by pleasant aroma without sharp alcoholic odour tones.PSP sample had slightly lower sensory quality (96,1).On the other hand, PSS sample was significantly lower evaluated (70,3) in comparison to PSA and PSP.

Chemical analyses
Tab. 2 shows the chemical composition of Williams pear brandies.Chemical composition of Williams pear brandies were within referential values [16,17,18].The results showed that brandies had ethanol content of 51,50 (PSS), 59,51 (PSA) and 59,60 % (PSP) which is within referential values [2,19].The amount of ethanol corresponds to the requirements on quality of brandies.Lower content of alcohol may cause the reduction of some aroma substance in brandies [20].
The content of total extract in all samples was within the recommended values [19] and was in range 0.834 -0.944 g/L.
The presence of free SO 2 in distillates (3.85 to 4.54 mg/L) is the result of added SO 2 prior fermentation which is important for the protection of pulp from no controlled fermentation process and oxidation.Free SO 2 may bind acetaldehyde which develops during distillation.
Methanol and furfural in all brandies were in insignificant amount or not detected.
Volatile aromatic compounds are the most important for the quality of brandies [10].Tab. 3 presents the content of important aroma substances of analyzed William pear brandies.The most abundant aromatic compound in all samples was isoamyl alcohol 98,80 ÷ 119,90 mg/L (higher alcohol) followed by esters: methyl 2-trans, 4-cis decadieonate (52,16 ÷ 53,18 mg/L) and ethyl 2-trans, 4cis decadieonate (57,49 ÷ 65,34 mg/L).PSA and PSP had almost the same quantity of isoamyl alcohol (118,79 and 119,91 mg/L) while PSS had significantly lower amount of 98,8 mg/L.Content of esters had the same trend (higher in PSA and PSP and significantly lower in PSS).
Results clearly demonstrated that the type of construction forms of pot still significantly content and ratio of primary aroma compounds.
Production process pot still-alambic may be highlighted as potentially the best procedure for production of quality Williams pear brandies.

Conclusion
The obtained results have shown that the type of construction forms of distillery pot still has a significant influence on quality of Williams pear distillates.Moreover, the aroma compounds that mostly contribute to the typical flavour of the Williams pear distillates, such as ethyl 2-trans, 4-cis decadieonate, methyl 2-trans, 4-cis decadieonate, isoamyl alcohol 2-phenylethyl ethanol, are affected by the construction forms of pot still.
Results clearly demonstrated that the type of construction forms of pot still significantly affected content and ratio of primary aroma compounds.The samples produced by distillation with alembic pot still had the best quality among all produced samples characterised by pleasant aroma without sharp alcoholic odour tones.

Table 1
Sensory analysis results of Williams pear brandies.Samples were produced by different pot still types.

Table 2
Chemical composition of Williams pear brandies produced by different pot still types Pot still-alembic (PSA), pot still-pot (PSP) and pot still-spherical (PSS)

Table 3
Content of aroma compounds of Williams pear brandies.(mg/L) produced by different pot still types.