Cutting The Curd
Proper cutting is extremely important to both quality and yield. Improper cutting and handling the curd results in the loss of fines, that is, small curd particles which are not recovered in the cheese. Unlike whey fat, fat trapped in fines; is not recovered by whey cream separation. Therefore, both fat and protein losses occur when shattered curd results in fines too small to be recovered in the cheese.
Determination of curd cutting time
Both early cutting when the curd is fragile and late cutting when the curd is brittle cause losses of fines. Several means are used to determine cutting time.
- Manual testing. The curd is ready to cut if it breaks cleanly when a flat blade is inserted at 45o angle to the surface and then raised slowly.
- Several mechanical devices based on oscillating viscometry, thermal conductance and sonication have been tested experimentally.
- Some plants cut by the clock. This may be OK as long as all conditions are uniform from day to day (is that every true??) and adjustments are made for any change in milk composition or properties.
- If setting temperature is high as for Swiss types, the curd firms rapidly and cutting must begin early when curd is still somewhat soft to prevent over setting. Agitation should begin immediately to prevent matting.
Curd size has a great influence on moisture retention. Hence, there is an obvious relationship between cheese moisture and the prescribed curd size:
- High temperature and low moisture varieties such as Italian hard cheese require the smallest curd. Cutting continues until the curd cutting is the size of rice grains.
- Medium moisture cheeses like most washed varieties and Cheddar are cut to Omega cm cubes.
- High moisture varieties like soft ripened cheese are cut with 2 cm knives or the curd is simply broken sufficiently to be dipped into forms.
Small curd size will result in greater fat and SNF recovery because large curds tend to get crushed resulting in the loss of 'fines'. Smaller curds will also dry out faster and, therefore, other factors such as cooking temperature and stirring out may have to be adjusted according to curd size.
Manual cutting is done with cutting harps, made by stretching stainless steel wire over a stainless steel frame. Total cutting time should not exceed 10 minutes (preferably less than 5 minutes) because the curd is continually changing (becoming overset) during cutting. The knives should be pulled (not pushed) quickly through the curd so has to cut the curd cleanly.
With mechanical knives, curd size is determined by the design of the vat and agitators, the speed of cutting (rpm) and the duration of cutting. In Double 'O' vats for Cheddar and American varieties, cutting is normally at a speed of about 4 rpm for 7 - 13 minutes, corresponding two a total of 30 to 50 revolutions. It is important that the knives are sharp and cut the curd cleanly rather than partially mashing the curd or missing some pieces altogether.
There is evidence (Johnston et al 1991, J. Dairy Res. 58:345) that curd particle size at draining in mechanized Cheddar cheese is influenced by cutting time, cutting speed, and subsequent agitation such that:
- Short cutting times and low rpm result in small particle size at draining and larger losses of fines.
- With increasing cutting time (more total revolutions), curd particle size at draining reaches a maximum which corresponds to a maximum in fat recovery.
- Further increased cutting time causes decreased curd size at draining with little effect on fat recovery.
Curd should be agitated gently or not at all after cutting to prevent formation of fines. The exterior of the freshly cut curd is fragile so some time is needed for the edges to close up (heal) and prevent the loss of fat and protein to the whey.
An index of cutting quality
The loss of fines is best monitored by accurate analysis of whey fat content. Whey fat for Cheddar types should be <0.3%;. Efficient operations may achieve levels near 0.2%.