Food poisoning and meat microbiology - Gracey's Meat Hygiene - page 272

for the spoilage of refrigerated foods. Some thermophilic
species can grow at 50°C and higher but not below 30°C.
Although most favour moist conditions, some, for example,
Candida , Rhodotorula , Cladosporium , Fusarium , Mucor ,
Penicillium , Rhizopus , Thamnidium and Sporotrichum ,
are relatively tolerant to water availability (limit of a w
value for growth is 0.88-0.80) and can grow in a pH
lower than 4.5.
Moulds first appear and grow most prolifically on
the cut surfaces of the lean meat. Although the spores
of moulds may have a ubiquitous existence, often in
the air attached to dust particles, they cannot germi-
nate without moisture. The growth of moulds can be
prevented by low temperatures and attention to
humidity; thus, proper ventilation in refrigerating and
storage works is necessary so that circulating air may
dry the surface of food and containers. The control of
moulds in food products with chemicals is neither
approved nor successful; the concentration of chemi-
cal required to inhibit growth increases rapidly with
humidity. The chief causes of mould on meat are expo-
sure to dust and variations in temperature causing
condensation on the meat surface. Intermittent freez-
ing or temperature fluctuations in a refrigerating
chamber are common predisposing causes to mould
growth.

authorities note, tibia sours are much more common.
Too large a sticking wound also facilitates deep-seated
contamination, for the ham-souring bacteria appear to
resist the high bactericidal properties of pigs' blood.

Phosphorescence
Phosphorescence is caused by a number of organisms,
for example, Pseudomonas phosphorescens , which are
widely distributed in nature, especially in seawater,
and may contaminate a chilling room. These organ-
isms are resistant to chilling room temperatures and
their invasion of cold stores can be a matter of consid-
erable inconvenience. At the commencement of phos-
phorescence, which occurs in 7-8 hours when the
condition is artificially produced, the surface of the
meat, when seen in the dark room, shows luminous
areas scattered over its surface and appears as if it were
studded with stars. If decomposition develops in the
meat, the phosphorescence disappears. Salted or stored
meat may show various changes in colour due to bacte-
rial action. Scattered areas, reddish in colour and not
unlike beetroot juice, are caused by Serratia marces-
cens , and a similar superficial change, but blue in col-
our, is seen as a result of surface contamination
by Pseudomonas cyanagenus. Pseudomonas cutirubra
appears to be the primary cause of ' red mould ' on
charque, the dried salted beef of South America. Meat
affected with phosphorescence or abnormal surface
colouration is unsightly and repugnant, but if no
putrefactive changes are present, it may safely be dealt
with by trimming.

Black spot
This is the most troublesome affection of imported meat
and is caused by the mould Cladosporium herbarum .
Aureobasidium and Rhizopus may also be involved. It is
liable to attack quarters of chilled beef taken from ships
and placed in cold store at a temperature above −8°C;
some varieties grow at −7.5°C, while all grow well at
around 0°C. In beef, black spot is commonly found on
the neck, diaphragm and pleura, and in frozen mutton
on the legs, inside the neck or in the thoracic or abdomi-
nal cavities. The spots are about 6-13 mm in diameter
and occur on the surface of the meat. The dark colour is
due to the fungal threads in the superficial layers of the
meat, from which the mould derives the moisture neces-
sary for growth.
Black spot cannot be removed by gentle scraping with
a knife, and microscopical examination shows that the
threads of the fungus, dark green or olive in colour, are
interlaced between the fat cells in the connective tissue
on the surface of the carcase; they do not penetrate to a
greater depth than 3 mm, and the contiguous muscular
and connective tissues are perfectly normal. Black spot
may at times be accompanied by bacterial spoilage, when
decomposition is manifested by a softening, darkening
and sliminess of the carcase surface and is associated
with the growth of micro-organisms of the Achromobacter

Moulds
In contrast with yeasts and bacteria, moulds are read-
ily seen with the naked eye, appearing typically as
fluffy growths on old damp newspapers, walls, rot-
ting fruits, cheese, jam, etc. They can occur in various
colours, for example, white, black, green and blue.
Unlike bacteria and yeasts, they are multicellular and
typically consist of a mass (mycelium) of branched
filaments or hyphae which bear reproductive bodies
or spores. Along with yeasts, mildews, rusts, smuts
and mushrooms, they belong to the class Mycota or
fungi. Like bacteria, they are present everywhere and
are responsible for many beneficial and harmful
activities.
Moisture, temperature and organic matter are
important factors in determining the presence and
activity of moulds. The majority are mesophilic and
have an optimum growth temperature of 20-30°C, but
several types can grow at or just below 0°C, for exam-
ple, the so-called snow moulds and those responsible