Lect 8 : Enology (Wine Fermentation) Flashcards
What are the 2 purposes of making wine from fruits?
- To convert more perishable fruit into less perishable wine
- For flavour and complexity
What kind of fermentation takes place during wine fermentation and what are the main end products?
Alcoholic fermentation
- end product : ethanol (+residual simple sugars)
[Not impt]
Why is there are virtual “non-existent” (grape) wine market in South East Asia?
Due to hot climate, which is unfavorable for growing grapes
[Not impt]
In SEA, there is a strong potential for tropical fruit wine.
There is no adaptation needed to make other fruit wines, and can just use the exact same methods to make other fruit wines as in grape wine. True or False?
False, adaptation is needed but the knowledge is transferable.
Wines can be classified by colours. What are the 3 main types of wine and what are the differences between these wines (materials and steps)?
- Red wine
- involves direct fermentation w grape + skin before filtering (maceration – soaking skin in juice) - White wine
- grape juice is filtered first
- fermentation without the skin - Rose wine
- also made from grape and their skin
- shorter maceration time (aka fermentation is shorter, and at lower temp to reduceROR –> reduced contact of grape wine w skin)
What is meant by ‘dryness’ of wine?
Dryness refers tosweetness, which correlates with the sugar content present in the wine. A ‘drier’ wine means that the wine is less sweet –> there are lesser residual sugars after fermentation
What are the 2 main measurements to determine when grapes are ripe enough to be made into wine?
- Sugar content (Brix°)
- must be sufficient enough for ferm, around 18-20 °Bx - Titratable acidity
- ripened grapes have higher pH –>reduce tartness of grapes (flavour) + if pH is too low, may inhibit yeasts (tho they prefer slightly acidic pH)
- measured in tataric acid (g/L) or equivalent units (e.g. mg/ml).
Outline the process in making red and white wine.
Refer to physical notes
What are 2 types of yeast used in wine fermentation (commercially)? Name the species of each type of yeast.
- Saccharomyces yeast
- Saccharomyces Cerevisae (most common) – moderate alc tolerance
- Saccharomyces Bayanus –higher ethanol tolerance
- Non-Saccharomyces yeast
- Torulaspora Delbrueckii
In the past, non-saccharomyces yeasts were thought to be unimportant/spoilage organisms in ferm. However, today, they are used in fermentation. Why?
Non-saccharomyces yeasts add to the flavour complexity of wines
- they contain glycosidases, and thus break down glycosidic bonds between sugar and aromatic compounds, which :
- adds to flavour due to release of aromatic volatiles
- they also improve stability of fermentation and reducing chances of stuck fermentation since they are known to be effective fermenters at initial stages (but die quickly due to low alcohol tolerance so inoculate w sacc yeast)
What are the benefits of using yeast starter cultures instead of doing spontaneous fermentation for wine making? [6]
- Cleaner flavour
- Greater consistency
- Faster rate
- Lower frequency of stuck fermentations (when ferm stops halfway or rate sudd become v slow)
- Yeast strains used are customisable to obtain desired properties (certain aromas etc)
- Immune to killer yeast (other yeast that may kill the yeast strains of interest in wine making)
[CRUSHING AND PRE-PROCESSING OF GRAPES]
What is 1 additional adjustment and 4 additional ingredients that can be added to grape juice when crushing?
- acidity / pH adjustments
- Sulfur dioxide
- Ascorbic acid
- prevent oxidation and browning (esp for white wine) - Enzymes
- pectinases, cellulases to break down cell wall and release nutrients
- Glycosidase : break down glycosides ( sugar bonded to other groups such as aromatic compounds through glycosidic bonding at anomeric carbon) –> ** release aromatic volatiles to contribute to flavour**
- Nutrients (optional)
- Disammonium phosphate/yeast extract –> source of N (growth) to prevent ferm from stopping too early
[CRUSHING AND PRE-PROCESSING OF GRAPES]
What are 2 purposes of adding sulfur dioxide, SO2 during grape crushing?
- inhibit wild yeast/bacterial growth –> reduce microbial spoilage
- Minimise oxidation / browning (esp for white wine)
[CRUSHING AND PRE-PROCESSING OF GRAPES]
What are the forms of SO2? Which is the most effective form?
- Molecular SO2 –> ** most effective **
- HSO3- : bisulfite
- SO3(2-) : sulfite
Refer to equations in notes
[CRUSHING AND PRE-PROCESSING OF GRAPES]
What state (solid/liquid/gas) is SO2 added into crushed grapes?
Dry powder form with 50% SO2 by weight
- using SO2 gas is inconvenient
[CRUSHING AND PRE-PROCESSING OF GRAPES]
The form of SO2 (SO2/HSO3- / SO3(2-)) depends on pH. At the pH of grapes (~3.5), which is the dominant form of SO2?
Dominant form of SO2 : HSO3-
Though there is little percentage of molecular SO2, it is effective enough
[CRUSHING AND PRE-PROCESSING OF GRAPES]
How does SO2 inhibit microbial growth?
SO2 is an antimicrobial constituent
[CRUSHING AND PRE-PROCESSING OF GRAPES]
How does SO2 prevent oxidation / browning in wine?
- Inhibits Polyphenol Oxidase (PPO)
- Scavenges for free O2 ( slow reaction)
- (SO2 / HSO3- / SO3(2-)) + O2 –> SO4(2-) - Scavenges for H2O2 (fast reaction)
- Phenolics react with O2 to form H2O2
- SO2 + H2O2 –> SO4(2-)
- HSO3- + H2O2 –> HSO4-
Outline the process of fermentation by yeast in wine, giving the important intermediate compounds and end product.
See notes
[WINE YEAST FERMENTATION]
What are the 2 inoculation methods to inoculate non-saccharomyces and saccharomyces yeast?
- Co-inoculation : use a higher ratio of non-sacc to sacc yeast to ferment simultaneously
- since sacc yeast will grow anyway
- Sequential / successive inoculation
- Non-sacc yeast is inoculated first to act as a fermentation starter for flavour (limited ability to produce alcohol)
- Sacc yeast is then inoculated to act as a fermentation finisher
[WINE YEAST FERMENTATION]
What are some significant compounds that are produced from grape wine fermentation? (and what sensory qualities do they contribute to?)[4]
1.Mainly alcohols
- glycerol (sweetness)
- Some esters
- Ethyl acetate
- Isoamyl acetate
- 2-phenylethyl acetate - Some acids
- Malic acid, Tartaric acid – acidity, harshness of wine (sour) - Diacetyl – buttery aroma
[POST-PROCESSING : MALOLACTIC FERM]
What is the purpose of malolactic ferm (MLF)?
To deacidify the wine, as the wine may taste too tart/harsh/acidic
[POST-PROCESSING : MALOLACTIC FERM]
What are the 2 ways to deacidify the wine?
- Calcium precipitation
- for Tartaric acid - Microbial fermentation
- mainly by LAB – Malolactic ferm, MLF
- by yeasts – maloethanolic ferm, MEF
[POST-PROCESSING : MALOLACTIC FERM]
What is the idea of MLF by microbes?
What are the 3 common species of LAB used to deacidify wine in MLF?
Idea : to convert malic acid, a stronger acid into lactic acid, a weaker acid
- Oenococcus Oeni
- Lactobacillus
- Pediococcus
[POST-PROCESSING : MALOLACTIC FERM]
Other than deacidification (main purpose), what other 2 purposes does MLF serve?
-
flavour development
- LAB can generate certain aroma compounds :- Diacetyl – break down of citric acid
- Acetic acid – anaerobic ferm of sugar by hetero LAB
- Esters (e.g. lactic acid + ethanol –> ethyl lactate)
-
Microbiological stability
- inhibit other spoilage MO by depleting nutrients
[POST-PROCESSING : MALOLACTIC FERM]
Some argue that MLF improves the microbiological stability of wine and thus extends its shelf life. Explain :
(a) how MLF improves microbiological stability
(b) why this statement is not entirely true
(a)
- LAB depletes nutrients, preventing growth of spoilage MO
- LAB degrades malic acid, which can cause growth of spoilage MO
(b)
- the pH increases with MLF, thus w higher pH, possibility that more spoilage microbes can grow
- LAB does not deplete nutrients that much, as it is less fastidous in growth requirements (do not require very complex nutritional requirements)
[POST-PROCESSING : AGEING]
In what kind of barrels does ageing of wine take place in?
Typically in oak barrels
- release of aroma compounds
- exposes wine to slow oxidation over time (desirable for orange / red wine)
[POST-PROCESSING : CLARIFICATION]
In what 2 ways can wine be clarified?
- Natural clarification
- “Racking off” : Allow precipitate to settle down and distill out bottom layer to obtain clear wine - Facilitated clarification
- add fining agents to promote rapid settling of ppt
- filtration
[POST-PROCESSING : CLARIFICATION]
How did the wine get cloudy?
Due to the production of CO2 during MLF – LAB respires anaerobically to produce lactic acid and CO2
[POST-PROCESSING : BLENDING]
What is blending and the purpose of blending?
It is to mix different wines together
Purpose is to increase flavour complexity
