Milk Treatment

A. Removal of Undesirable Microorganisms

The fermentation of the milk itself will often inhibit the growth of many undesirable bacteria (spoilage or pathogenic bacteria), but in order to be sure and to avoid disadvantageous bacterial enzymatic activities, many producers prefer to inactivate or remove contaminating flora. This may prolong the shelf life of the cheese and protect consumers from illness. Inactivation is commonly performed by heat treatment (pasteurization) of the milk (e.g., 70-72°C [158-162°F] for 15-20 sec) using a plate heat exchanger. However, spores in particular may survive the heat treatment and cause severe problems during cheese ripening, particularly spores of Clostridium tyrobutyricum. The spores grow during cheese ripening and produce H2 and CO2, which may cause ''blowing'' of the cheese as well as butyric acid and other fermentation products with unpleasant smell. Methods such as bactocentrifugation or microfiltration can be used to overcome this problem. The bacto-

Fermentation Milk Produce Cheese
Figure 1 Process flow in production of hard and semihard cheese. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

centrifuge can separate bacteria and spores from the milk. Depending on the number of outlets on the apparatus top, they are classified into two types:

The two-phase bactocentrifuge has two outlets at the top: one for continuous discharge of bacteria concentrate and one for the bacteria-reduced phase.

The one-phase bactocentrifuge has only one outlet at the top: an outlet for the bacteria-reduced milk. (The bacteria concentrate is collected in the sludge space of the bowl and discharged at preset intervals.) (1)

Microfiltration is another method for separating bacteria and spores from the milk. In microfiltration a membrane with a pore size of approximately 1 micron is used to filter out bacteria from the milk. In order to avoid fouling the filter with milk fat, the milk is first separated into skim milk and cream, and only the skim milk is microfiltrated. The cream is heat-treated together with the retentate (e.g., at 118°C [244°F] for 4 sec). Afterward the cream is used for standardization of the milk (1).

B. Standardization

After the bacteria and spores have been removed, the milk is standardized by addition of cream after the separator by in-line mixing or by mixing with skim milk in tanks, followed by pasteurization. The composition of the milk varies over the year, influencing coagulation and syneresis. This can be overcome by the addition of calcium chloride (5-20 grams/ 100 kg); however, this is not always legal.

III. CURD MANUFACTURING A. Addition of Starter Culture

Fermentation of the lactose to lactic acid gives cheese curds the desired pH. This fermentation is due mainly to addition of mesophilic lactic acid bacteria (e.g., Lactococcus lactis subsp. lactis and Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis biovar diacetylactis, Leuconostoc lactis and Leuconostoc mesenteroides subsp. cremoris) or thermophilic bacteria (e.g. Streptococcus thermophilus, Lactobacillus helveticus, Lactobacillus delbrueckii subsp. lactis and Lactobacillus delbrueckii subsp. bulgaricus) or by a combination of both. The activity of the starter cultures is very important in determining the different steps and their duration. The time used for milk ripening will be 30 min for most cheeses, during which only small amount of lactose is metabolized.

The starter cultures can be propagated locally as a mother culture and intermediate cultures before being added to the cheese milk or it can be added as a frozen or lyophilized concentrated culture directly to the cheese milk. Often the starter culture is added at the same time as the tanks are filled with milk in order to obtain uniform distribution and to quickly initiate the acidification as fast as possible. Acidification can be performed in open vats (Fig. 2); however, large modern tanks are closed and equipped with HEPA filters (in order to avoid contamination with bacteriophage), an automatically operated whey strainer, spray nozzles for proper distribution of rennet, and spray nozzles connected to cleaning systems (Fig. 3). All the tanks and tubes are designed so that they include an efficient cleaning-in-place (CIP) system, crucial to avoid bacteriophages. Today, tanks containing up to 22,000 liters are used.

Curd Treatment Starter Tank

Figure 2 Conventional cheese vat with tools for cheese manufacture. 1, Jacketed cheese vat with beam and drive motor for tools; 2, stirring tool; 3, cutting tool; 4, strainer to be placed inside the vat at the outlet; 5, whey pump on a trolley with a shallow container; 6, prepressing plates for round-eyed cheese production; 7, support for tools; 8, hydraulic cylinders for prepressing equipment; 9, cheese knife. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

Figure 2 Conventional cheese vat with tools for cheese manufacture. 1, Jacketed cheese vat with beam and drive motor for tools; 2, stirring tool; 3, cutting tool; 4, strainer to be placed inside the vat at the outlet; 5, whey pump on a trolley with a shallow container; 6, prepressing plates for round-eyed cheese production; 7, support for tools; 8, hydraulic cylinders for prepressing equipment; 9, cheese knife. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

B. Addition of Rennet

Coagulation of milk casein is critical for proper curd formation, and it is achieved by the addition of rennet. The characteristic rennet enzymes are chymosine and bovine pepsin, which can be added either as purified standardized enzyme or as an extract of the fourth stomach of calves. The optimal temperature for rennet activity is approx 42°C (107°F). In cheesemaking, normal renneting temperature is around 30°C (86°F); renneting normally takes 20-30 min.

C. Cutting of the Curd

Renneting is followed by the cutting of the curd. This is a very important and effective procedure for whey release. When the coagulum has reached the required degree of firmness, it is carefully cut up and separated into small pieces (cheese grains) by tools equipped with knife blades or wires. By this separation, significantly better drainage of the whey inside the cheese grains is obtained (short distance to the surface). The cutting tools can be designed in different ways. Conventional open cheese vats are normally equipped with exchangeable pairs of tools for stirring and for cutting (Fig. 2). In a modern enclosed cheese-

Vibratory Strainer Cheese

Figure 3 Horizontal enclosed cheese tank with combined stirring and cutting tools and hoisted whey drainage system. 1, Combined cutting and stirring tools; 2, strainer for whey drainage; 3, frequency-controlled motor drive; 4, jacket for heating; 5, manhole; 6, CIP nozzle. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

Figure 3 Horizontal enclosed cheese tank with combined stirring and cutting tools and hoisted whey drainage system. 1, Combined cutting and stirring tools; 2, strainer for whey drainage; 3, frequency-controlled motor drive; 4, jacket for heating; 5, manhole; 6, CIP nozzle. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

making tank, stirring and cutting are done with dual-purpose tools (cut or stir) depending on the direction of rotation (Fig. 3) (1). The cutting commonly takes about 10-15 min.

D. Stirring

After the cutting is finished, the curd is handled gently so that the cheese grains are not broken apart, which leads to loss of the fat in the whey. Stirring is done to assist in whey release (also named syneresis); the mechanical effect of stirring causes the grains to collide, and the pressure that results from the collision causes whey to be pressed out of the grains.

Obviously the effect is increased with a higher stirring speed and a decreasing distance between the grains (increasing drainage degree) (2). Stirring proceeds for one to two hours.

For some cheese varieties, a large portion of the whey is drained off before scalding is performed—for example, for semihard cheeses such as Gouda and Danbo, which requires about 15 min. Draining equipment is shown in Figs. 2 and 3.

E. Heating/Cooking/Scalding

The purpose of heat treatment is to regulate the pH and the moisture content of the final cheese. An increase of the cooking temperature will inhibit the growth of the starter culture, resulting in a higher cheese pH. Heating to temperatures above 40°C (104°F), is typically called cooking, and heating beyond 44°C (111°F) is called scalding). Also, the velocity of the heating influences whey release. Ordinarily, high velocity (short heating time) will result in a minor whey release and vice-versa. A high velocity increases the risk for blockage in the channel system of the cheese grains. The water-binding ability for the bound water in the curd is temperature dependent; water-binding ability decreases with increasing temperature, and vice-versa. Thus, high temperature will result in low moisture content of the cheese and vice versa. A too-low cooking temperature will cause the cheese to become soft and sour, while a too high temperature may result in cheeses that are dry and rubbery. For some cheeses, heating may only require adjusting the temperature to the original level; for other cheeses, a considerable temperature increase is necessary. The cooking of semihard cheeses made with mesophilic starter cultures is typically done at 34-40°C (93°-104°F) depending on the fat content, whereas scalding for hard cheeses made with thermophilic cultures is done at 50-56°C (122-133°F). In general, it can be said that it is best to heat gradually, approximately 1°C (1.8°F) in the first 5 min, 2°C (3.6°F) in the next 5 min, etc. The final temperature also has a significant influence on the later forming of the curd. The lower the temperature at which the cheese is formed, the more difficult it is to achieve proper pressing and to get the cheese grains to stick together (2). Depending on the type of cheese, heating can be performed in the following ways:

By introducing steam into only the vat/tank jacket

By introducing steam into the jacket in combination with the addition of hot water to the curd/whey mixture

By hot water addition to the curd/whey mixture only

Heating/ cooking is often ended by a final stirring that mechanically assists syneresis. The process, from the beginning of the heating to the start of the molding, takes about one hour for many types of cheeses, although, it can vary from 30-90 min.

During heating, the amount of lactose in the curd is regulated by the addition of water or the draining of the whey. Increased water addition decreases the concentration of lactose, resulting in a higher pH minimum. Addition of water also increases the moisture content. Therefore, water addition during the cooking of the cheese in the vat increases both the moisture content and the pH value of the cheese. If the water addition is exaggerated, however, a soft and foul-tasting cheese may result (3,4).

IV. MOLDING AND PRESSING OF THE CHEESE A. Molding of the Cheese

After heating/cooking and stirring, the temperature gradually decreases, except for a few pasta filata-type cheeses. When the curd has reached its desired firmness and acidity, the

Figure 4 Cheese with granular texture. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

residual whey has to be removed and the cheese shaped. This is done via molding. Depending on how this is done, the final texture of cheese can be varied. If the curd grains are pressed under the whey, the grains will stick closely together and the texture will be very close and uniform. If the grains are separated from the whey before pressing, the grains will be intermixed with air, so that they cannot stick together and the resulting texture will be more open and granular. During ripening, the CO2 produced from the citrate metabolism by Lactococcus lactis subsp. lactis biovar diacetylactis, Leuconostoc lactis, and Leuconostoc mesenteroides subsp. cremoris, or produced from the lactate metabolism by Propionibacte-rium, will make the holes/eyes in the cheeses. Molding is in general made in the following ways (1):

1. The curd grains are transferred directly to molds, producing open-texture cheeses (soft and granular texture cheeses (see Fig. 4).

2. The curd grains are gently prepressed under the whey before molding, producing cheese with a closed texture (round-eyed cheese) (see Fig. 5).

Figure 5 Cheese with round eyes. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

Figure 6 Closed-texture cheese with typical mechanical holes. (From Ref 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

3. The cheese curds are drained and salted before being piled and continually repiled to squash the curd, producing cheese with a closed texture, as shown for Cheddar in Fig. 6.

4. The cheese curds are drained before milling, warmed, kneaded and stretched before molding, producing a cheese with a compact texture (pasta filata cheese).

Procedures 1-4 in detail:

1. The whey and curd mixture is pumped across a vibrating or rotating strainer, where the grains are separated from the whey and discharged directly into molds (Fig. 7). Another method is to withdraw whey directly from manually operated open cheese vats.

2. The pressure during prepressing is applied gradually and should be at least 1-1.5 times the weight of the cheese curd at the end. The pressure is normally sustained

Figure 7 Curd and whey are separated in a rotating strainer. 1, Curd/whey mixture; 2, drained curd; 3, whey outlet. (From Ref. 1, courtesy of Tetra Pak Processing Systems AB, Sweden.)

Was this article helpful?

0 0
Bread Making

Bread Making

Discover How To Surprise Family and Friends With Homemade Bread? Is Your Bread Coming Out Doughy Or Crumbly? Well, you don't have to be frustrated anymore by baking bread that doesnt rise all of the way or just doesn't have that special taste.

Get My Free Ebook


Post a comment