VWT 272 Class 6

Quiz 4

Number of quizzes taken 20

Min 2

Max 30

Mean 21.7

Median 26

Mode 26

Lecture 6 Some Chemical Structures and the “Sulfur

Dioxide Family”

The difference between professional winemakers and armature winemakers is how much SO2 they use.

Ed Moody Retired Senior Winemaker – Bronco Winery

Plan of Study • Review pH & TA

– pH and [H+]

– TA and Titrations

• More chemical structures

– Alcohols

– Carbonyl Groups

– Aromatic Rings

– Phenols

• The “Sulfur Dioxide Family”

– Sulfite Bisulfite Molecular SO2

DON’T pHANIC!

pH = - log10 ([H+ ])

• Water = 1.0x10-7 [H+ ] • pH 7.00

• 1 M HCl = 1 [H+ ] = 1x100 [H+ ] • pH 0.00

• 1 M Acetic Acid = 4.2x10-3 [H+ ] • pH 2.38

• 1 M NaOH = 1.0x10-14 [H+ ] • pH 14.00

• Sparkling Wine ~ 1.3x10-3 [H+ ] • pH 2.88

• Low Acid Red Wines ~ 6.3x10-5 [H+ ] • pH 4.20

TA (Total/Titratiable Acidity)

• A different way to quantify the amount of acid in a wine

– Based on titration

• Acid/Base neutralization – HA + BOH → HOH + AB

• Titration Equation – Ma x Va = Mb x Vb

• A measure of the [H+] at pH 8.2

• Reported as “g Tartaric Acid/L”

More Fun with Chemical Structures! • Alcohol

– A compound with a hydroxyl group ( -OH ) bound to a saturated carbon atom • Methanol

– Methyl Alcohol, Wood Alcohol – Poisonous

» Metabolized to Formaldehyde

– Used to “denature ethanol”

• Ethanol – Ethyl Alcohol, EtOH, Alcohol – Cause Intoxication – Found in fermented foods

• Propanol – Two forms

» n-propyl alcohol • solvent

» iso-propyl alcohol • Rubbing alcohol

More Fun with Chemical Structures! • Carbonyl Group

– A functional group with a double bonded Oxygen atom ( =O ) bound to a carbon atom • Aldehyde

– Strong Odor (Sherry, Green Apple)

– Hangover

• Ketone – Strong Odor (Fruty)

– Hangover Breath

• Carboxylic Acid – Vinegar

– Amino Acids

• Ester – Strong Odor (Fruty/Floral)

– Lipids (Fats & Oils)

Even More Fun with Chemical Structures! • Aromatic Ring Structures

– A closed ring structure of 6 Carbon atoms with a single Hydrogen attached to each Carbon • Benzene

– Sweet/Spicy odors

• NOT cyclohexane

Even More Fun with Chemical Structures! • Aromatic Ring

Structures

– Common in Nature • Lots of groups can attach

to the ring

• Polycyclic – lots of rings can attach to each other

Yet More Phun with Chemical Structures! • Phenol

– A compound with a hydroxyl group ( -OH ) bonded to a Aromatic Ring • Phenol

• Polyphenol

– Flavonoid

» Quercetin

Yet Even More Phun with Chemical Structures!

• Phenol

– Huge structures • Polymers

– Lignin

– Bakelite

A brief History of Sulfur Dioxide (SO2)

• Ancient Egypt – used to bleach Linen

• Ancient Greece - Used as a fumigant

– The Odyssey Book XXII “Odysseus himself spoke to Eurycleia, his faithful nurse: ‘Old woman, bring some sulphur, and make a fire, so I can purge the hall from this pollution.”

• 1487 - Prussian royal decree officially permitted the use of the wine additive sulfur dioxide

• Mid-1600s - Introduced into Bordeaux by Dutch traders

Why Add Sulfur Dioxide (SO2) to Wine?

• Killing and growth inhibition of yeast

• Killing and growth inhibition of bacteria

• Inhibition of browning enzyme (polyphenol oxidase)

• Interaction with wine phenols (bleaching)

• Binding of acetaldehyde (oxidation taint)

• Antioxidant

• Get to use the warning label

Meet The Sulfur Dioxide (SO2) Family

• SO2 when added to an acid solution (like wine) exists in an equilibrium between three forms

– Sulfite

• SO32-

– Bisulfite

• HSO3-

– Molecular

• SO2 (aq)

Sulfite Bisulfite Molecular SO2

The (SO2) Family Equilibrium

Sulfite Bisulfite Molecular SO2

• The concentration of each family member depends upon pH

Sulfite ion (SO32-)

• At wine pH is virtually nonexistent – 1 to 3 μM

– < 0.01% at pH 3.4

• Is a good Antioxidant (O2 grabber) – BUT requires much higher pH

– Example • It took 30 days to trap ½ of the dissolved oxygen in an

O2 saturated model wine (tartaric acid + ethanol + SO2) solution

• It took 2 days to trap ½ of the dissolved oxygen in an O2 saturated white wine with the same SO2 concentration

Bisulfite ion (HSO3-)

• At wine pH is Most common form – 94.4% at pH 3.0 – 99.4% at pH 4.0

• Is not a good Antioxidant (O2 grabber)

• Is not a good Antimicrobial • Binds with the carbonyl oxygen in

– Acetaldehyde – Glucose – Keto acids

• Pyruvate

• Inhibits polyphenol oxidase

Bisulfite ion (HSO3-)

• Binds with the carbonyl oxygen in Acetaldehyde

• Binds with other carbonyl group containing molecules to form “Bound SO2”

Where does Acetaldehyde Come From? • Produced by Saccharomyces cerevisiae

– Then Re-utilized by the yeast

• More SO2 at crush increases Acetaldehyde production – Red with 50 mg/L SO2 added before Fermentation – Black with no SO2 added before Fermentation

• Yeast nutrients increase reutilization of Actaldehyde • Malolactic bacteria degrade Acetaldehyde during MLF • “Coupled” chemical reaction

– O2 reacts with a phenol to make Quinone + Hydrogen Peroxide (H2O2)

– Hydrogen Peroxide oxidizes Ethanol to Acetaldehyde

• On average, red wines contain 30 mg/L, white 80 mg/L, and

Sherries 300 mg/L • Sensory threshold is between 100-125 mg/L

Molecular SO2 • At wine pH is present in small amounts

– 5.6% at pH 3.0 – 0.6% at pH 4.0

• Is responsible for antimicrobial action – Not ionic – can pass through cell membranes – Disrupt enzyme activity – Disturb protein structure

• Reacts with Hydrogen Peroxide (H2O2) to make Sulphate SO4

2- before it can react with ethanol SO2 + H2O2 → 2H+ + SO4

2- “Fast” CH3CH2OH + H2O2 → CH3CHO + 2 H2O “Slow”

• Sensory threshold – 10 mg/L in air – 15 to 40 mg/L in wine

Molecular SO2 The Table

Free vs. Bound SO2

• At wine pH

– Bound (BSO2) is all SO2 in the Bisulfite ion (HSO3-)

form that is bound to

• Acetaldehyde

• Glucose

• Keto Acids

– Free (FSO2) is all SO2 in the Bisulfite ion (HSO3-)

form that is NOT bound + all Molecular SO2

– Total SO2 (TSO2) = FSO2 + BSO2

Free & Total SO2 Measurement

• Ripper Method

– Iodine added to wine

– Starch indicator goes from colorless to blue

– Difficult with red wines • Yellow light helps

– Inexpensive

– Approximate values of • TSO2 & FSO2

Free & Total SO2 Measurement

• Aeration-Oxidation Method – SO2 is trapped in a in hydrogen

peroxide solution after it has been blown/sucked out of an acidified wine sample

– Methylene Blue/Methyl Red indicator goes from colorless to green

– Complex setup including vacuum pump and condenser

– Easy analysis for red and white wines

– Precise values of • TSO2 & FSO2

Forms of SO2

• Gas / High Pressure SO2

– Exact addition • Direct measure of grams of pure SO2

– Expensive and potentially dangerous equipment

• 6% to 10% SO2 Solution – Commercially available – Fresh Stock Solutions are accurate – Simple to add

• Potassium Metabisulfate (KMB) – Easy to measure by weight – Quickly degrades with time

• Keep package sealed and dry!

– Inexpensive – Simple to add – Source of Potassium- cold stability concerns

Free SO2 Addition Calculations

• Imprecise “GUIDE” for SO2 Addition in Juice/Must

– Assume ~ 60 % of your added SO2 will be bound (BSO2)

• 100 mg added SO2/40 mg free SO2

• Concentration will drop rapidly to unmeasurable

– Damaged fruit will require significantly more SO2

Free SO2 Addition Calculations

• Imprecise “GUIDE” for SO2 Addition in Wine – ~ 50% of SO2 Addition becomes quickly bound in

young wines and wines below 60 mg/L TSO2

• 100 mg added SO2/ 50 mg free SO2

– ~ 30% of SO2 Addition becomes quickly bound in wines between 60 mg/L and 100 mg/L TSO2

• 100 mg added SO2/ 70 mg free SO2

– ~ 15% of SO2 Addition becomes quickly bound in older wines above 100 mg/L TSO2

• 100 mg added SO2/ 85 mg free SO2

• KMB has 0.576 g of SO2 for every g of KMB

Free SO2 Addition Calculations

1. Determine the amount of Molecular SO2 you need and the volume of wine you will be sulfuring

2. Estimate the amount of binding that will occur after addition

3. Calculate the amount of Free SO2 needed to get the required Molecular SO2 from “The Table” or on-line calculator

4. Calculate the additional Free SO2 needed given the Free SO2 already in the wine

5. Use the correct concentration factor for the method of SO2 addition (KMB vs. pure gas vs. 10% or 6% solution)

Free SO2 Addition Example 1000 gal tank of Syrah, pH 3.40, FSO2=10 mg/L, TSO2=67 mg/L, using pure SO2 gas, goal is 0.8 mg/L Molecular SO2. How much SO2 gas are you going to add?

– At pH 3.40 table says FSO2 should be about 35 mg/L

– With 10 mg/L already in tank (35 – 10) you need to add 25 mg/L as Free

– With 67 mg/L TSO2 you would expect about 30% of any addition will be bound. That means for every 100 mg added SO2 you will get 70 mg “free” SO2

– 25 mg free SO2/L x 100 mg added SO2/70 mg free SO2/L 3.785 L/gal x 1000 gal x 1 g added SO2/1000 mg added SO2 = 135 g added SO2

Another Free SO2 Addition Example 264 gal tank of Chardonnay, pH 3.25, FSO2=3 mg/L, TSO2=32 mg/L, using KMB, goal is 0.8 mg/L Molecular SO2. How much KMB are you going to add?

– At pH 3.25 table says FSO2 should be about 25 mg/L

– With 3 mg/L already in tank (25 – 3) you need to add 22 mg/L as Free SO2

– With 32 mg/L TSO2 you would expect about 50% of any addition will be bound.

– 22 mg free SO2/L x 100 mg added SO2/70 mg free SO2/L 3.785 L/gal x 264 gal x 1 g added SO2/1000 mg added SO2 x 1 g KMB/0.576 g added SO2 / = 54.5g KMB

Next Week

• The Other Sulfurs

– Elemental

– Sulfide

• Copper

– Disulfide