Question: Can yeasts grow food that has a low PH?
About the article "Yeast", section "Food spoilage".

Candidate passages:
Yeasts are able to grow in foods with a low pH (5.0 or lower) and in the presence of sugars, organic acids, and other easily metabolized carbon sources.
During their growth, yeasts metabolize some food components and produce metabolic end products.
This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled.
The growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as syrups and jams.
The yeast of the "Zygosaccharomyces" genus have had a long history as spoilage yeasts within the food industry.
This is mainly because these species can grow in the presence of high sucrose,ethanol, acetic acid, sorbic acid, benzoic acid, and sulphur dioxide concentrations, representing some of the commonly used food preservation methods.
Methylene blue is used to test for the presence of live yeast cells.
In oenology, the major spoilage yeast is "Dekkera bruxellensis ".
The pH level of a wine is the measurement of the amount of free (H+) hydrogen ions.
It is related to the titratable acidity level of a wine but differs in significant ways.
Low pH numbers indicate a high concentration of acids in a solution.
While pure water is neutral with a pH of 7, wine tends to be more acidic with a pH between 3 and 4.
As the acid levels in ripening grapes fall, the concentration of acids are lessening which means the pH level is rising.
Yeasts, bacteria, phenolic compounds such as anthocyanins which influence color all have varying tolerance to wines with high pH levels.
In general, wines with high pH value tend to have duller colors and less developed flavors and be more prone to wine faults caused by spoilage organisms which makes monitoring the pH levels of grapes during ripening a priority for viticulturists and winemakers.
While the rudimentary method of testing pH is to expose the grape juice to a pH indicator such as the strips used for a standard litmus test, the results are usually not as detailed and accurate as what is needed to evaluate ripeness.
Therefore, most wineries will us a pH meter that can give readings to an accuracy of plus or minus 0.1.
As with sugars and acids, the ideal pH levels to determine ripeness will vary.
For white wines, winemakers often look for pH readings between 3.1 and 3.2, while would be a maximum of 3.4.
If the pH is too high, it may be a sign that the grapes are overripe (or that the soil has too much potassium which will also influence pH readings).
While there are risks to letting the pH go too high, winemakers can counter high pH by adding more tartaric or malic acid during the winemaking.
However many viticulturists and winemakers uses pH readings as a strong boundary line for when to start the harvest. 
An outstanding feature of "Z. bailii" is its exceptional resistance to weak acid preservatives commonly used in foods and beverages, such as acetic, lactic, propionic, benzoic, sorbic acids and sulfur dioxide.
In addition, it is reported that the yeast is able to tolerate high ethanol concentrations (≥ 15% (v/v)).
The ranges of pH and aw for growth are wide, 2.0 - 7.0 and 0.80 - 0.99, respectively.
Besides being preservative resistant, other features that contribute to the spoilage capacity of "Z. bailii" are: (i) its ability to vigorously ferment hexose sugars (e.g.
glucose and fructose), (ii) ability to cause spoilage from an extremely low inoculum (e.g.
one viable cell per package of any size), (iii) moderate osmotolerance (in comparison to "Zygosaccharomyces rouxii").
Therefore, foods at particular risk to spoilage by this yeast usually have low pH (2.5 to 5.0), low aw and contain sufficient amounts of fermentable sugars.
The extreme acid resistance of "Z. bailii" has been reported by many authors.
On several occasions, growth of the yeast has been observed in fruit-based alcohols (pH 2.8 - 3.0, 40 - 45% (w/v) sucrose) preserved with 0.08% (w/v) benzoic acid, and in beverages (pH 3.2) containing either 0.06% (w/v) sorbic acid, 0.07% (w/v) benzoic acid, or 2% (w/v) acetic acid.
Notably, individual cells in any "Z. bailii" population differ considerably in their resistance to sorbic acid, with a small fraction able to grow in preservative levels double that of the average population.
In some types of food, the yeast is even able to grow in the presence of benzoic and sorbic acids at concentrations higher than those legally permitted and at pH values below the pKa of the acids.
For example, according to the European Union (EU) legislation, sorbic acid is limited to 0.03% (w/v) in soft drinks (pH 2.5 - 3.2); however "Z. bailii" can grow in soft drinks containing 0.05% (w/v) of this acid (pKa 4.8).
Particularly, there is strong evidence that the resistance of "Z. bailii" is stimulated by the presence of multiple preservatives.
Hence, the yeast can survive and defeat synergistic preservative combinations that normally provide microbiological stability to processed foods.
It has been observed that the cellular acetic acid uptake was inhibited when sorbic or benzoic acid was incorporated into the culture medium.
Similarly, ethanol levels up to 10% (v/v) did not adversely influence sorbic and benzoic acid resistance of the yeast at pH 4.0 - 5.0.
Moreover, Sousa et al.
(1996) have proved that in "Z. bailii", ethanol plays a protective role against the negative effect of acetic acid by inhibiting the transport and accumulation of this acid intracellularly.
Like other microorganisms, "Z. bailii" has the ability to adapt to sub-inhibitory levels of a preservative, which enables the yeast to survive and grow in much higher concentrations of the preservative than before adaptation.
In addition, it seems that "Z. bailii" resistance to acetic, benzoic and propionic acid is strongly correlated, as the cells which were adapted to benzoic acid also showed enhanced tolerances to other the preservatives.
Some studies have revealed the negligible effects of different sugars on preservative resistance of "Z. bailii", e.g.
comparable sorbic and benzoic acid resistance was observed regardless whether the cells were grown in culture medium containing glucose or fructose as fermentable substrates.
However, the preservative resistance of the yeast is influenced by glucose level, with maximum resistance obtained at 10 - 20% (w/v) sugar concentrations.
As "Z. bailii" is moderately osmotolerant, the salt and sugar levels in foods are usually insufficient to control its growth.
The highest tolerance to salt has been observed at low pH values, e.g.
the maximum NaCl allowing growth was 12.5% (w/v) at pH 3.0 whereas this was only 5.0% (w/v) at pH 5.0.
Moreover, the presence of either salt or sugar has a positive effect on the ability of "Z. bailii" to initiate growth at extreme pH levels, e.g.
the yeast showed no growth at pH 2.0 in the absence of NaCl and sucrose, but grew at this pH in 2.5% (w/v) NaCl or 50% (w/v) sucrose.
Most facultatively fermentative yeast species cannot grow in the complete absence of oxygen.
That means limitation of oxygen availability might be useful in controlling food spoilage caused by fermentative yeasts.
However, it has been observed that "Z. bailii" is able to grow rapidly and ferment sugar vigorously in a complex medium under strictly anaerobic condition, indicating that the nutritional requirement for anaerobic growth was met by the complex-medium components.
Therefore, restriction of oxygen entry into foods and beverages, which are rich in nutrients, is not a promising strategy to prevent the risk of spoilage by this yeast.
Besides, Leyva et al.
(1999) have reported that "Z. bailii" cells can retain their spoilage capability by producing a significant amount of gas even in non-growing conditions (i.e.
presence of sugars but absence of nitrogen source). 
According to Thomas and Davenport (1985), early reports of spoilage in mayonnaise and salad dressing due to "Z. bailii" date back to the beginning of the 20th century.
More detailed investigations in the 1940s and 1950s confirmed that "Z. bailii" was the main spoiler in cucumber pickles, sundry pickled vegetable mixes, acidified sauces, etc.
Around the same time, fermentation spoilage incidents occasionally appeared in fruit syrups and beverages preserved with moderate benzoic acid levels (0.04 - 0.05% (w/w)).
Again, "Z. bailii" was identified as the spoilage source.
Nowadays, despite great improvements in formulation control, food processing equipment and sanitation technologies (e.g.
automated clean-in-place), the yeast remains highly problematic in sauces, acidified foods, pickled or brined vegetables, fruit concentrates and various non-carbonated fruit drinks.
"Z. bailii" is also well recognized as one of the main spoilers in wines due to its high resistance to combinations of ethanol and organic acids at low pH.
Furthermore, the spoilage by this yeast has been expanding into new food categories such as prepared mustards, fruit-flavoured carbonated soft drinks containing citrus, apple and grape juice concentrates.
The ability of "Z. bailii" in spoiling a wide range of foods is a reflection of its high resistance to many stress factors.
Therefore, it has been included in the list of most dangerous spoilage yeasts by several authors.
Spoilage by "Z. bailii" often occurs in acidic shelf-stable foods, which rely upon the combined effects of acidity (e.g.
vinegar), salt and sugar to suppress microbial growth.
The spoiled foods usually display sensorial changes that can be easily recognized by consumers, thus resulting in significant economic losses due to consumers’ complaints or product recalls Observable signs of spoilage include product leakage from containers, colour change, emission of unpleasant yeasty odours, emulsion separation (in mayonnaises, dressings), turbidity, flocculation or sediment formation (in wines, beverages) and visible colonies or brown film development on product surfaces.
The specific off-flavour that has been attributed to "Z. bailii" is related to H2S.
In addition, the taste of spoiled foods can be modified by the production of acetic acid and fruity esters.
It has been reported that growth of "Z. bailii" also results in significant gas and ethanol formation, causing a typical alcoholic taste.
The excessive gas production is a direct consequence of high fermentable ability of this yeast and in more solid food, gas bubbles can appear within the product.
Under extreme circumstances, the produced gas pressure inside glass jars or bottles can reach such a level that explosions may take place, creating an additional hazard of injuries from broken glass.
It should be mentioned that in general, detectable spoilage by yeasts requires the presence of a high number of cells, approximately 5 - 6 log CFU/ml.
Apart from spoiling foods, as a direct consequent of growth, "Z. bailii" can modify the product texture and composition such that it may be more readily colonized by other spoilage microorganisms.
For example, by utilizing acetic acid, the yeast can raise the pH of pickles sufficiently to allow the growth of less acid-tolerant bacteria.
Besides, as with other yeasts, the concentration of fermentable sugar in a product affects the rate of spoilage by "Z. bailii", e.g.
the yeast grows faster in the presence of 10% (w/w) than 1% (w/w) glucose.
Particularly, "Z. bailii" can grow and cause spoilage from extremely low inocula, as few as one viable cell in ≥ 10 liters of beverages.
That means detection of low numbers of yeast cells in a product does not guarantee its stability.
No sanitation or microbiological quality control program can cope with this degree of risk.
Hence, the only alternatives would be reformulation of food to increase the stability and/or application of high-lethality thermal-processing parameters. 
Drying is a method of food preservation that works by removing water from the food, which inhibits the growth of bacteria and has been practiced worldwide since ancient times to preserve food.
Where or when dehydration as a food preservation technique was invented has been lost to time, however the earliest known practice of food drying is 12,000 B.C.
by inhabitants of the modern Middle East and Asia regions.
A solar or electric food dehydrator can greatly speed the drying process and ensure more consistent results.
Water is usually removed by evaporation (air drying, sun drying, smoking or wind drying) but, in the case of freeze-drying, food is first frozen and pressure is reduce to a level that sublimation will take place then the water is removed.
Bacteria, yeasts and molds need the water in the food to grow, and drying effectively prevents them from surviving in the food.