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Effect of Various Cutting and Stirring Conditions on Curd Particle Size and Losses of Fat to the Whey during Cheddar Cheese Manufacture in Ost Vats
PII : S0958-6946(98)00050-8
Int. Dairy Journal 8 (1998) 281—288 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0958-6946/98/$19.00#0.00
Effect of Various Cutting and Stirring Conditions on Curd Particle Size and Losses of Fat to the Whey during Cheddar Cheese Manufacture in Ost Vats K. A. Johnstona,*, M. S. Luckmana, H. G. Lilley b and B. M. Smale b aNew Zealand Dairy Research Institute, Private Bag 11 029, Palmerston North, New Zealand bAlpine Dairy Products Ltd, P.O. Box 33, Temuka, New Zealand (Received 10 December 1997; accepted 17 April 1998) ABSTRACT Nine combinations of speed and duration of cutting the coagulum were varied during the commercial manufacture of Cheddar cheese in 27 Ost IV cheese vats. The effects of cut speed ramp-up rate, vat overfilling and stirring rate after cutting were also evaluated. The effects of the variations were assessed by determining the curd particle size distribution and fat losses to the whey at draining. The results of varying the speed and duration of cutting were compared with the results of a similar investigation carried out using vertical, double O, Damrow vats. Although the results showed similar trends, compared with the Damrow vat at the same speed the Ost vat’s cutting system reduces curd particles to a constant size more rapidly, thereby avoiding excessive curd shattering and high whey fat losses during subsequent stirring. Based on these results and comparisons with the results from the Damrow trial, a model is proposed. It explains how variation in speed and duration of continuous cutting, followed by a constant stirring speed, determines curd particle size distribution in an Ost IV cheese vat. The model can be used to maximise moisture retention and minimise whey fat losses. Reducing the cut speed ramp-up rate and the stirring speed significantly increased the curd particle size. Overfilling Ost IV vats significantly increased curd the particle size but had no significant effect on whey fat losses. ( 1998 Elsevier Science Ltd. All rights reserved Keywords: cutting; cut speed ramp up rate; overfilling; stirring; cheddar; ost vat; curd particle size; yield
on final cheese moistures. The larger the curd particle following cutting, the higher was the final cheese moisture. They also evaluated the effect speed of stirring following cutting had on moisture levels (water in the non-fat substance) but no significant effect was established. In mechanised Cheddar cheese production in New Zealand, curd particle size is one of the major means by which cheesemakers can control the moisture content of their cheese. A number of different cheese vats are now being used in the New Zealand industry including Damrow (both the double O and the latest horizontal version), Ost and Scherping vats. Most of the modern vats are 30 000 L capacity. In 1990, Johnston et al. (1991) showed how changes in the speed and duration of cutting in Damrow cheese vats affected the curd particle size distribution at draining. They also demonstrated how inadequate cutting can lead to curd particle shattering and heavy whey fat losses during stirring. Both would result in reduced cheese yields. In addition, their investigation supported Whitehead and Harkness’ (1954) results in that large curd particles at draining produced higher moistures in the milled unsalted curd. The results of Johnston et al. (1991) were also used to formulate a hypothesis and to propose a model that explained changes in the size distribution of
INTRODUCTION Many published studies have investigated the effect of various parts of the cheesemaking process on yield so that the contribution of cheesemilk composition and the major steps within both the pilot plant and commercial scale cheese-making processes are mostly well known and understood. A number of reviews on the subject have also been written. Factors affecting the yield of cheese (IDF, 1991) and Lucey and Kelly’s (1994) Cheese yield stand out as being particularly thorough. Both reviews provide comprehensive lists of references relating to how specific parts of the cheese-making process influence yield. Forming a gel and cutting that gel into cubes to allow syneresis to occur are the first major steps in the cheesemaking process by which many of the valuable nutritional components of fresh milk are concentrated. After cutting, optimising losses of milk components and maximising moisture levels during cheese production are paramount concerns when maximising cheese yields. Whitehead and Harkness (1954) established that, in pilot plant trials, curd particle size had a major influence *Corresponding author. 281
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and times following this initial sequence depended on the treatment being evaluated. Curd particle size and levels of fat in the whey were estimated for all experimental vats at draining, halfway through pumpout. With the exception of trial (d), stirring in all vats following cutting was as shown in Table 4 under ‘normal’. In total, samples of curd and whey from 53 vats were evaluated over 3 consecutive days. The generalised linear model (GLM) was used to test for significant main effects, interactions and treatment differences as was appropriate for the different trials. Least square means were also calculated. The statistical package SAS (SAS Institute Inc., Cary, NC; Version 6.11, 1995) was used to carry out these analyses.
curd particles and variations in the levels of fat and fines in the whey in terms of the cutting programme used. Johnston et al. (1991) carried out their investigation in a large New Zealand cheese plant during commercial production of 35% fat Cheddar cheese at a time when large quantities of New Zealand’s cheese were being made in vertical, double O, 20 000 L Damrow vats. Since 1990, continuing amalgamation of dairy companies, increased milk production and economies of scale have put pressure on New Zealand cheese plants to increase production capacity. Consequently, many cheese plants have undergone extensive rebuilding and change over the past 7 years. As part of that change, cheese vat capacity in most plants has been increased to 30 000 L. Ost IV vats have been installed in new plants and have replaced some vertical, double O, Damrow vats during refurbishment of old plants. Only 22% of the 245 000 t of cheese New Zealand will produce during the 1997/1998 dairy season will be produced in vertical, double O, Damrow vats. Most of the remaining tonnage will be made in 30 000 L Ost IV vats. As much more New Zealand cheese is now being made in Ost vats, this investigation covers four trials and looks at the effect of speed and duration of (continuous) cutting, cut speed ramp-up rate, overfilling and stirring speed in Ost vats on Cheddar cheese curd particle size and whey fat levels. The results of the effect of speed and duration of cutting are compared with the results of the earlier evaluation of the vertical, double O, Damrow vat (Johnston et al., 1991). Where such comparisons of results are made, particularly in the Discussion section, the vertical, double O, Damrow vat is simply referred to as the Damrow vat. Based on the results of this work and on the comparison with the Damrow evaluation, a model is proposed. It explains changes in the size distribution of curd particles and variations in the levels of fat in the whey in terms of the cutting/stirring programme used in the Ost vat. Fill volumes, cut speed ramp up rate and stirring speeds following cutting are aspects of Ost vat operation that are often changed. Cheese managers wanted more information on how changes in these three parameters affected curd particle size and whey fat losses.
Effect of speed and duration of cutting The speed and the duration of cutting were varied in each of nine Ost vats. After the initial cutting sequence had been completed, the coagulum was cut according to the regime shown in Table 1. A 3 x 3 factorial design was used to determine the effect variation in speed and duration of cutting had on the response variables: curd particle size and fat in the whey. The three replications of the nine combinations were randomised over all 27 consecutive vats during manufacture on 1 day. Effect of cut speed ramp-up rate The rate at which the cut speed was increased during the cutting cycle was varied in two vats as shown in Table 2. For a slow rate of increase, the cut speed was increased by 1 rev min~1 for increasing durations over 10 min following the completion of the initial cutting sequence. In comparison, for a fast rate of increase, the speed was immediately increased to 7 rev min~1 for 8 min. Although the slow ramp-up rate took longer to complete compared with the fast treatment, both treatments involved 61 revolutions of the cutting knives. The two treatments were run three times over six vats. Effect of overfilling To determine the effect of overfilling, one vat was filled to 33 000 L, 3000 L more than the control vat. In addition, three cut speeds and times were also evaluated for both fill volumes (Table 3). Treatments were randomised and replicated in a total of 12 vats over 1 day.
METHODS AND MATERIALS
Effect of stirring speed To determine the effect of stirring speed, the stirring speed during cooking and draining following cooking was reduced compared with normal. In addition, two cut
Experimental design In all experimental vats, the coagulum in each vat was initially cut at 2.5 rev min~1 for 2 min. Cutting speeds
Table 1. Speed and duration of cutting Cheese vat
Speed (rev min~1)
Duration (min)
Speed (rev min~1)
Duration (min)
No. of revolutions
1 2 3 4 5 6 7 8 9
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
2 2 2 2 2 2 2 2 2
3 3 3 5 5 5 7 7 7
2 5 8 2 5 8 2 5 8
11 20 29 15 30 45 19 40 61
Cutting and stirring in ost cheese vats
283
Table 2. Cut speed ramp-up rate Cheese vat
Speed (rev min~1)
Slow cut speed ramp-up rate 1 2.5
Fast cut speed ramp-up rate 2 2.5
Duration (min)
Speed (rev min~1)
Duration (min)
No. of revolutions
2
3 4 5 6 7
1 1 2 3 3
61
7
8
61
2
Table 3 Overfilling Cheese vat
Fill volume (L)
Speed (rev min~1)
Duration (min)
Speed (rev min~1)
Duration (min)
No. of revolutions
1 2 3 4 5 6
30 30 30 33 33 33
2.5 2.5 2.5 2.5 2.5 2.5
2 2 2 2 2 2
3 5 7 3 5 7
8 8 8 8 8 8
29 45 61 29 45 61
000 000 000 000 000 000
speeds and times were also evaluated for both stirring speeds (Table 4). Treatments were completely randomised and replicated in a total of eight vats on 1 day. Cheese manufacture The trials were carried out in a modern mechanised cheese plant during commercial production of 37% fat Cheddar cheese. Ost IV cheese vats (Alfa Laval, Lund, Sweden), a two-layered draining belt, two cheddaring towers and Wincanton towers (Alfa Laval, Lund, Sweden) were used to process the curd from 1.5]106 L of wholemilk per day prior to vacuum packaging and storage. A standard Cheddar cheese manufacturing procedure was used. Milk standardised to constant protein (3.90%) and fat (5.10%) levels by the addition of whole milk retentate, previously concentrated by membrane treatment, and skim milk to whole milk was pasteurised and continuously pumped (32°C) to the (30 000 L) Ost cheese vats. During filling, 0.7% of a pH-controlled starter (¸actococcus lactis subsp. cremoris strains) was injected into the filling line. As soon as the vat was filled, calf rennet (Hansen’s double strength, 290 International Milk Clotting Units mL~1; Chr. Hansen, Melbourne, Australia) was added to the milk (9.7 mL 100 L~1) which was left to set for 35 min. Duration and speed of cutting were as shown in Tables 1—4. After cutting, the direction of rotation of the knife panels was reversed and stirring commenced. Stirring continued during cooking to 38°C, holding at that temperature until the whey pH reached the drain target. After draining, cheddaring, milling and salting, the curd was pressed in 14 Wincanton towers, vacuum packaged into 20 kg blocks and placed in storage.
Curd particle size distribution The same procedure used by Johnston et al. (1991) to determine curd particle size in their investigation was used in this study. The curd and whey mixture from each experimental vat was sampled (+145 g curd) midway through pumpout. A stack of five stainless-steel mesh sieves (aperture sizes 12, 7.5, 5, 3 and (3 mm) was used to separate individual curd particles into their various size fractions. The curd retained on each sieve was estimated and expressed as a percentage of the total curd retained on all five sieves. To avoid confusion and to simplify future discussion, a condensed version of the curd particle size distribution was also calculated. The cumulative % of curd particles sized (7.5 mm (% CPS), i.e. the sum of the individual % of curd retained on the bottom three sieves, was used to summarise individual distributions. The % CPS value continued to reflect the original distribution in that an increasing or decreasing % CPS value indicated an increase or decrease in numbers of small particles. Whey fat content Samples of whey were taken from behind the drain screen at the same time as the curd samples were taken for particle size analysis. To determine the fat content, whey samples were analysed by means of a calibrated A/S N Foss Electric S52 milkoscan (Foss Electric, Hiller+d, Denmark). RESULTS Least-squares mean values for the % CPS and % fat in the whey for all four trials are shown in Tables 5—8.
K. A. Johnston et al.
23 23 23 23
% Fat Pr(F
42.6 45.6 60.2
0.0001
0.34 0.31 0.30
0.0001
51.2 49.5 47.7
0.280
0.33 0.31 0.31
0.0021
59.0 38.5 30.2 46.6 48.1 42.1 47.9 62.0 70.9
0.0001
0.37 0.33 0.32 0.33 0.29 0.30 0.31 0.30 0.30
0.1755
3.5 3.5 2.5 2.5 Normal Normal Slow Slow 3 3 3 3 2.5 2.5 2.5 2.5 1 2 3 4
2 2 2 2
Speed (rev min)~1 Vat
Duration (min)
2 8 2 8
Speed (rev min)~1
Ramp-up rate Slow Fast
% Fat
LSM
Pr(F LSM
Pr(F
59.9 73.4
0.004
0.101
0.35 0.34
Table 7. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each cutting speed (over 8 min cutting time) and overfilling an Ost Cheese Vat to 33 000 L Note: *cut time was 6 min shorter in duration than **.
Speed (rev min)~1
53* 47** 53* 47** 2 2 2 2
% CPS
Stirring speed
Duration (min)
7.5 7.5 5 5
Table 6. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each ramp-up rate
Duration (min)
7.5 7.5 5 5
LSM
40 40 40 40
Pr(F
7 7 4 4
LSM
Speed (rev min)~1
Speed (rev min)~1
% CPS
Speed (rev min~1) 3 5 7 Duration (min) 2 5 8 Speed-duration 3]2 3]5 3]8 5]2 5]5 5]8 7]2 7]5 7]8
Speed Duration (rev min)~1 (min)
Cooking and stirring Cutting
Table 4. Cut speeds and times prior to stirring and stirring speeds following cutting
Table 5. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each cutting speed, duration of cutting and speedduration interaction
Duration (min)
Duration
Duration (min)
284
%CPS
Speed, (rev min~1) 3 5 7 Over filling, (L) 30 000 33 000
% Fat
LSM
Pr(F
LSM
Pr(F
40.5 56.4 64.4
0.0003
0.31 0.30 0.30
0.3372
63.3 44.3
0.0006
0.31 0.30
0.0348
The value in the column Pr(F is the probability value associated with the statistical F-test. It refers to the likelihood that the true (population) mean values for a particular factor or interaction are all equal. A p-value of less than or equal to 0.05 (5%) is usually taken to indicate that this factor is having a real influence on the response variable.
Cutting and stirring in ost cheese vats Table 8. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each cutting time (at 3 rev min~1) and stirring speed following cutting % CPS
Duration, (min) 2 8 Stirring speed (rev min~1) Normal Slow
% Fat
LSM
Pr(F
LSM
Pr(F
36.3 29.2
0.1778
0.35 0.31
0.0201
41.2 24.3
0.0183
0.35 0.31
0.0129
285
curd particle size. In addition, overfilling the Ost vat by 10% beyond its nominal 30 000 L capacity significantly increased the curd particle size without any practical change in levels of fat lost to the whey. There was no significant interaction at the 5% level between speed and overfilling (p-value"0.45). (d) Effect of stirring speed. As shown in Table 4, the effect of stirring speed was evaluated by reducing the stirring speed normally used by the factory when making Cheddar cheese. The effect of reducing the stirring speed was evaluated over two cut times: 2 and 8 min. Table 8 shows not only that there was a significant increase in curd particle size with a decrease in stirring speed but also that there was a significant reduction in fat lost to the whey when the stirring speed following cutting was reduced. There was also a significant reduction in whey fat levels when the cutting time was increased. The interaction was not significant (p-value"0.94) DISCUSSION Effect of speed and duration of cutting
Fig. 1. The effect of speed of cutting and overfilling an Ost cheese vat beyond nominal capacity on the cumulative % of curd particles size(7.5 mm (%CPS).
An interaction p-value of 40.05 indicates that the main effects are not independent so the individual main effect information is not sufficient to fully understand the response. (a) Effect of speed and duration of cutting. Table 5 shows the average % CPS and % fat in the whey values for each variable. The statistical analysis of curd particle size shows a large, significant speed-duration interaction effect, suggesting that curd particle size is strongly influenced by the combination of speed and time. Conversely, as there is no significant interactive effect, the significant effects of speed and time on fat levels in the whey are independent. These results are evaluated in more detail and compared with those obtained from the Damrow investigation later, in the Discussion section. (b) Effect of cut speed ramp-up rate. Table 6 shows the significant effect of ramp-up speed during cutting on curd particle size. A slow ramp-up to the final cut speed of 7 rev min~1 produced significantly larger curd particles than cutting the curd at 7 rev min~1 immediately following the initial cutting sequence. Fat losses to the whey were not significantly affected by the ramp-up speed (p-value"0.101). (c) Effect of overfilling. As established in (a), Table 7 and Fig. 1 show the significant effect of speed of cutting on
¹he hypothesis. Johnston et al. (1991), in their investigation, demonstrated a relationship between the % CPS, % fat in the whey and the number of revolutions of the cutting panels in a Damrow cheese vat. Those relationships are reproduced in Figs 2 and 4. Their hypothesis was that the curd particle size and the level of fat in the whey at draining were not determined by the cutting sequence alone but that they were determined by a combination of the cutting programme used and the subsequent speed of stirring prior to cooking. They concluded that, where there had been insufficient cutting, large curd particles remained after cutting and those large curd particles were further reduced in size by shattering during subsequent stirring. Curd particle shattering during stirring was accompanied by high whey fat losses. The greater the degree of shattering, the smaller was the curd particle size and the higher was the level of
Fig. 2. The effect of speed and duration of cutting in Damrow and Ost cheese vats on the cumulative % of curd particles sized(7.5 mm (%CPS).
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K. A. Johnston et al.
Fig. 3. The effect of speed and duration of cutting]knife density]tip speed in Damrow and Ost cheese vats on the cumulative % of curd particles sized (7.5 mm (%CPS).
Fig. 4. The effect of speed and duration of cutting in Damrow and Ost cheese vats on % fat in the whey.
fat lost to the whey. Longer periods of cutting reduced the curd particle size so that shattering during stirring after cutting decreased. Curd particle size at draining increased to a maximum and whey fat levels decreased to a minimum for any one cutting and stirring speed combination. Beyond the maximum curd particle size, the curd particle size could be further reduced by longer cutting but, as shattering during stirring had been minimised, fat losses to the whey remained low. To compare results, the Ost vat data from this study have also been plotted in Figs 2 and 4. Divergence of Ost vat data points from the estimated trend plotted in Fig. 2 was much greater than for the Damrow, however the curd particle size data (Fig. 2) followed a similar trend to that established for cutting in a Damrow vat. Curd particle size initially increased and then decreased with continued cutting. An explanation for the divergence of Ost vat data points from the estimated trend plotted in Fig. 2 is that there are other influences which have a much greater effect on curd particle size in the Ost vat than they do in the Damrow vat.
Curd particle size at draining in any cheese vat, is determined principally by the speed and duration of cutting (number of revolutions) and the stirring speed following cutting. Knife density (amount of cutting area), knife tip speed, vat configuration and plane of cut (vertical, horizontal) will also influence curd particle size. Accounting for these other influences would produce a closer fit of the data to the trend. Fig. 3 shows a closer fit for the Ost vat data when just knife density and tip speed are accounted for. The number of knives per unit area (knife density) in the Ost vat is approximately 3.6 times greater than that found in the Damrow and the tip speeds of the knife panels are 0.730, 1.216 and 1.703 km/h~1 at 3, 5 and 7 rev min~1. In the Damrow study, tip speeds of the knife panels were 0.556, 1.414 and 2.263 km h~1 at 2, 5 and 8 rev min~1 respectively. Redefining of the x-axis of Fig. 2 to include factors for knife density and knife tip speed (total revolutions x knife density x knife tip speed) and replotting of the % CPS data for both vat types is shown in Fig. 3. The rapid increase in particle size as duration increases for the slower cut speed, the decrease in curd particle size as duration increases for the faster speed and the overall much larger maximum curd particle size (minimum % CPS) in Figs 2 and 3 suggest that the amount of curd shattering occurring in an Ost vat is significantly less than that established for the Damrow vat. This can be seen in practice when observing cutting in an Ost vat. Compared with the Damrow vat, and cutting at any one speed, curd particles in an Ost vat are reduced in size markedly faster. Consequently, fewer particles are shattered during stirring and curd particle size is larger at draining. A greater knife density would be one explanation for the difference in cutting performance. Damrow knife blades are mounted approximately 145 mm apart whereas individual knives in the Ost vat are mounted only 40 mm apart. The much closer knife arrangement in the Ost vat would lead to proportionately more cutting per revolution of the cutting system. Reduced shattering of curd particles in the Ost vat is also supported by the reduction of losses of fat in the whey. Figure 4 shows the whey fat losses from the Damrow evaluation along with the results of this investigation. In the region 6 to 20 revolutions of the cutting system, where curd shattering and hence fat lost to the whey are high, whey fat losses in the Ost vat are lower than those recorded for the Damrow evaluation. The difference has greater impact when the % fat lost in the whey is expressed as a percentage of the initial fat content of the cheesemilk. In the Damrow investigation, the standardised cheesemilk fat content was 4.0%. The fat level in the milk used in this study was 5.10%. Therefore, a loss/retention expressed as a percentage of the original cheesemilk fat level (where the whey fat level at draining was 0.30% for both) would be 7.5% loss/92.5% retention and 5.8% loss/94.2% retention, respectively. ¹he model Using their hypothesis as a platform, Johnston et al. (1991) proposed a model that explained how variation in cutting speed and duration of cutting, followed by a constant stirring speed, determined curd particle size distribution in a Damrow cheese vat. Their model is reproduced in Fig. 5. Each of the five Damrow curves represents the variation in curd particle size distribution with duration of
Cutting and stirring in ost cheese vats
287
Secondly, less curd shattering in the Ost vat allows for a larger curd particle overall especially at slower speeds of 3 and 5 rev min~1. This would be important for achieving high moisture targets in high moisture cheese. Effect of cut speed ramp-up rate A commonly reported observation by cheesemakers is that cutting at one continuous speed in an Ost vat invariably results in the development of cyclic flow of uncut curd being pushed in front of the knives. To break this pattern, managers either use an intermittent cutting sequence to allow the movement of curd to slow down or ramp up the cut speed to effectively allow the knives to catch up with the uncut curd. The results of this study (Table 6) show that, despite a longer duration of cutting, ramping up the cut speed slowly in 1 rev min~1 steps produces significantly larger curd particles but no change in whey fat levels. Effect of overfilling
Fig. 5. Model of the change in the cumulative (%) of curd particles sized (7.7 mm (%CPS) with increasing duration of cutting.
cutting, for a constant speed of cut. The dotted lines represent an estimated trend. The solid lines are based on curd particle size measurements. Each trend is characterised by a specific cutting duration at which % CPS is at a minimum or curd particle size is at a maximum. Shorter times produce curd particles that shatter during stirring, causing higher fat losses in the whey and a higher % CPS at draining. A longer duration of cutting also produces smaller curd particles but does not significantly elevate whey fat losses. Continued cutting beyond a certain time, depending on the speed of cut, will not further reduce the curd particle size. As the cutting speed is reduced and the duration of cutting is increased to avoid shattering, the curd particle size increases. Using the data gained from this evaluation, the authors have proposed a model that explains how variation in cutting speed and duration of cutting, followed by a constant stirring speed, determines curd particle size distribution in an Ost cheese vat. For comparison purposes, the Ost model has been overlayed on the Damrow model in Fig. 5. The comparison highlights two major differences between the two types of vat. Firstly, the Ost model shows a much greater effect of speed of cutting. Because marked differences exist in the design and operation of the 2 vat types, significant differences in % CPS values for the same speed and duration of cutting result. In the Ost vat curd shattering and therefore higher whey fat losses will occur only if the curd is cut for very short times at very low speeds. For example, after the initial slow cut of 2 min at 2.5 rev min~1 further cutting at 3, 5 or 7 rev min~1 for more than approximately 8, 4 and 2 min, respectively, is all that would be required to avoid curd shattering during stirring. In comparison, the Damrow model predicts that substantially more cutting is required to avoid shattering, especially at low speeds.
Filling 30 000 L Ost vats to 33 000 L allows a cheese plant manager to process more milk without any further capital expenditure assuming that the rest of the process, such as the draining system and Wincanton towers, can handle the extra curd. Although not a routine occurrence, this practice is often used in some New Zealand cheese plants when seasonal milk flows peak unexpectedly higher in late October/early November or when other product plant breakdowns put pressure on cheese manufacturing capacity. The results of this study show that, although overfilling has no practical effect on losses of fat to the whey, curd particle size significantly increases with overfilling. An explanation for this result is that the dimensions of the 30 000 L Ost vat have been determined not only on volume considerations alone but also on what level of milk can be tolerated above the centre of the vat without compromising cutting efficiency. Consequently, filling above this level results in less cutting at 3, 5 and 7 rev min~1. This is an important result in that it will not be advantageous for cheese managers, looking to reduce final cheese moistures, to overfill their vats. Effect of stirring As Johnston et al. (1991) point out in their Damrow vat study, stirring after cutting can have a marked effect on curd particle size and fat losses to the whey at draining. In this study, the effect of stirring speed was evaluated after the curd had been cut, after the initial cutting sequence, at 3 rev min~1 for 2 or 8 min. These times and speed (Table 4) were chosen because in the earlier trial (see the Effect of speed and duration of cutting) cutting at 3 rev min~1 for 2 min had produced small curd particles and high whey fat losses at draining. Cutting for a longer duration of 8 min had produced less curd shattering during stirring, resulting in larger curd particles and lower fat losses (Table 5). Although the curd particle sizes after cutting at 3 rev min~1 for 2 min were not as small as those measured in the initial trial at the same speed, fat levels in the whey were significantly lower where the curd had been cut for 8 min (Table 8).
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Reducing the stirring speed following cutting, during cooking and during draining significantly reduced fat losses to the whey and increased curd particle size at draining. General Speed and time of intermittent cutting (where the Ost vat knives come to a stationary position at the top and/or bottom of each revolution) were not evaluated in this trial. The intermittent cutting option is often chosen by cheesemakers as the middle stage of their total cutting programme to avoid excessive movement of curd during cutting. Long cutting times in Ost vats are common for this reason. The results of this investigation suggest that for Cheddar cheesemaking the intermittent cutting option is of little benefit. For example, the standard cutting sequence used in the cheese plant prior to this investigation was 3.2 rev min~1 continuous cutting for 10 min followed by 3.2 rev min~1 intermittent cutting for 9 min plus a further 6.0 rev min~1 for 8 min, i.e. a total of 27 min cutting time. The results of these trials show that the same curd particle size at draining can be achieved by cutting continuously for 2 min at 2.5 rev min~1 followed by 8 min at 3 rev min~1, i.e. a total of 10 min cutting time which is approximately one-third of the standard cutting time. Nevertheless, intermittant cutting is used extensively in the production of other cheese types. Therefore the effect of intermittant cutting and the confirmation of the predicted trends in Fig. 5 beyond cutting for 10 min at 3, 5 and 7 rev min~1 are two areas where further work could be done.
ACKNOWLEDGEMENTS The authors would like to acknowledge the assistance given by Peter Cross, Operations Manager; Mike Lawson, Dry Salt Cheese Plant Manager; Neil Maloney, Shift Supervisor and their staff for their assistance in carrying out these trials. Changing cutting speeds, cutting times, cut speed ramp-up rates, stirring times and fill volumes are not easy tasks in a modern cheese plant processing +1.5]106 L of wholemilk per day. The authors would also like to thank the New Zealand Dairy Board for funding the project and Anne Price, Project Manager, New Zealand Dairy Board, for her assistance. Thanks also to Garnett Davy, New Zealand Dairy Research Institute, for the graphics shown in this publication. REFERENCES International Dairy Federation (1991) Factors affecting the yield of cheese. IDF Special Issue No. 9301. International Dairy Federation, Brussels. Johnston, K. A., Dunlop, F. P. and Lawson, M. F. (1991) Effects of speed and duration of cutting in mechanized Cheddar cheesemaking on curd particle size and yield. Journal of Dairy Research 58, 345—354. Lucey, J. and Kelly, J. (1994) Cheese yield. Journal of the Society of Dairy ¹echnology 47(1), 1—14. Whitehead, H. R. and Harkness, W. L. (1954) The influence of variations in cheese-making procedure on the expulsion of moisture from Cheddar cheese curd. Australian Journal of Dairy ¹echnology 9, 103—107.
Int. Dairy Journal 8 (1998) 281—288 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0958-6946/98/$19.00#0.00
Effect of Various Cutting and Stirring Conditions on Curd Particle Size and Losses of Fat to the Whey during Cheddar Cheese Manufacture in Ost Vats K. A. Johnstona,*, M. S. Luckmana, H. G. Lilley b and B. M. Smale b aNew Zealand Dairy Research Institute, Private Bag 11 029, Palmerston North, New Zealand bAlpine Dairy Products Ltd, P.O. Box 33, Temuka, New Zealand (Received 10 December 1997; accepted 17 April 1998) ABSTRACT Nine combinations of speed and duration of cutting the coagulum were varied during the commercial manufacture of Cheddar cheese in 27 Ost IV cheese vats. The effects of cut speed ramp-up rate, vat overfilling and stirring rate after cutting were also evaluated. The effects of the variations were assessed by determining the curd particle size distribution and fat losses to the whey at draining. The results of varying the speed and duration of cutting were compared with the results of a similar investigation carried out using vertical, double O, Damrow vats. Although the results showed similar trends, compared with the Damrow vat at the same speed the Ost vat’s cutting system reduces curd particles to a constant size more rapidly, thereby avoiding excessive curd shattering and high whey fat losses during subsequent stirring. Based on these results and comparisons with the results from the Damrow trial, a model is proposed. It explains how variation in speed and duration of continuous cutting, followed by a constant stirring speed, determines curd particle size distribution in an Ost IV cheese vat. The model can be used to maximise moisture retention and minimise whey fat losses. Reducing the cut speed ramp-up rate and the stirring speed significantly increased the curd particle size. Overfilling Ost IV vats significantly increased curd the particle size but had no significant effect on whey fat losses. ( 1998 Elsevier Science Ltd. All rights reserved Keywords: cutting; cut speed ramp up rate; overfilling; stirring; cheddar; ost vat; curd particle size; yield
on final cheese moistures. The larger the curd particle following cutting, the higher was the final cheese moisture. They also evaluated the effect speed of stirring following cutting had on moisture levels (water in the non-fat substance) but no significant effect was established. In mechanised Cheddar cheese production in New Zealand, curd particle size is one of the major means by which cheesemakers can control the moisture content of their cheese. A number of different cheese vats are now being used in the New Zealand industry including Damrow (both the double O and the latest horizontal version), Ost and Scherping vats. Most of the modern vats are 30 000 L capacity. In 1990, Johnston et al. (1991) showed how changes in the speed and duration of cutting in Damrow cheese vats affected the curd particle size distribution at draining. They also demonstrated how inadequate cutting can lead to curd particle shattering and heavy whey fat losses during stirring. Both would result in reduced cheese yields. In addition, their investigation supported Whitehead and Harkness’ (1954) results in that large curd particles at draining produced higher moistures in the milled unsalted curd. The results of Johnston et al. (1991) were also used to formulate a hypothesis and to propose a model that explained changes in the size distribution of
INTRODUCTION Many published studies have investigated the effect of various parts of the cheesemaking process on yield so that the contribution of cheesemilk composition and the major steps within both the pilot plant and commercial scale cheese-making processes are mostly well known and understood. A number of reviews on the subject have also been written. Factors affecting the yield of cheese (IDF, 1991) and Lucey and Kelly’s (1994) Cheese yield stand out as being particularly thorough. Both reviews provide comprehensive lists of references relating to how specific parts of the cheese-making process influence yield. Forming a gel and cutting that gel into cubes to allow syneresis to occur are the first major steps in the cheesemaking process by which many of the valuable nutritional components of fresh milk are concentrated. After cutting, optimising losses of milk components and maximising moisture levels during cheese production are paramount concerns when maximising cheese yields. Whitehead and Harkness (1954) established that, in pilot plant trials, curd particle size had a major influence *Corresponding author. 281
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and times following this initial sequence depended on the treatment being evaluated. Curd particle size and levels of fat in the whey were estimated for all experimental vats at draining, halfway through pumpout. With the exception of trial (d), stirring in all vats following cutting was as shown in Table 4 under ‘normal’. In total, samples of curd and whey from 53 vats were evaluated over 3 consecutive days. The generalised linear model (GLM) was used to test for significant main effects, interactions and treatment differences as was appropriate for the different trials. Least square means were also calculated. The statistical package SAS (SAS Institute Inc., Cary, NC; Version 6.11, 1995) was used to carry out these analyses.
curd particles and variations in the levels of fat and fines in the whey in terms of the cutting programme used. Johnston et al. (1991) carried out their investigation in a large New Zealand cheese plant during commercial production of 35% fat Cheddar cheese at a time when large quantities of New Zealand’s cheese were being made in vertical, double O, 20 000 L Damrow vats. Since 1990, continuing amalgamation of dairy companies, increased milk production and economies of scale have put pressure on New Zealand cheese plants to increase production capacity. Consequently, many cheese plants have undergone extensive rebuilding and change over the past 7 years. As part of that change, cheese vat capacity in most plants has been increased to 30 000 L. Ost IV vats have been installed in new plants and have replaced some vertical, double O, Damrow vats during refurbishment of old plants. Only 22% of the 245 000 t of cheese New Zealand will produce during the 1997/1998 dairy season will be produced in vertical, double O, Damrow vats. Most of the remaining tonnage will be made in 30 000 L Ost IV vats. As much more New Zealand cheese is now being made in Ost vats, this investigation covers four trials and looks at the effect of speed and duration of (continuous) cutting, cut speed ramp-up rate, overfilling and stirring speed in Ost vats on Cheddar cheese curd particle size and whey fat levels. The results of the effect of speed and duration of cutting are compared with the results of the earlier evaluation of the vertical, double O, Damrow vat (Johnston et al., 1991). Where such comparisons of results are made, particularly in the Discussion section, the vertical, double O, Damrow vat is simply referred to as the Damrow vat. Based on the results of this work and on the comparison with the Damrow evaluation, a model is proposed. It explains changes in the size distribution of curd particles and variations in the levels of fat in the whey in terms of the cutting/stirring programme used in the Ost vat. Fill volumes, cut speed ramp up rate and stirring speeds following cutting are aspects of Ost vat operation that are often changed. Cheese managers wanted more information on how changes in these three parameters affected curd particle size and whey fat losses.
Effect of speed and duration of cutting The speed and the duration of cutting were varied in each of nine Ost vats. After the initial cutting sequence had been completed, the coagulum was cut according to the regime shown in Table 1. A 3 x 3 factorial design was used to determine the effect variation in speed and duration of cutting had on the response variables: curd particle size and fat in the whey. The three replications of the nine combinations were randomised over all 27 consecutive vats during manufacture on 1 day. Effect of cut speed ramp-up rate The rate at which the cut speed was increased during the cutting cycle was varied in two vats as shown in Table 2. For a slow rate of increase, the cut speed was increased by 1 rev min~1 for increasing durations over 10 min following the completion of the initial cutting sequence. In comparison, for a fast rate of increase, the speed was immediately increased to 7 rev min~1 for 8 min. Although the slow ramp-up rate took longer to complete compared with the fast treatment, both treatments involved 61 revolutions of the cutting knives. The two treatments were run three times over six vats. Effect of overfilling To determine the effect of overfilling, one vat was filled to 33 000 L, 3000 L more than the control vat. In addition, three cut speeds and times were also evaluated for both fill volumes (Table 3). Treatments were randomised and replicated in a total of 12 vats over 1 day.
METHODS AND MATERIALS
Effect of stirring speed To determine the effect of stirring speed, the stirring speed during cooking and draining following cooking was reduced compared with normal. In addition, two cut
Experimental design In all experimental vats, the coagulum in each vat was initially cut at 2.5 rev min~1 for 2 min. Cutting speeds
Table 1. Speed and duration of cutting Cheese vat
Speed (rev min~1)
Duration (min)
Speed (rev min~1)
Duration (min)
No. of revolutions
1 2 3 4 5 6 7 8 9
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
2 2 2 2 2 2 2 2 2
3 3 3 5 5 5 7 7 7
2 5 8 2 5 8 2 5 8
11 20 29 15 30 45 19 40 61
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283
Table 2. Cut speed ramp-up rate Cheese vat
Speed (rev min~1)
Slow cut speed ramp-up rate 1 2.5
Fast cut speed ramp-up rate 2 2.5
Duration (min)
Speed (rev min~1)
Duration (min)
No. of revolutions
2
3 4 5 6 7
1 1 2 3 3
61
7
8
61
2
Table 3 Overfilling Cheese vat
Fill volume (L)
Speed (rev min~1)
Duration (min)
Speed (rev min~1)
Duration (min)
No. of revolutions
1 2 3 4 5 6
30 30 30 33 33 33
2.5 2.5 2.5 2.5 2.5 2.5
2 2 2 2 2 2
3 5 7 3 5 7
8 8 8 8 8 8
29 45 61 29 45 61
000 000 000 000 000 000
speeds and times were also evaluated for both stirring speeds (Table 4). Treatments were completely randomised and replicated in a total of eight vats on 1 day. Cheese manufacture The trials were carried out in a modern mechanised cheese plant during commercial production of 37% fat Cheddar cheese. Ost IV cheese vats (Alfa Laval, Lund, Sweden), a two-layered draining belt, two cheddaring towers and Wincanton towers (Alfa Laval, Lund, Sweden) were used to process the curd from 1.5]106 L of wholemilk per day prior to vacuum packaging and storage. A standard Cheddar cheese manufacturing procedure was used. Milk standardised to constant protein (3.90%) and fat (5.10%) levels by the addition of whole milk retentate, previously concentrated by membrane treatment, and skim milk to whole milk was pasteurised and continuously pumped (32°C) to the (30 000 L) Ost cheese vats. During filling, 0.7% of a pH-controlled starter (¸actococcus lactis subsp. cremoris strains) was injected into the filling line. As soon as the vat was filled, calf rennet (Hansen’s double strength, 290 International Milk Clotting Units mL~1; Chr. Hansen, Melbourne, Australia) was added to the milk (9.7 mL 100 L~1) which was left to set for 35 min. Duration and speed of cutting were as shown in Tables 1—4. After cutting, the direction of rotation of the knife panels was reversed and stirring commenced. Stirring continued during cooking to 38°C, holding at that temperature until the whey pH reached the drain target. After draining, cheddaring, milling and salting, the curd was pressed in 14 Wincanton towers, vacuum packaged into 20 kg blocks and placed in storage.
Curd particle size distribution The same procedure used by Johnston et al. (1991) to determine curd particle size in their investigation was used in this study. The curd and whey mixture from each experimental vat was sampled (+145 g curd) midway through pumpout. A stack of five stainless-steel mesh sieves (aperture sizes 12, 7.5, 5, 3 and (3 mm) was used to separate individual curd particles into their various size fractions. The curd retained on each sieve was estimated and expressed as a percentage of the total curd retained on all five sieves. To avoid confusion and to simplify future discussion, a condensed version of the curd particle size distribution was also calculated. The cumulative % of curd particles sized (7.5 mm (% CPS), i.e. the sum of the individual % of curd retained on the bottom three sieves, was used to summarise individual distributions. The % CPS value continued to reflect the original distribution in that an increasing or decreasing % CPS value indicated an increase or decrease in numbers of small particles. Whey fat content Samples of whey were taken from behind the drain screen at the same time as the curd samples were taken for particle size analysis. To determine the fat content, whey samples were analysed by means of a calibrated A/S N Foss Electric S52 milkoscan (Foss Electric, Hiller+d, Denmark). RESULTS Least-squares mean values for the % CPS and % fat in the whey for all four trials are shown in Tables 5—8.
K. A. Johnston et al.
23 23 23 23
% Fat Pr(F
42.6 45.6 60.2
0.0001
0.34 0.31 0.30
0.0001
51.2 49.5 47.7
0.280
0.33 0.31 0.31
0.0021
59.0 38.5 30.2 46.6 48.1 42.1 47.9 62.0 70.9
0.0001
0.37 0.33 0.32 0.33 0.29 0.30 0.31 0.30 0.30
0.1755
3.5 3.5 2.5 2.5 Normal Normal Slow Slow 3 3 3 3 2.5 2.5 2.5 2.5 1 2 3 4
2 2 2 2
Speed (rev min)~1 Vat
Duration (min)
2 8 2 8
Speed (rev min)~1
Ramp-up rate Slow Fast
% Fat
LSM
Pr(F LSM
Pr(F
59.9 73.4
0.004
0.101
0.35 0.34
Table 7. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each cutting speed (over 8 min cutting time) and overfilling an Ost Cheese Vat to 33 000 L Note: *cut time was 6 min shorter in duration than **.
Speed (rev min)~1
53* 47** 53* 47** 2 2 2 2
% CPS
Stirring speed
Duration (min)
7.5 7.5 5 5
Table 6. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each ramp-up rate
Duration (min)
7.5 7.5 5 5
LSM
40 40 40 40
Pr(F
7 7 4 4
LSM
Speed (rev min)~1
Speed (rev min)~1
% CPS
Speed (rev min~1) 3 5 7 Duration (min) 2 5 8 Speed-duration 3]2 3]5 3]8 5]2 5]5 5]8 7]2 7]5 7]8
Speed Duration (rev min)~1 (min)
Cooking and stirring Cutting
Table 4. Cut speeds and times prior to stirring and stirring speeds following cutting
Table 5. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each cutting speed, duration of cutting and speedduration interaction
Duration (min)
Duration
Duration (min)
284
%CPS
Speed, (rev min~1) 3 5 7 Over filling, (L) 30 000 33 000
% Fat
LSM
Pr(F
LSM
Pr(F
40.5 56.4 64.4
0.0003
0.31 0.30 0.30
0.3372
63.3 44.3
0.0006
0.31 0.30
0.0348
The value in the column Pr(F is the probability value associated with the statistical F-test. It refers to the likelihood that the true (population) mean values for a particular factor or interaction are all equal. A p-value of less than or equal to 0.05 (5%) is usually taken to indicate that this factor is having a real influence on the response variable.
Cutting and stirring in ost cheese vats Table 8. Least-squares mean (LSM) values for the cumulative % of curd particles sized (7.5 mm (% CPS) and % fat in the whey for each cutting time (at 3 rev min~1) and stirring speed following cutting % CPS
Duration, (min) 2 8 Stirring speed (rev min~1) Normal Slow
% Fat
LSM
Pr(F
LSM
Pr(F
36.3 29.2
0.1778
0.35 0.31
0.0201
41.2 24.3
0.0183
0.35 0.31
0.0129
285
curd particle size. In addition, overfilling the Ost vat by 10% beyond its nominal 30 000 L capacity significantly increased the curd particle size without any practical change in levels of fat lost to the whey. There was no significant interaction at the 5% level between speed and overfilling (p-value"0.45). (d) Effect of stirring speed. As shown in Table 4, the effect of stirring speed was evaluated by reducing the stirring speed normally used by the factory when making Cheddar cheese. The effect of reducing the stirring speed was evaluated over two cut times: 2 and 8 min. Table 8 shows not only that there was a significant increase in curd particle size with a decrease in stirring speed but also that there was a significant reduction in fat lost to the whey when the stirring speed following cutting was reduced. There was also a significant reduction in whey fat levels when the cutting time was increased. The interaction was not significant (p-value"0.94) DISCUSSION Effect of speed and duration of cutting
Fig. 1. The effect of speed of cutting and overfilling an Ost cheese vat beyond nominal capacity on the cumulative % of curd particles size(7.5 mm (%CPS).
An interaction p-value of 40.05 indicates that the main effects are not independent so the individual main effect information is not sufficient to fully understand the response. (a) Effect of speed and duration of cutting. Table 5 shows the average % CPS and % fat in the whey values for each variable. The statistical analysis of curd particle size shows a large, significant speed-duration interaction effect, suggesting that curd particle size is strongly influenced by the combination of speed and time. Conversely, as there is no significant interactive effect, the significant effects of speed and time on fat levels in the whey are independent. These results are evaluated in more detail and compared with those obtained from the Damrow investigation later, in the Discussion section. (b) Effect of cut speed ramp-up rate. Table 6 shows the significant effect of ramp-up speed during cutting on curd particle size. A slow ramp-up to the final cut speed of 7 rev min~1 produced significantly larger curd particles than cutting the curd at 7 rev min~1 immediately following the initial cutting sequence. Fat losses to the whey were not significantly affected by the ramp-up speed (p-value"0.101). (c) Effect of overfilling. As established in (a), Table 7 and Fig. 1 show the significant effect of speed of cutting on
¹he hypothesis. Johnston et al. (1991), in their investigation, demonstrated a relationship between the % CPS, % fat in the whey and the number of revolutions of the cutting panels in a Damrow cheese vat. Those relationships are reproduced in Figs 2 and 4. Their hypothesis was that the curd particle size and the level of fat in the whey at draining were not determined by the cutting sequence alone but that they were determined by a combination of the cutting programme used and the subsequent speed of stirring prior to cooking. They concluded that, where there had been insufficient cutting, large curd particles remained after cutting and those large curd particles were further reduced in size by shattering during subsequent stirring. Curd particle shattering during stirring was accompanied by high whey fat losses. The greater the degree of shattering, the smaller was the curd particle size and the higher was the level of
Fig. 2. The effect of speed and duration of cutting in Damrow and Ost cheese vats on the cumulative % of curd particles sized(7.5 mm (%CPS).
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K. A. Johnston et al.
Fig. 3. The effect of speed and duration of cutting]knife density]tip speed in Damrow and Ost cheese vats on the cumulative % of curd particles sized (7.5 mm (%CPS).
Fig. 4. The effect of speed and duration of cutting in Damrow and Ost cheese vats on % fat in the whey.
fat lost to the whey. Longer periods of cutting reduced the curd particle size so that shattering during stirring after cutting decreased. Curd particle size at draining increased to a maximum and whey fat levels decreased to a minimum for any one cutting and stirring speed combination. Beyond the maximum curd particle size, the curd particle size could be further reduced by longer cutting but, as shattering during stirring had been minimised, fat losses to the whey remained low. To compare results, the Ost vat data from this study have also been plotted in Figs 2 and 4. Divergence of Ost vat data points from the estimated trend plotted in Fig. 2 was much greater than for the Damrow, however the curd particle size data (Fig. 2) followed a similar trend to that established for cutting in a Damrow vat. Curd particle size initially increased and then decreased with continued cutting. An explanation for the divergence of Ost vat data points from the estimated trend plotted in Fig. 2 is that there are other influences which have a much greater effect on curd particle size in the Ost vat than they do in the Damrow vat.
Curd particle size at draining in any cheese vat, is determined principally by the speed and duration of cutting (number of revolutions) and the stirring speed following cutting. Knife density (amount of cutting area), knife tip speed, vat configuration and plane of cut (vertical, horizontal) will also influence curd particle size. Accounting for these other influences would produce a closer fit of the data to the trend. Fig. 3 shows a closer fit for the Ost vat data when just knife density and tip speed are accounted for. The number of knives per unit area (knife density) in the Ost vat is approximately 3.6 times greater than that found in the Damrow and the tip speeds of the knife panels are 0.730, 1.216 and 1.703 km/h~1 at 3, 5 and 7 rev min~1. In the Damrow study, tip speeds of the knife panels were 0.556, 1.414 and 2.263 km h~1 at 2, 5 and 8 rev min~1 respectively. Redefining of the x-axis of Fig. 2 to include factors for knife density and knife tip speed (total revolutions x knife density x knife tip speed) and replotting of the % CPS data for both vat types is shown in Fig. 3. The rapid increase in particle size as duration increases for the slower cut speed, the decrease in curd particle size as duration increases for the faster speed and the overall much larger maximum curd particle size (minimum % CPS) in Figs 2 and 3 suggest that the amount of curd shattering occurring in an Ost vat is significantly less than that established for the Damrow vat. This can be seen in practice when observing cutting in an Ost vat. Compared with the Damrow vat, and cutting at any one speed, curd particles in an Ost vat are reduced in size markedly faster. Consequently, fewer particles are shattered during stirring and curd particle size is larger at draining. A greater knife density would be one explanation for the difference in cutting performance. Damrow knife blades are mounted approximately 145 mm apart whereas individual knives in the Ost vat are mounted only 40 mm apart. The much closer knife arrangement in the Ost vat would lead to proportionately more cutting per revolution of the cutting system. Reduced shattering of curd particles in the Ost vat is also supported by the reduction of losses of fat in the whey. Figure 4 shows the whey fat losses from the Damrow evaluation along with the results of this investigation. In the region 6 to 20 revolutions of the cutting system, where curd shattering and hence fat lost to the whey are high, whey fat losses in the Ost vat are lower than those recorded for the Damrow evaluation. The difference has greater impact when the % fat lost in the whey is expressed as a percentage of the initial fat content of the cheesemilk. In the Damrow investigation, the standardised cheesemilk fat content was 4.0%. The fat level in the milk used in this study was 5.10%. Therefore, a loss/retention expressed as a percentage of the original cheesemilk fat level (where the whey fat level at draining was 0.30% for both) would be 7.5% loss/92.5% retention and 5.8% loss/94.2% retention, respectively. ¹he model Using their hypothesis as a platform, Johnston et al. (1991) proposed a model that explained how variation in cutting speed and duration of cutting, followed by a constant stirring speed, determined curd particle size distribution in a Damrow cheese vat. Their model is reproduced in Fig. 5. Each of the five Damrow curves represents the variation in curd particle size distribution with duration of
Cutting and stirring in ost cheese vats
287
Secondly, less curd shattering in the Ost vat allows for a larger curd particle overall especially at slower speeds of 3 and 5 rev min~1. This would be important for achieving high moisture targets in high moisture cheese. Effect of cut speed ramp-up rate A commonly reported observation by cheesemakers is that cutting at one continuous speed in an Ost vat invariably results in the development of cyclic flow of uncut curd being pushed in front of the knives. To break this pattern, managers either use an intermittent cutting sequence to allow the movement of curd to slow down or ramp up the cut speed to effectively allow the knives to catch up with the uncut curd. The results of this study (Table 6) show that, despite a longer duration of cutting, ramping up the cut speed slowly in 1 rev min~1 steps produces significantly larger curd particles but no change in whey fat levels. Effect of overfilling
Fig. 5. Model of the change in the cumulative (%) of curd particles sized (7.7 mm (%CPS) with increasing duration of cutting.
cutting, for a constant speed of cut. The dotted lines represent an estimated trend. The solid lines are based on curd particle size measurements. Each trend is characterised by a specific cutting duration at which % CPS is at a minimum or curd particle size is at a maximum. Shorter times produce curd particles that shatter during stirring, causing higher fat losses in the whey and a higher % CPS at draining. A longer duration of cutting also produces smaller curd particles but does not significantly elevate whey fat losses. Continued cutting beyond a certain time, depending on the speed of cut, will not further reduce the curd particle size. As the cutting speed is reduced and the duration of cutting is increased to avoid shattering, the curd particle size increases. Using the data gained from this evaluation, the authors have proposed a model that explains how variation in cutting speed and duration of cutting, followed by a constant stirring speed, determines curd particle size distribution in an Ost cheese vat. For comparison purposes, the Ost model has been overlayed on the Damrow model in Fig. 5. The comparison highlights two major differences between the two types of vat. Firstly, the Ost model shows a much greater effect of speed of cutting. Because marked differences exist in the design and operation of the 2 vat types, significant differences in % CPS values for the same speed and duration of cutting result. In the Ost vat curd shattering and therefore higher whey fat losses will occur only if the curd is cut for very short times at very low speeds. For example, after the initial slow cut of 2 min at 2.5 rev min~1 further cutting at 3, 5 or 7 rev min~1 for more than approximately 8, 4 and 2 min, respectively, is all that would be required to avoid curd shattering during stirring. In comparison, the Damrow model predicts that substantially more cutting is required to avoid shattering, especially at low speeds.
Filling 30 000 L Ost vats to 33 000 L allows a cheese plant manager to process more milk without any further capital expenditure assuming that the rest of the process, such as the draining system and Wincanton towers, can handle the extra curd. Although not a routine occurrence, this practice is often used in some New Zealand cheese plants when seasonal milk flows peak unexpectedly higher in late October/early November or when other product plant breakdowns put pressure on cheese manufacturing capacity. The results of this study show that, although overfilling has no practical effect on losses of fat to the whey, curd particle size significantly increases with overfilling. An explanation for this result is that the dimensions of the 30 000 L Ost vat have been determined not only on volume considerations alone but also on what level of milk can be tolerated above the centre of the vat without compromising cutting efficiency. Consequently, filling above this level results in less cutting at 3, 5 and 7 rev min~1. This is an important result in that it will not be advantageous for cheese managers, looking to reduce final cheese moistures, to overfill their vats. Effect of stirring As Johnston et al. (1991) point out in their Damrow vat study, stirring after cutting can have a marked effect on curd particle size and fat losses to the whey at draining. In this study, the effect of stirring speed was evaluated after the curd had been cut, after the initial cutting sequence, at 3 rev min~1 for 2 or 8 min. These times and speed (Table 4) were chosen because in the earlier trial (see the Effect of speed and duration of cutting) cutting at 3 rev min~1 for 2 min had produced small curd particles and high whey fat losses at draining. Cutting for a longer duration of 8 min had produced less curd shattering during stirring, resulting in larger curd particles and lower fat losses (Table 5). Although the curd particle sizes after cutting at 3 rev min~1 for 2 min were not as small as those measured in the initial trial at the same speed, fat levels in the whey were significantly lower where the curd had been cut for 8 min (Table 8).
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Reducing the stirring speed following cutting, during cooking and during draining significantly reduced fat losses to the whey and increased curd particle size at draining. General Speed and time of intermittent cutting (where the Ost vat knives come to a stationary position at the top and/or bottom of each revolution) were not evaluated in this trial. The intermittent cutting option is often chosen by cheesemakers as the middle stage of their total cutting programme to avoid excessive movement of curd during cutting. Long cutting times in Ost vats are common for this reason. The results of this investigation suggest that for Cheddar cheesemaking the intermittent cutting option is of little benefit. For example, the standard cutting sequence used in the cheese plant prior to this investigation was 3.2 rev min~1 continuous cutting for 10 min followed by 3.2 rev min~1 intermittent cutting for 9 min plus a further 6.0 rev min~1 for 8 min, i.e. a total of 27 min cutting time. The results of these trials show that the same curd particle size at draining can be achieved by cutting continuously for 2 min at 2.5 rev min~1 followed by 8 min at 3 rev min~1, i.e. a total of 10 min cutting time which is approximately one-third of the standard cutting time. Nevertheless, intermittant cutting is used extensively in the production of other cheese types. Therefore the effect of intermittant cutting and the confirmation of the predicted trends in Fig. 5 beyond cutting for 10 min at 3, 5 and 7 rev min~1 are two areas where further work could be done.
ACKNOWLEDGEMENTS The authors would like to acknowledge the assistance given by Peter Cross, Operations Manager; Mike Lawson, Dry Salt Cheese Plant Manager; Neil Maloney, Shift Supervisor and their staff for their assistance in carrying out these trials. Changing cutting speeds, cutting times, cut speed ramp-up rates, stirring times and fill volumes are not easy tasks in a modern cheese plant processing +1.5]106 L of wholemilk per day. The authors would also like to thank the New Zealand Dairy Board for funding the project and Anne Price, Project Manager, New Zealand Dairy Board, for her assistance. Thanks also to Garnett Davy, New Zealand Dairy Research Institute, for the graphics shown in this publication. REFERENCES International Dairy Federation (1991) Factors affecting the yield of cheese. IDF Special Issue No. 9301. International Dairy Federation, Brussels. Johnston, K. A., Dunlop, F. P. and Lawson, M. F. (1991) Effects of speed and duration of cutting in mechanized Cheddar cheesemaking on curd particle size and yield. Journal of Dairy Research 58, 345—354. Lucey, J. and Kelly, J. (1994) Cheese yield. Journal of the Society of Dairy ¹echnology 47(1), 1—14. Whitehead, H. R. and Harkness, W. L. (1954) The influence of variations in cheese-making procedure on the expulsion of moisture from Cheddar cheese curd. Australian Journal of Dairy ¹echnology 9, 103—107.