The Role of Starter Culture and Enzymes/
Rennet for Fermented Dairy Products
Manufacture- A Review
Assefa Bezie* and Hailu Regasa
Milk and Milk Product Research Processing Industry Development, Ethiopian Meat and Dairy Industry Development Institute (EMDIDI), Ethiopia
Submission: June 21, 2019; Published: August 08, 2019
*Corresponding author: Assefa Bezie, Milk and Milk Product Research Processing Industry Development Directorate. Ethiopian Meat and Dairy Industry Development Institute (EMDIDI), P.O.Box: 1573, Debre Zeit,Ethiopia
How to cite this article: Assefa Bezie, Hailu Regasa. The Role of Starter Culture and Enzymes/Rennet for Fermented Dairy Products Manufacture- A Review. Nutri Food Sci Int J. 2019. 9(2): 555756. DOI:10.19080/NFSIJ.2019.09.555756.
This review is concerning the role of starter culture and enzymes/rennet for fermented dairy product manufacture, types and sources of enzymes and starter culture, factors affects the activity of both starter cultures and rennet. Fermented milk products are known for their taste, nutritive value and therapeutic properties. Dairy starters are the ‘heart’ of fermented milk products, the most crucial component in the manufacture of high-quality fermented milks. In the dairy industry, some enzymes are required for the production of cheeses, yogurt, and other dairy products, while others are used in a more specialized fashion to improve texture or flavor. Starters are obtained by using traditional and growth media methods. Rennet is an extract from the fourth stomach (abomasum or rennet-bag) of ruminant animals, principally calves and adult cattle, with the capability of clotting milk by enzymic action. Starter cultures can be classified into mesophilic cultures, which grow best at 25-30 ℃, and thermophilic cultures, which grow at higher temperatures (37-45 ℃). Rennet (chymosin and pepsin), Proteases, lactase and catalase are some indigenous enzymes that used for fermented dairy products manufactured. Starter culture changes the milk sugar, lactose, into acid (lactic acid) so that the milk becomes sour (fermented or cultured). Starter cultures of LAB are responsible for the formation of cheese flavor. Main role of starter cultures is to produce acid during manufacture and also contribute to the ripening process. Rennets (rennin, a mixture of chymosin and pepsin obtained mainly from animal and microbial sources) are used for coagulation of milk in the first stage of cheese production. Rennet is an essential clotting agent used during the manufacture of many cheeses. Its main function is to coagulate milk proteins such as casein. Rennet is mainly used in hard cheese making and little is used in the manufacture of soft cottage cheese or fromage frais. heat treatment, Quantity of additives / sanitizing chemicals, culture, saltwater, rennet and Quantity of additives /culture, salt, water, rennet/ are the main factors which affects the activity of starter cultures and enzaymes/rennet.
Raw milk for processing generally comes from cows, but occasionally from other mammals such as goats, sheep, and water buffalo . Milk is processed into different dairy
products. So that, several dairy products such as cream, butter,
yogurt, kefir, and cheese have been produced and consumed worldwide for millennia . Dairy products are generally defined as food products that are produced from milk. They are rich sources of energy . The inclusion of dairy products adds diversity to plant-based diets. However, the role of milk and dairy products in human nutrition has been increasingly debated in recent years, both in the scientific literature and in popular science literature . Consequently, the quantity and types of dairy products manufacture in the world becomes increased (Figure 1).
The main dairy product is fresh milk (32.3 million tons/year), followed by cheese (4.1 million tons/year), yogurt
and other fermented milks (2.8 million tons/year), small quantities of butter (0.8 million tons/year) and cream (0.5 million tons/year) . Yoghurt and fermented milks
havereceived considerable attention as carriers of live
probiotic cultures . Besides, so many kinds of fermented
milk and milk products are manufactured by using starter
cultures in the dairy industry with their known properties.
Fermented milk products are known for their taste,
nutritive value and therapeutic properties. Fermented
milks are products prepared from milks, entire, mostly or
completely skimmed, concentrated or milk substituted from
partially or fully skimmed dried milk, somewhat or completely
skimmed pasteurized or sterilized and fermented by means
of specific microorganisms [7,8]. Milk products also serve as
the important delivery vehicles for probiotic bacteria. The
probiotic bacteria have a long history of association with dairy
products . Now a day, milk processors used dairy starter and
enzymes/rennet for dairy product production/manufacture.
Dairy starters are the ‘heart’ of fermented milk products,
the most crucial component in the manufacture of high-quality
fermented milks. The cultures are harmless food-grade
microorganisms, such as active bacteria, that are intentionally
grown in milk or whey or other formulated media to impart
desirable and predictable flavor and texture to fermented milk
products. The microorganisms employed in milk fermentation
are single-strain or multiple-strain cultures of lactic acid
bacteria, producing different types of fermented milk products
The leading factors in the cheese-making process are the
lactic bacteria. They are necessary for the lactic fermentation:
they transform lactose (the milk sugar) into lactic acid; this
acidification process helps the rennet’s work, the draining
of the whey after the curd has been broken and the cheese
In the dairy industry, some enzymes are required for
the production of cheeses, yogurt, and other dairy products,
while others are used in a more specialized fashion to improve
texture or flavor . Therefore; the objective of this paper is
to identify The Role of Starter Culture and Enzymes/Rennet
for the Manufacture of Dairy Products, determine the types
of starter culture and Enzymes that used for fermented dairy
product production and to differentiate sources of Starter
Culture and Enzymes/Rennet.
It can bed efined as one or more strains of one or more
species of desirable bacteria used to inoculate a raw or
pasteurized product to start a fermentation to produce a
fermented food by accelerating and steering its fermentation
Starter cultures are also those microorganisms (bacteria,
yeasts, and molds or their combinations) that initiate and
carry out the desired fermentation essential in manufacturing
cheese and fermented dairy products .
Rennet and rennin are general terms for any enzyme
used to coagulate milk. Technically rennet is also the term
for the lining of a calf's fourth stomach. Animal rennet is a
milk clotting enzyme isolated from calf stomachs. The major
components of rennet are Chymosin (EC220.127.116.11) but in
commercial preparations of rennet other proteases, typically
bovine pepsin, are found in various concentrations [12,13].
In addition, the terminology, knowing the sources of starter
cultures and enzymes/rennet are relevant for sustainable
fermented dairy products manufacture.
Milk is the usual growth medium for bulk starters, but
other media may also be used. These may contain nonfat milk
powder, phosphate salts, sodium citrate, dextrose, dextrin,
pancreatin, dried autolyzed yeast, lactose, and sucrose. Starter
cultures for fermented milk manufacture are also available in
freeze-dried or frozen concentrated form, and either as directvat-
set (DVS) type or as cultures for bulk starter production.
The popularity of DVS type cultures is increasing. Application
of DVS cultures eliminates the risk of phage contamination
during starter preparation in the plant and ensures
appropriate strain balance . Starters are obtained by using
traditional and growth media methods.
Traditionally, a starter is obtained via growth of lactic
acid bacteria in milk at a suitable temperature. The starter
is subsequently maintained by propagating and growing it
in a fresh portion of milk. Currently, special growth media
other than milk are also utilized to avoid multiplication of
bacteriophages during starter manufacture .
Rennet is an extract from the fourth stomach (abomasum
or rennet-bag) of ruminant animals, principally calves and
adult cattle, with the capability of clotting milk by enzymic
action . A study by Moschpoulou  showed that Rennet
is the enzymatic preparation of two main acid proteolytic
enzymes (chymosin and pepsin) secreted in the fourth stomach
(abomasum) of unweaned ruminants (calves, lambs or kids).
Chymosin can also be obtained from several other animals,
microbial or vegetable sources, but indigenous microbial
chymosin (from fungi or bacteria) is ineffective for making
cheddar and other hard cheeses .
Milk contains scores of enzymes. The native or indigenous
enzymes are those known to be excreted by the mammary
gland. Most of these are synthesized by the secretory cells,
others derive from blood, for example, plasmin. Moreover,
several enzymes are present in the leukocytes, e.g., catalase.
Moreover, the native enzymes can be present at different
locations in the milk. Many of them are associated with the fat
globule membrane .
Diverse lactic starter cultures are used in the manufacture
of commercial fermented milk products in the world. They
can be classified into mesophilic cultures, which grow best
at 25-30 ℃, and thermophilic cultures, which grow at higher
temperatures (37-45 ℃)  (Figure 2).
Mesophilic cultures are widely used in the fermented milk
industry in the manufacture of products such as ‘filmjo¨lk’
and ‘lactofil’ (in Sweden) and ‘ymer’ (in Denmark). Mesophilic
starters will almost certainly contain Lactococcus lactis
subsp. cremoris, but rarely will this species be used alone.
Buttermilk, ‘la˚ngfil’, and ‘viili’, which are popular in Norway,
Sweden, and Finland,combine Lc. lactis subsp. cremoris with
Leuconostoc species. Other fermented milk products made
with mesophilic starters include sour cream, cultured butter
milk,and kefir . In addition, mesophilic cultures are used
for the production of stirred types of yoghurt.
Thermophilic starter cultures are used for the manufacture
of yogurt, Bulgarian buttermilk, and the whole range of
products made with intestinal bacteria, primarily lactobacilli
and bifidobacteria . Currently, thermophilic starter
cultures are used for the production of so many types of dairy
products such as set yoghurt and different variety cheese
(mozzarella cheese, provolone, feta, gauda e.t.c).In addition to
starter cultures, Different types of enzymes are applicable for
different purposes .
Are used to produce hydrolyzed whey protein, which is
whey protein broken down into shorter polypeptide sequences.
Hydrolyzed whey is less likely to cause allergic reactions and is
used to prepare supplements for infant formulas and medical
Lactase (-galactosidase, EC 18.104.22.168) is used to hydrolyze
lactose to glucose and galactose as a digestive aid and to
improve the solubility and sweetness in various dairy
products. Many people do not have sufficient lactase to digest
milk sugar. Lactose hydrolysis helps these lactose-intolerant
people to drink milk and eat various dairy products . It is
also used in the preparation of ice cream, to make a creamier
and sweeter tasting product. Lactase is usually prepared from
Kluyveromyces sp. of yeast and Aspergillus sp. of fungi .
The enzyme Catalase has found limited use in one
particular area of cheese production. Hydrogen peroxide
is a potent oxidizer and toxic to cells. It is used instead of
pasteurization, when making certain cheeses such as Swiss,
in order to preserve natural milk enzymes that are beneficial
to the end product and flavor development of the cheese.
These enzymes would be destroyed by the high heat of
pasteurization. However, residues of hydrogen peroxide in the
milk will inhibit the bacterial cultures that are required for the
actual cheese production, so all traces of it must be removed.
Catalase enzymes are typically obtained from bovine livers
or microbial sources and are added to convert the hydrogen
peroxide to water and molecular oxygen .
Lipases are used to break down milk fats and give
characteristic flavors to cheeses. The flavor comes from the
free fatty acids produced when milk fats are hydrolyzed.
Animal lipases are obtained from kid, calf, and lamb, while microbial lipase is derived by fermentation with the fungal
species Mucor meihei . the result agrees with the finding
of  who indicated that Lipases are used mainly in cheese
ripening for development of lipolytic flavors.
There are several types of fermented milk products and
each product is made with different lactic starter bacteria .
Dairy starter cultures are prepared cultures of microorganism
employed in the manufacture of a variety of dairy products
including butter, cheese, yoghurt and cultured milk. All dairy
starter cultures produce acids; the most important group of
microorganisms are the lactic acid bacteria which are included
in almost all dairy starter cultures .
Starter culture changes the milk sugar, lactose, into acid
(lactic acid) so that the milk becomes sour (fermented or
cultured). Lactic acid bacteria have much to offer within
food preservation and flavour generation . A finding by
Walstra et al.  revealed that Lactic acid bacteria are the
prime agents in producing soured (fermented) milk and dairy
products. A great majority of the flavour compounds produced
in yogurt result from the activity of microorganisms in starter
cultures. The predominant organisms in these starter cultures
are lactic acid bacteria (LAB), for example, Lactococcus
lactis, Lactobacillus species, Streptococcus thermophilus,
Bifidobacterium species, and Leuconostoc species .
Starter cultures of LAB are responsible for the formation of
cheese flavor. Several LAB are widely used and their role can
be divided into starters, and non-starters, including adjunct,
cultures. Main role of starter cultures is to produce acid during
manufacture and also contribute to the ripening process. Nonstarter
cultures do not responsible to the production of acid,
but they contribute more during ripening process [23,24].
Moreover, Starter cultures are important to produced different
milk/dairy products such as many cheeses, butter, and yoghurt
and to controlled preservation and fermentation processes.
According to Panesar  during fermentation, certain
physical and chemical changes occur in the milk due to the
growth and fermentative activities of lactic acid bacteria used
as starter cultures. The main role of lactic acid bacteria used as
starter during cheese production is the production of lactic acid
through metabolism of lactose. This action improves the milk
coagulation process, makes the curd stronger and protects the
final product against contamination . Berry  reported
that Lactic acid bacteria ferment milk’s inherent lactose
to lactic acid, providing desirable sour notes. By lowering
the product’s pH, shelf life is also extended, as the acidic
environment controls the growth of spoilage microorganisms.
Natural cheese making uses lactic acid produced by the
fermentation of lactic acid bacteria (LAB) to control the pH.
Lactic acid bacteria are Gram-positive, non-motile, and nonspore
forming . Yogurt taste is mainly characterized by an
acid character due to the presence of lactic acid in the product. Yogurt aroma is characterized by about hundred
volatile compounds that consist of carbonyl compounds
(mainly acids and esters), alcohols, and heterocyclic and sulfurcontaining
compounds. Among them, acetaldehyde is the major
flavor compound of yogurt, where it confers a pleasant fresh
and fruity aroma. It is produced by the lactic acid bacteria .
The acid produced during fermentation helps to form curds,
also called a gel, and contributes to syneresis of the curd,
removal of water held within the milk proteins .
In the meantime, many enzymes are responsible for dairy
Rennets (rennin, a mixture of chymosin and pepsin
obtained mainly from animal and microbial sources) are used
for coagulation of milk in the first stage of cheese production
. Milk coagulation properties (MCP) are an important
aspect in assessing cheese making ability . The production
process of cheese coagulation of milk proteins to ‘curds’ is
achieved by the proteolytic activity of rennet added to milk
. This confirms the study of Kevany et al.  who obtained
that the concentration of residual rennet in the cheese can
be used to indicate the potential proteolytic activity during
ripening. Rennet is an essential clotting agent used during the
manufacture of many cheeses. Its main function is to coagulate
milk proteins such as casein. Rennet is mainly used in hard
cheese making and little is used in the manufacture of soft
cottage cheese or fromage frais .
Rennet plays significant role in the sensory characteristics
of the produced cheese, because it contains also lipolytic
enzymes that release free fatty acids (FFA) during ripening.
Consequently, several cheeses have special sensory
characteristics that are attributed to the rennet used and
this is mainly the case of the cheeses made with rennet from
small ruminants although, commercial lipase preparations
from small ruminants are frequently used to enhance lipolysis
in cheese. Chymosin, pepsin A and gastriscin (or pepsin B
or pepsin C), which are secreted in the abomasum of young
ruminants, belong to the group of aspartic proteinases having
two residues of Asp in their active centre. These gastric
aspartic proteinases are secreted in the mucus of abomasum
in the form of zymogens and their action is to clot and digest
the milk .
Commercial rennet extracts used for the manufacture of
most cheese varieties are free from lipase activity. However,
rennet pastes used in the manufacture of certain hard Italian
cheese varieties, such as Provolone and the various Pecorino
cheeses, and often in traditional Greek feta, contain much
lipase activity . On the other hand, several factors inhibit
/affect the functionality or activity of starter cultures as
well as enzymes during fermented milk and dairy products
A number of factors may adversely affect the activity
of starters, leading to poor quality of fermented milks and
financial losses to the manufacturer. When lactic acid is not
produced by a starter culture at the desired rate, the culture
is called ‘slow’. The slowness can be due to either the genetic
makeup of the strains or extrinsic factors. The latter include,
among others, (1) bacteriophage, (2) residues of antibiotics and
sanitizing agents, (3) inhibitory compounds naturally found
in milk, (4) variations in milk composition due to mastitis or
seasonal factors, and (5) metabolites of spoilage bacteria. A
lower rate of acid production can also be caused by irregular
culture transfers, by fluctuations in incubation temperature,
and by over acidification .
Heat treatment is the most widely used processing
technology in the dairy industry. Its main purpose is to destroy
microorganisms, both pathogenic and spoilage, to ensure the
milk is safe and has a reasonable shelf-life [3,34,35]. Each
microorganism has a specific temperature for optimal growth
and production rates (g/L/h) . Temperature in the range
of 65-72 °C destroys 60-80 % of bacteria. The delay of growth
of the additional heat-treated culture is positive from the
perspective of rennet cheese production, as the additional
biomass of bacterial culture - though incapable of growth
at the initial stages of cheese making but possessing active
peptidolytic enzymes - may accelerate the ripening process
of cheese and prevent the accumulation of bitter peptides in
the cheese bulk that may be synthesized at the first stage of
ripening under the influence of rennin or proteinases bound
with a cell wall . A study by Simsek et al.  showed that
high temperature led to reduction of bacterial content, this
reduction has been effective in reducing the content of the
enzymes. Microbial enzymes derived from Rhizomucor miehei,
for example, are less heat-sensitive than other enzymes and
may be preferred for cheese operations with higher cooking
The time needed for fermentation may be as short as
2.5 h for the classic yogurt starter culture . As could be
expected on the basis of the chemical acidification reaction
that underlies the fermentation process, pH dropped during
the 3-5 hr to values 4.6. Moreover, it was observed that samples
produced by strains with low proteolytc activity had sigmoidal
pH decrease but starter cultures with high proteolytic activity
showed different acidification profiles and fermentation
times had been longer . The time required for the gel with
low rennet concentration to reach a medium firmness, as
determined by an experienced cheese maker, was found to be
50 min .
Increased the ratio of rennet in addition to the
recommendation with little amount is positively affects the
activity of rennet i.e. it speeds up the formation of curd/
coagulation during cheese manufactured). Depending on the
manufactured date and quantity of rennet, curd will be formed
from 30-45 minutes.
Many sanitizing chemicals, such as quaternary ammonium
compounds, iodine and chlorine compounds, retard acid
development by starter cultures. Colostrum and late lactation
milk contain non-specific agglutinins, which clump and
precipitate sensitive strains of the starter. The agglutinins may
possibly retard the rate of acid production by interfering with
the transport of lactose and other nutrients. Seasonal variation
of the solids-not-fat fraction of milk affects the growth and the
balance of strains in culture. Another cause of slow starters
may be attributed to antibiotics in milk. Concentrations as low
as 0.005-0.05 international units (IU) of antibiotics per mL
of milk, used in mastitis therapy, are high enough to impact
partial or full inhibition of the culture. Proper use of sanitizers
includes verification of sanitizer concentration as well as
removal of residual sanitizers from the pipes and vessels prior
to the use of equipment for processing of fermented dairy
The formation of the coagulum depends upon on the
quantity and type of rennet selected. Coagulation may be
inhibited by residual chlorine in the cheese vat; as little as 2
ppm of residual chlorine will destroy 40% of rennet activity
in 3 minutes. Typically, hard water (pH> 7.0) also decreases
rennet activity .
Starter cultures are perishable and have a finite shelf-life!
Freeze-dried cultures can be stored at 4ºC for longer than 12
months. Successful cheese making depends on healthy, active
starter bacteria which produce acid consistently from day to
day. To ensure that starter bacteria perform predictably; it is
important to handle and stores them correctly (Table 1).
Dairy products are generally defined as food products
that are produced from milk. The quantity and types of dairy
products manufacture in the world becomes increased. The
leading factors in the cheese-making process are the lactic
bacteria. They are necessary for the lactic fermentation:
they transform lactose (the milk sugar) into lactic acid; this
acidification process helps the rennet’s work, the draining
of the whey after the curd has been broken and the cheese
In the dairy industry, some enzymes are required for the
production of cheeses, yogurt, and other dairy products, while
others are used in a more specialized fashion to improve texture
or flavor. The production process of cheese coagulation of milk
proteins to ‘curds’ is achieved by the proteolytic activity of
rennet added to milk.
The author would like to thank Ms Betelhem Tarkegn for
her valuable support during writing this paper. Our glory
also going to Ethiopian Meat and Dairy Industry Development
Institute staff as a whole, particularly dairy sub-sector
professionals for their comments.
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