Lab 5:
Enumeration of yeasts and
moulds in foods
Fungi
Fungi
Eukaryotic
Aerobic
Slow growth
Grow in both
acidic and basic
condition
Filamentous Mycelia
 Fungi kingdom (yeasts and molds)
-Subkingdom: based on septation of mycelia
- Phylum: based on ability to produce and type
of sexual spores
Molds
Yeasts
unicellular
multicellular
non-filamentous
filamentous
Enumeration of yeasts and moulds in foods
 Application
• This method is applicable to the
enumeration of viable yeasts and moulds in
foods and food ingredients.
• It may also be used to confirm the viability
of apparent yeast and mould material
scraped from food plant equipment and the
manufacturing environment.
Media
 Acidified Potato Dextrose
Agar
 PDA (nonselective) + tartaric acid
• low pH inhibits most bacteria & permits
growth of fungi.
• Note: Some acid-tolerant bacteria can
still grow leading to an over-estimate of
fungi count
Media
Antibiotic Plate Count Agar
 PCA (nonselective) + antibiotics
(chloramphenicol & tetracycline)
• antibiotics prevent bacterial growth by
inhibiting protein synthesis
Media preparation
1. Label empty plates
3. Pour plates
PCA
PDA
Antibiotic
Tartaric acid
50C
2. Add selective agents
2 ml
Ab-PCA
1.85 ml
Acid-PDA
Y/M period 2
Mon,Oct 7
1 ml
1 ml
Peptone
water 9 ml
100
10-1
10-2
Acidified PDA
0.1 ml
each
Antibiotics PCA
Y/M petrifilm
1 ml
each
Defination of terms
 Scrapings: Suspected yeast and mould material scraped from
food plant equipment and the manufacturing environment.
 Xerophilic: Moulds capable of growing at reduced water
activity (aw). (Yeasts preferring reduced aw are also
sometimes referred to as xerophilic.)
 Osmophilic: Yeasts preferring high osmotic pressures for
growth.
Materials and special equipment
 The following media and reagents (1-8) are commercially available and are to be
prepared and sterilized according to the manufacturer's instructions.
 Enumeration of yeasts and moulds in foods (not specified below)
These agars are suitable for fresh foods (fruit, vegetables, meat and dairy).
1. Dichloran rose bengal chloramphenicol agar (DRBC)
2. Plate count agar with chloramphenicol (PCA-C)
3. Potato dextrose agar with chloramphenicol (PDA-C)
4. Potato dextrose salt agar with chloramphenicol (PDSA-C)
(for analysis of 'spreader' moulds).
Materials and special equipment
Enumeration of xerophilic yeasts and moulds in grains, flours, nuts, and
spices
5. Dichloran-glycerol DG 18 agar (DG-18)
Enumeration of xerophilic yeasts and moulds in jams, jellies, fruit
concentrates, and dried fruits
6. 20% sucrose (diluent additive for osmophiles)
7. Malt extract agar containing 50% (w/w) sucrose
Materials and special equipment
Other:
8. Peptone water (0.1%) (PW)
9. 2% sodium citrate tempered to 45oC (diluent for high fat foods, such
as cheese) (optional)
10. 1N HCl and 1N NaOH
11. Gram stain solutions
12. Stomacher, blender or equivalent
13. pH meter or paper capable of distinguishing to 0.3 to 0.5 pH units
within a range of 5.0 to 8.0
14. Light microscope
15. Colony counting device (optional)
16. Incubator (darkened) capable of maintaining 22 to
25oC, 55oC water bath (and 45oC water bath if sodium citrate is
to be used).
Procedure
 Each sample unit shall be analyzed individually.
 The test shall be carried out in accordance with the
following instructions:
 Handling of Sample Units and Scrapings
1. During storage and transport, the following shall apply: with the exception
of shelf- stable products, keep the sample units refrigerated (0-5oC). Sample
units of frozen products shall be kept frozen.
2. Thaw frozen samples in a refrigerator or under time and
temperature conditions which prevent microbial
growth or death.
3. Analyze the sample units as soon as possible after
receipt at the laboratory.
Procedure
 Preparation of Medium
1. Prepare the appropriate media for the analysis being carried out .
NOTE: DRBC agar should not be exposed to light, since photo-degradation of
rose bengal produces compounds that are toxic to fungi.
2. Temper melted agar in a 55oC water bath, ensuring that the water level is 1
cm above the level of the medium in the bottles.
3. Clean surface of working area with a suitable disinfectant.
4. Mark clearly the duplicate petri plates identifying sample, sample unit,
dilution and date of inoculation.
Procedure
 Preparation of Dilutions
1. Prepare 0.1% peptone water as diluent.
• An appropriate solute, such as 20% sucrose, should be added
to the diluent when enumerating osmophiles in foods such
as syrups and fruit juice concentrates.
• In addition, a 2% solution of sodium citrate, pre-warmed to
45°C, can be used as diluent for high-fat foods such as
cheese.
2. To ensure a representative analytical portion, agitate liquid or
free flowing materials until the contents are homogeneous.
• If the sample unit is a solid, obtain the analytical unit by
taking a portion from several locations within the sample
unit.
Preparation of Dilutions
3. Some degree of soaking may be beneficial for the recovery of yeasts and
moulds from dried or intermediate-moisture foods.
• Soaking may allow for the repair of sub-lethally damaged cells
(resuscitation).
• Rehydrate dried foods for 1 h with an equal amount of distilled water or
peptone water and store at room temperature.
4. Prepare a 1:10 dilution of the food by aseptically blending 25 g or mL (the
analytical unit) into 225 mL of the required diluent. If a sample size other
than 25 g or mL is used, maintain the 1:10 sample to dilution ratio, such as 11
(10) g or mL into 99 (90) mL.
Preparation of Dilutions
5. Stomach, blend or shake according to the type of food .
• Blend or stomach for the minimum time required to produce a
homogeneous suspension.
• To prevent over-heating, blending time should not exceed 2.5 min.
• With foods that tend to foam, use blender at low speed and remove
aliquot from below liquid/foam interface.
6. Verify the pH of the suspension. If the pH is not between 5.5 and 7.5,
adjust the pH to 7.0 with a sterile solution of 1N NaOH or 1N HCl.
Plating
1. Agitate each dilution bottle to resuspend material that may
have settled out during preparation.
2. Moulds should be enumerated by a surface spread-plate
technique rather than with pour plates.
• This technique provides maximal exposure of the cells to
atmospheric oxygen and avoids heat stress from molten agar.
• Spread 0.1 mL onto duplicate plates.
Incubation
• Incubate plates undisturbed in an upright
position at 22 to 25oC for 3-5 days.
• Incubate plates in the dark.
• Normally, count colonies on plates after 5 days.
• Handle the plates as little as possible when
counting on day 3 so spores will not be
dislodged, which may result in secondary
growth.
Counting Colonies and Examining Growth
1. Count colonies, distinguishing, if required, yeast
colonies from mould colonies, according to their
colonial morphology.
• Microscopic examination with crystal violet stained
smears may be necessary to distinguish yeast colonies
from some bacterial colonies that may look like yeast.
2. If possible, select plates with 10-150 colonies.
• Determine the identity of pin-point colonies
microscopically.
• If counts do not fall within this range, select plates that
fall nearest to the 10-150 range.
Counting Colonies and Examining Growth
3. If plates contain colonies which spread, select a representative portion of
the plates free from spreaders, if possible, and count colonies in this area.
• The total count of the whole plate is estimated by multiplying the count
for the representative area by the reciprocal of the fraction of the plate
counted, e.g., 30 colonies counted on 1/4 of the area of the plate; count
for the whole plate: 30 x 4 = 120 colonies.
• Results are expressed as an estimated count.
4. Wet mounts and gram stains of several diverse types of cells per sample
should be examined to confirm that bacteria are not present.
• Yeast cells and asexual mould spores are generally
gram-positive, whereas mould mycelia are gram-negative.
Table 1
Type of Food
Preparation*
Treatment
Liquids:
milk, water
etc.
pipette directly into Petri dishes and/or
into peptone water diluent
Shake
viscous lipids
weigh into peptone water diluent
Shake
Water Solids
soluble weigh into peptone water diluent
Shake
powder,
meats
weigh into peptone water diluent
Stomach
or Blend
powder,
meats
weigh into previously warmed (45oC) 2% sodium
citrate(Na3C6H5O7.2H2O)
Stomach
or Blend
Spices
weigh into diluent
Shellfish
weigh into peptone water diluent
Solids:
Shake
Stomach
or Blend
Note/
*Sample may be
weighed into a
stomacher or
blender jar with
the diluent
added prior to
mixing.
END OF LECTURE