Yeast assimilable nitrogen (YAN), comprised of primary amino nitrogen (PAN) and ammonia (NH₃), is a measurement of the sources of nitrogen that can be assimilated by yeast cells to maintain a healthy fermentation. Without enough nutrients, stressed yeast cells may produce sulfurous odors or shut down and die. YAN can be supplemented through the addition of diammonium phosphate or other nitrogenous amino acids; however, excessive additions can be problematic by providing a nutrient source for spoilage organisms.

Volatile acidity is a measurement of a wine’s volatile acids, the primary of which is acetic acid. The volatility of these acids contributes negatively to a wine’s aroma, imparting smells of vinegar and nail polish remover. Even at low levels, is it important to monitor VA as increases suggest activity of spoilage bacteria that may require intervention.

Ethanol is a key component in wine formed from the metabolism of sugars by yeast during fermentation. Concentration of alcohol in a wine can impact sensory, the ability of yeast and bacteria to complete primary and malolactic fermentation, and the tax rates imposed. Near infrared spectroscopy is one of several methods used to measure alcohol and is beneficial due to its precision and rapid results.

Assimilable nitrogen are nutrients required by yeast cells to survive and perform fermentation. Yeast assimilable nitrogen (YAN) is a measurement of the primary organic and inorganic sources of nitrogen that can be assimilated by S. cerevisiae. YAN is comprised of two components: primary amino nitrogen (PAN) and ammonia (NH₃⁺). Ammonia is the primary source of assimilable nitrogen throughout fermentation.

Brettanomyces bruxellensis is a yeast strain typically considered a spoilage organism in winemaking due to its production of off-flavors, such as barnyard, leather and smokiness. Its presence is particularly troublesome in a winery due to its ability to form cleaning-resistant biofilms. If caught early, contamination may be contained and further spread avoided. PCR-based analysis allows for the sensitive and specific detection of Brettanomyces in musts and wines, enabling rapid intervention in cases of contamination.

Brix approximates the sugar concentration of a solution, expressed as a percentage by mass. One degree Brix (°Bx) is equal to 1 gram of sucrose in 100 grams of solution. Measurement by refractometer relies on the ability of dissolved sugars in water to change the refractive index of a solution and rotate a plane of linearly polarized light. A DMA uses the density and temperature of a solution to approximate the sugar concentration.

The level of dissolved carbon dioxide (DCO2) in a wine has significant impacts on sensory. CO2 is known to enhance the perception of freshness and acidity, decrease sweetness and intensify bitterness and astringency. A balance is important, as too little CO2 can make a wine flat, while too much can make a wine harsh.

Oxygen is vital to the health of a fermentation due to its role in sterol synthesis, however, its presence in finished wines can lead to oxidation and rapid degradation. Oxidation of polyphenols can start a domino effect in wines, ultimately producing acetaldehyde and browning in color. Monitoring oxygen levels in wine can help prevent this degradation, particularly during racking and bottling, when oxygen exposure can be difficult to avoid.

Sulfur dioxide (SO₂) is used in the production of most wines due to its antioxidant and antimicrobial properties. Once in solution, SO₂ exists in free and bound forms—only the free forms, however, have these important properties. There are many different compounds that can bind SO₂, and the wine’s pH determines the percentage of SO₂ in each form. It is, therefore, very important to monitor the amount of SO₂ in free form to ensure the protection of the wine.

Malic acid is an organic acid found in grapes and impacts the tartness of a wine, often its depletion is necessary to soften a particularly acidic wine. This depletion can also lead to greater microbial stability, as malic acid can be used as a substrate for some spoilage organisms. Malic acid is converted to the less tart lactic acid during malolactic fermentation. Measuring malic acid is the most efficient way to monitor this process and enzymatic analysis allows for accurate and rapid results.

Microscopic examination is used to identify yeast and bacterial cell types, and characterize them by concentration, viability, percent budding, and cell shape. Samples can be used to gauge yeast cell populations to time nutrient additions and tank heating. Additionally, suspected spoilage organisms can potentially be identified by microscan.

Turbidity, measured in nephelometric turbidity units (NTU), is the cloudiness or haziness of a liquid caused by suspended solid particles. Turbidity can vary based on a wine’s style but may impact the commercial value of a wine as well as its ability to be filtered. A turbidimeter measures the loss of intensity of transmitted light due to scattering by the particles suspended in it.

Capable of metabolizing a wide range of carbon sources found in wine, many types of lactic acid bacteria can produce off-flavors, such as acetic acid, acrolein and mousiness, and can additionally lead to other stability issues. It is, therefore, helpful to be aware of the presence of LAB in wines to prevent spoilage. PCR-based analysis allows for the sensitive and specific detection of Pediococcus and Lactobacillus in musts and wines, enabling rapid intervention in cases of contamination.

pH is a logarithmic scale used to measure the concentration of hydrogen ions in solution. The scale ranges from 0 to 14; acidic solutions have a pH lower than 7, while basic solutions have a pH greater than 7. The pH of musts and finished wines is key to winemaking, as it can impact microbial activity, equilibrium of tartrates, effectiveness of sulfur dioxide additions, solubility of proteins, color of red wine and oxidative and browning reactions.

For winemaking purposes, potassium is monitored in incoming grapes due to its effect on pH. Potassium, a positively charged ion, can bind bitartrate, the negatively charged species of tartaric acid. Potassium bitartrate salts then precipitate out of the solution, lowering the acidity of the solution and raising pH. Increases in pH consequently lead to decreased color stability of red wines and increased risk of microbial instability.

Glucose and fructose are two sugars that are consumed during the fermentation process. Residual sugar refers to the levels of these two sugars left unfermented. RS is usually taken near or after the end of primary fermentation to evaluate the dryness of a wine. Glucose and fructose also serve as substrates for spoilage organisms; therefore, RS is also an important measurement to understand the microbial stability of a finished wine.

Tartaric acid is the organic acid found in highest concentrations in grapes and contributes significantly to the taste and mouthfeel of a finished wine. Unfortunately, tartaric acid can also cause instabilities in wine due to its tendency to precipitate out of solution at low temperatures. While measuring titratable acidity via autotitrator can give an estimate of tartaric acid levels in wine, this method also measures other organic acids and free hydrogen ions present in the wine. Enzymatic analysis is capable of specific and rapid measurement of tartaric acid.

Titratable acidity is a measurement of the concentration of total available hydrogen ions in solution, including free hydrogen ions and protonated acids. Although both pH and TA measure acidity, TA can vary within wines of the same pH due to the different buffering capacities of wines. While pH can inform a winemaker of important stability issues in wine, TA is a useful measurement in determining acid content for sensory description.

Sulfur dioxide (SO₂) is used in the production of most wines due to its antioxidant and antimicrobial properties. Once in solution, SO₂ exists in free and bound forms—bound forms are no longer available to protect the wine. Acetaldehyde and microorganisms bind strongly to SO₂, therefore large amounts of bound SO₂ can indicate that oxidation or microbial spoilage has occurred. It is also important to measure total SO₂ to keep it below the legal limit set by the TTB of 350 ppm.