7. Descrierea Principalelor Caracteristici Fizico-chimice Ale Strugurilor

8/12/2019 7. Descrierea Principalelor Caracteristici Fizico-chimice Ale Strugurilor

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C H A P T E R

2Composition of Grape Must

O U T L I N E

1. Grape Must 13

2. Chemical Families Present in Must 152.1. Sugars 15

Disaccharides 172.2. Organic Acids 172.3. Nitrogen Compounds 182.4. Minerals 192.5. Polyphenols 19

2.6. Vitamins 20

2.7. Aromatic Compounds 20Terpenes 20Carotenoids 21Pyrazines 21Alcohols and Aldehydes 21

1. GRAPE MUST

Grape must is the liquid obtained by the gentle crushing or pressing of grapes. Pressingtakes place once the grapes (either destemmed or still in clusters) have been gently crushed.Even within the same winemaking region, must composition varies according to severalfactors, including:

The type and variety of grapes used, The ripeness and health of the grapes (ripeness depends on a range of factors, such as the

climate during the growing season, the type of soil, and the fertilizers used), The pressure exerted on the grapes.

Musts are classied as free-run must (or juice), which is obtained by the simple crushing of grapes, or press-fraction must, which is obtained by subjecting the grapes to increasing levelsof pressure. There are therefore many types of must.

13Enological Chemistry. DOI: 10.1016/B978-0-12-388438-1.00002-9 Copyright 2012 Elsevier Inc. All rights reserved.


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Quality white wines are made only from free-run must (known in Spain as mosto or omosto yema) or rst-press fractions. Subsequent fractions can be used to make other products,such as the more intense, deeper-colored press wines.

As the press fractions, and logically, the pressure exerted on the grapes increase toimprove the yield, the resulting juice becomes increasingly rich in substances derivedfrom the solid parts of the grape, such as the stems (when the grapes are crushed in clusters),the skins, and the pips.

Figure 2.1 shows variations in must pH and the concentration of several compoundsderived from grape solids over successive press fractions. As can be seen, polyphenol andpotassium levels increase after several presses, as the increased pressure on the solid partsof the grape extracts a greater proportion of these compounds. The pH of successive pressfractions is related to the amount of free acids and acid salts in the different parts of the grapeand is therefore also a reection of the pH of the tissue of the solid parts of the grape (which ishigher than that of the pulp). pH is also inuenced by potassium levels, as potassium ions

neutralize most of the acids in the berry. Iron levels are also directly related to the numberof presses, but it should be noted that pressing equipment, which is generally made of iron or stainless steel, can also contribute to these levels.

It is thus clear that there is no such thing as a single must, and that to understand mustcomposition we must take into account the different treatments that both the grape and its juice undergo in order to obtain a reliable raw material for fermentation.

The following substances or groups of substances, shown in order of abundance, arepresent in must:

WaterSugarsOrganic acidsNitrogen compoundsMineralsPolyphenolsVitaminsAromatic compounds

TABLE 2.1 Composition of Musts Obtained at Increasing Press Pressures

LiquidVolume (%)

Dry Extract(g/100 mL)

Sugars(g/L)

Acidity(g/L)

Ash(g/L)

Alkalinity ofAsh (meq/L)

TartaricAcid (g/L)

MalicAcid (g/L

Free run 60 21 194 7.5 3.4 32 5.6 3.8

First press 25 22 192 7.7 3.4 34 5.8 4.0

Second press 10 22 191 6.5 3.8 34 4.4 4.2

Third press 4 25 187 5.4 4.9 40 3.6 4.3

Fourth press 1 31 176 5.1 5.6 46 3.7 4.3

Total 100 22 193 7.3 3.6 34 5.3 3.9

Adapted from De Rosa, 1988.

2. COMPOSITION OF GRAPE MUST14

ENOLOGICAL CHEMISTRY


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2. CHEMICAL FAMILIES PRESENT IN MUST

All chemical families are characterized by a functional group, which is a collection of oneor more atoms within a molecule that provide the molecule with a unique chemical function,specic to the functional group.

2.1. Sugars

The sugars present in must are polyalcohols with a carbonyl group. These are organiccompounds that contain several OH groups along with an aldehyde or ketone group.According to the established nomenclature, the sufx -ose should be used to refer to thesecompounds, which can be aldoses or ketoses. The D and L prexes used in sugars are relatedto the ( ) or ( ) enantiomer of the glyceraldehyde from which they are derived. Accordingly,monosaccharides in which the chiral center that is furthest from the aldehyde or ketone grouphas the same conguration as the corresponding D-glyceraldehyde are members of the D series,while those with the opposite conguration are members of the L series. In a Fischer projection,

the OH group is to the right of the chiral center of D-glyceraldehyde. The D or L congurationis not related to the ability of the enantiomers to rotate polarized light to the right or left.The most abundant sugars found in must are monosaccharides with six carbon atoms:

Glucose: 6-carbon aldose (dextrose) Fructose: 6-carbon ketose (levulose)

0

750

1500

2250

3000

3750

0

5

10

15

20

25

% of must in successive press fractions

P o l y p

h e n o

l s ( m g /

L )

p H

P o t a s s i u m

( m g /

L )

I r o n

( m g /

L )

% of must in successive press fractions

% of must in successive press fractions % of must in successive press fractions

2.75

3.25

3.75

4.25

0

0

0.4

0.8

1.2

1.6

0 20 40 60 80 10020 40 60 80 100

0 20 40 60 80 100 0 20 40 60 80 100

FIGURE 2.1 Must composition according to number of presses.

2. CHEMICAL FAMILIES PRESENT IN MUST 15



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The chemical structures for D-glucose and D-fructose are shown below:

The most relevant 5-carbon monosaccharides are the D-aldoses:

CH 2 OH

C

C

C

C

CH 2 OH

O

HHO

OHH

OHH

C

C

C

C

C

CH 2 OH

OH

OHH

HHO

OHH

OHH

Glucose Fructose

C

C

C

C

CH 2 OH

OH

OHH

HHO

OHH

C

C

C

C

CH 2 OH

OH

OHH

OHH

OHH

C

C

C

C

CH 2 OH

OH

HHO

OHH

OHH

Xylose Ribose Arabinose

C1

OCCH 2

HH

OH

H

OH

OH

HH

OH

OH

C1

OC

CH2

HH

OHH

OH

OH

HOH

H

OH

C1

OHC

CH 26

HH

OH

H

OH

OH

HH

O

OH

C1

OHC CH 26

H

H

OH

H

OH

OH

HH

O

OH

C1C

C

C

C

O

H

H

H

H

CH 26

OH

OH

OH

OH

OH

H

D-Glucose(Fischer projection)

-Glucopyranose

-GlucopyranoseD-Glucose

(Haworth projection)

FIGURE 2.2 Haworth projection of glucose.




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DisaccharidesDisaccharides are formed by joining pairs of various monosaccharides via a - or

b -glycosidic bonds. A hemiacetal hydroxyl group formed from the oxygen of the carbonylgroup ( C O) always participates in the formation of these bonds. In certain cases, all thecarbonyl groups in the molecule are used. This means that the resulting product (e.g.,sucrose) lacks reducing power, because the two monosaccharide units are linked by thehemiacetal hydroxyls of the two molecules, hence both are blocked.

In the case of maltose, which is a disaccharide formed by two glucose molecules, only onehemiacetal hydroxyl group is blocked, meaning that the second one retains the reducingproperties characteristic of aldehydes.

Glycosidic bonding of these disaccharides to other monosaccharides gives rise topolysaccharides.

2.2. Organic Acids

Organic acids possess a COOH functional group. In must, however, acids also possessother groups, such as the OH group of alcohols.

C1 C1

O

C

CH 2OH

H

H

OH

H

OH

OH

Maltose

H

H H

O

C

O

CH 2OH

H

H

OH

OH

H

OH

H

C 1

O

C

CH 2 OH

H

H

OH

H

OH

OH

H

H

O

C 2

O

CH 2 OH6

HO

OH

CH 2 OH

Sucrose

CH 2

C

CH 2

OH

COOH

COOH

HOOC

C

C H

COOH

HO

OHH

COOH

C

C OH

COOH

H

HH

COOH

Malic acidCitric acid Tartaric acid




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Other sugar-related compounds are uronic acids, which are the result of the oxidation of the OH group on carbon 6 of a sugar to form a carboxyl group, and aldonic acids, which areformed by the oxidation of the aldehyde group of an aldose. Other acids of interest are gal-acturonic acid, which is the main component of grape pectins, and gluconic acid, which ishighly abundant in rotten grapes. Must produced from rotten grapes can also contain largequantities of acetic acid.

2.3. Nitrogen CompoundsThe main nitrogen compounds found in must are amino acids, either in free form or as

polypeptides or proteins.The characteristic functional group of amino acids is shown below:

All naturally occurring amino acids are a -amino acids, because the amine group (NH 2 ) i bound to the carbon immediately adjacent to the one bearing the acid group (which is pref-erentially used to name these compounds). Amino acids are joined by peptide bonds to formpeptides and proteins.

Nitrogen in the form of ammonium ions is the most assimilable form of nitrogen for yeasts,and its decit in must can cause stuck fermentation.

C

C

C

C

OHH

HHO

HHO

OHH

COOH

COOH

C

C

C

C

OHH

HHO

HHO

OHH

COOH

CH 2 OH

Galacturonic acid Gluconic acid

C C

O

OH

H

NH 2

R

R1 CCO

OHNH 2

H

R2

C C

O

OHNH 2

H

R1 CC

O

NH

NH 2

H

C

R2

C

H

OH

O

+

H 2 O

Peptide bond




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2.4. Minerals

The mineral content of must and wine refers to the cations and elements that these contain.Musts contain many mineral substances, which can be classied according to their electriccharge and abundance.

Cations

Abundant: K

, Ca2

, Mg2

, Na

, and Si4

(plant macronutrients)Less abundant: Fe 3 , Mn2 , Zn 2 , Al3 , Cu2 , Ni2 , Li , Mo4 , Co2 , and V 3

(micronutrients)Trace levels: Pb 2 , As3 , Cd 2 , Se4 , Hg 2 , and Pt 2 (ppb)

AnionsAbundant: PO 4 3 , SO4 2 , ClLess abundant: Br , I

2.5. Polyphenols

Grapes acquire their color from different compounds in the berries. The most noteworthyof these are: Chlorophyll Carotenoids Betalains Polyphenols

Anthocyans 0 redYellow avonoids 0 yellowTannins 0 brown

Polyphenolic compounds play an essential role in both grapes and wine, as theyare responsible for a range of sensory properties, such as appearance (color), taste(astringency, bitterness), and aroma (volatile phenols). They can be classied asfollows:

Simple (non-avonoid) polyphenols Flavonoids Tannins Others (stilbenes)

OH COOH

R1

R2

C

O

C

Simple phenol Flavonoid phenol




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2.6. Vitamins

Grapes contain approximately 90 mg of vitamin C (ascorbic acid) per kilogram, as well assmall quantities of B-group vitamins (of which there are 10). Vitamins are particularly impor-tant in wine making as they are very useful to yeasts and therefore essential for successfulalcoholic fermentation.

2.7. Aromatic CompoundsTwo types of compounds confer aroma to must: grape-derived compounds (terpenes,

carotenoids, and pyrazines) and compounds that arise during avor extraction and pre-fermentation treatments (alcohols and C 6 -aldehydes).

TerpenesTerpenes are derived from isoprene units (2-methyl butadiene).

HO

HO

HO HO

HO

Linalool Diendiol I Diendiol II

TABLE 2.2 Vitamin Content of Grapes and Must

Grapes ( m g/1000 grapes) Must ( m g/L)

Thiamin 253 160 e 450

Riboavin 3.6 3 e 60

Pantothenic acid 660 0.5 e 1.4

Nicotinamide 700 0.68 e 2.6

Pyridoxine 260 0.16 e .50

Biotin 2.2 1.5e 4.2

Myo-inositol 297 380 e 710

Aminobenzoic acid 14 15 e 92

Folic acid 1.3 0e 1.8

Choline 24 19 e 45

Cyanocobalamin 0 e 0.2

Ascorbic acid 30,000 e 50,000




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CarotenoidsThe main carotenoids found in must are b-carotene and lutein. When the grape berry

bursts, they can break down into compounds with 9, 10, 11, or 13 carbon atoms that aremore powerful odorants than their precursors. Of particular note are the C 13 -norisoprenoidderivatives. These are divided into two groups: megastigmane forms and non-megastigmaneforms.

PyrazinesMethoxypyrazines are nitrogen heterocyclic compounds with the following general

structure:

They are responsible for the vegetal aroma of certain grape varieties, such as Cabernet Sau-

vignon, Sauvignon Blanc, and Merlot.

Alcohols and AldehydesThe most important aromatic alcohols and aldehydes are those with 6 carbon atoms (satu-

rated and unsaturated). These compounds originate enzymatically during prefermentation

Megastigmane forms

Non-megastigmane forms

-Damescenone -Ionone

Trimethyldihydronaphthalene Vitispirane

O O

OO

OHActinidol

N

N

O

CH 3

R




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treatments via the aerobic oxidation of linoleic and linolenic acid (C 18:2 and C 18:3 ). Thfollowing compounds have been identied: hexanal, (E)-2-hexenal, (Z)-3-hexenal, hexanol-1,(E)-2-hexen-1-ol, and (Z)-3-hexen-1-ol.

The list below shows the average quantitative composition of must, shown by groups of compounds:

pH 3 e 4.5Water 700 e 850 g/LSugars * 140e 250 g/LOrganic acids 4 e 17 g/LNitrogen compounds 4 e 7 g/LPolysaccharides 3 e 5 g/LMinerals 0.8 e 2.8 g/LPolyphenols 0.5 g/LVitamins 0.25 e 0.8 g/LAromatic compounds < 0.5 g/L

* These levels can be much higher in certain musts, such as those made from raisined grapes or grapes with noble rot.

Other compounds of interest in musts are the wax and oleanolic acid present in the grape bloom. This bloom retains traces of products used during the wine-growing process, such aspesticides and other mineral compounds, e.g., copper sulfate. The bloom also contains yeaststhat participate in alcoholic fermentation.

H 3 CCH 2 OH

Hexanol-1 (Z)-3-hexen-1-ol (E)-2-hexen-1-ol

H 3 C CH 2 OHH 3 C

CH 2 OH



7. Descrierea Principalelor Caracteristici Fizico-chimice Ale Strugurilor

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