Effect of storing milk at 3 and 7C on the yield and quality of Cheddar cheese

Introduction

Refrigerated storage of raw milk is used to limit the growth of microorganisms in milk prior to processing. It has been known for some time that the quality and yield of cheese produced from bulk cooled milk may be adversely affected by this procedure (Weatherup et al., 1988; Weatherup and Mullan, 1993). The reduced yield and poor quality may be due to physico-chemical changes in the state of several milk components e.g. dissociation of micellar casein, mainly Κ-casein into a soluble phase, occurs during the first 48 h of storage at 4° and 7° C. This results in losses of fat and curd fines, weaker curd, more moist curd and a slightly lower yield. Partial reversal of dissociation occurs after further storage. The reduced yield and quality can also be due to the activity of proteases and lipases produced by psychrotrophic bacteria.

Despite the work that has been done over many years milk is still being stored for extended periods (1-3+ days on some farms) and cheesemakers are again (2019) reporting problems with the yield and quality of cheese produced using this milk.

Following several queries related to milk quality and cheese manufacture I am providing a report written by Wilf Weatherup and me some years ago that may be helpful.

A simple calculator has been provided using the total viable count of milk prior to pasteurisation and a regression equation to predict the grade value of Cheddar cheese.

THE EFFECT OF STORAGE OF MILK AT REFRIGERATION TEMPERATURES ON THE QUALITY AND YIELD OF CHEDDAR CHEESE

WILFRED WEATHERUP AND  W MICHAEL A MULLAN

SUMMARY

It has been suspected for some time that the practice of storing milk at refrigerated temperatures may adversely affect the quality and yield of Cheddar cheese.  The objective of this project was to determine the effect of storing milk at refrigerated temperatures on the quality and yield of Cheddar cheese.  Milk was stored at 3° and 7°C for periods up to 5 days prior to cheese manufacture.  Cheese quality was assessed at 1, 3 and 6 months by a commercial cheese grader and the weights of milk and final product was recorded in order to assess yield.  All process variables were monitored and detailed chemical and microbiological analyses of the milk, whey and cheese were undertaken.  Storage of milk at 3° and 7°C resulted in a highly significant reduction in yield (p < 0.001).  Fat and total solids recovery from the stored milks were reduced.  Storage at 7°C resulted in a highly significant reduction in yield (p < 0.001).  Fat and total solids recovery from the stored milks were reduced.  Storage at 7°C for 3 or more days adversely affecting cheese quality (p < 0.01).  Quality was also reduced by storage at 3°C for 5 days while storage at 3°C for 3 days resulted in an improvement in quality.  The effects were greater at 6 months than 3 months.  The psychrotrophic, lipolytic, proteolytic and total viable counts all increased rapidly during storage at both 3° and 7°C and there was a high degree of correlation between grade score and the psychrotrophic count (p < 0.001).  There was also a high degree of correlation between the grade score and the total viable count (p < 0.001).  Levels of free fatty acids (FFA) increased with storage and were highly correlated with the grade score (p < 0.001).  This evidence suggests that the reduction in quality was due to the activity of extracellular lipolytic enzymes produced by psychrotophic bacteria.  FFA profiles of the cheeses were consistent with this theory.  There was also a significant increase in the levels of phosphotungstic acid soluble amino nitrogen (PTA–sol N) in the pressed curd produced from stored milks (p < 0.001) and the levels of PTA–sol N in the pressed curd were correlated with the grade score (p < 0.01).  No significant differences in PTA–sol N were observed during ripening.  The commercial significance of these results is discussed in this report.

 

INTRODUCTION

Refrigerated storage of raw milk is used extensively to limit the growth of micro–organisms in milk prior to processing.  It has been suspected for some time that the quality and yield of cheese produced from bulk–cooled milk may be adversely affected by this procedure.  Ali et al. (1980) have shown that cold storage modifies the physico–chemical state of several milk components.  They found that storage at 4° and 7°C was accompanied by a dissociation of micellar caseins, particularly β–casein , into the soluble phase during the first 48 hrs but on further storage there was a partial reversal of this process.  At higher temperatures (10–20°C) the contents of all the individual caseins in the soluble phase decreased throughout storage.  During cheesemaking, losses of fat and curd fines in the whey were greater with increased levels of soluble phase casein and clotting times were also prolonged.  Curd structure was weaker, curds were more moist and slightly lower cheese yields were obtained in stored milks with elevated soluble–phase casein.  Other workers have also found that the soluble casein content of milk is increased by lowering the temperature (Creamer et al. 1977).  In the conversion of milk to cheese, casein aggregates during clotting to form a network that entraps some of the water and most of the milk fat.  Any alterations in the milk protein which forms this network might be expected to affect both cheese yield and quality.

The quality and yield of Cheddar cheese may also be adversely affected by the action of extracellular enzymes produced by psychrotrophic bacteria.  The latter are capable of growth at refrigeration temperatures and produce heat stable proteases, lipases and phospholipases which may cause quality problems in dairy products (Law, 1979; Fox and Stepaniak, 1983; Law et al., 1979).  O’Leary et al. (1983) reported reduced yields of Cottage cheese and Cheddar cheese with milk which had been stored at refrigeration temperatures.  Reduction in yield was correlated with increased proteolytic and lipolytic counts and decreased fat and casein in the stored milk.  They also noted an increase in cheese moisture and reduced grade scores for cheeses made from stored milk.  Burlingame–Frey and Marth (1984) reported that proteolytic psychrotophs caused a decrease in the size of casein micelles during refrigerated storage of skim milk.  Hicks et al. (1980) indicated that the quality of Cheddar cheese and Cottage cheese decreased with storage time of the raw milk.  Cousin and Marth (1977) precultured milk with psychrotrophic bacteria prior to cheese manufacture and found that cheese quality was inferior.  The quality defects became more marked after ripening of the cheese for 3 months.  Law et al. (1979) concluded that reduced quality in Cheddar cheese manufactured from stored milk was due mainly to the action of lipolytic enzymes and not proteolytic enzymes.  This was further confirmed by Kalogridou–Vassiliadou and Alichanidis (1984) in the manufacture of Teleme cheese.  They found unacceptable rancidity after 60 days in cheeses made from milk stored for >4 days at 4–5°C.  Off–flavours related to excessive protein breakdown were less marked.

Cheddar cheese is an important product of the Northern Ireland Dairy Industry, the annual production being in the order of 20,000 tonnes (note in the 1980s).  In a highly competitive market it becomes increasingly important to produce a high quality product and ensure that yields are optimised.  The objective of this project was to determine the effect of storing milk at refrigerated temperatures on the quality and yield of Cheddar cheese.  If storage of milk at certain temperatures resulted in cheese of inferior quality and/or yield, it was envisaged that further work would be initiated aimed at counteracting the deleterious effects of cold storage or developing alternative or supplementary preservation techniques for milk intended for cheesemaking.

MATERIALS AND METHODS

Origin of Milk

Raw unstandardised milk was obtained from the bulk silo of a local factory.  The silo which had a capacity of approximately 114 x 103 litres contained at least 8.0 x 104 litres immediately prior to milk removal.  The silo contents were agitated periodically by means of a mechanical impeller.  An attempt was made to ensure that fresh (1 day old) milk was used but it is probable that the supply contained some alternate day collected milk which would have been two days old at the point of reception.

Experimental Plan

The experimental plan is shown in Figure 1.  The raw milk was divided into three equal lots on Day 1.  One lot was used to produce two vats of Cheddar cheese while the remaining lots were stored at 3° and 7°C respectively and used to produce cheese on Day 3 and Day 5.  These storage temperatures were chosen because the mean temperature of storage of milk at cheese factories in N Ireland was 7°C and at this temperature psychrotrophic bacteria proliferate rapidly whereas at lower temperatures (e.g. 3°C) growth is reduced. Cheesemaking experiments were replicated 7-times over a 1-year period to take account of seasonal variation.

Experimental plan for Cheddar cheese manufacture

Figure 1:   Experimental Plan. Seven replications were used.

Bulk Starter Preparation

A defined bacteriophage–insensitive multiple strain starter was used.  The starter, designated MA–012, was obtained from Eurozyme, London, England.  It was distributed in 0.5 g lots in sterile vials and stored at –25°C until required.

Prior to bulk starter manufacture the contents of one vial were suspended in 10 ml of sterile, reconstituted (12% total solids, TS), antibiotic–free, skim milk (RSM).

Starter milk was inoculated using 0.3 ml of the mixture per 4 litres of starter milk.  Bulk starter was produced from the same batch of skim milk powder (SMP) and contained 12% TS.  The SMP was stored at <10°C throughout the trial.  The RSM was heat–treated at 88°C for one hour and cooled to 25°C prior to inoculation.  The inoculated milk was incubated at 25°C for 19 hrs.  This gave cultures with a total titratable acidity (TA) of approximately 0.7% (w/v) lactic acid (LA) and relatively constant acid producing potential throughout the project.

Cheese Manufacture

Milk was pasteurised at 71.7°C for 15 seconds using a modified APV paraflow, type – HX–P 345 pasteuriser (APV, Crawley, Sussex).  The unit had been modified by the manufacturer to increase its throughput to approximately 1,800 litres/hr.  The pasteurised milk was weighed into rectangular 225 litre vats.  The pasteuriser was stopped, rinsed and drained between heat–treatment of different batches of milk.

Cheddar cheese was produced from approximately 180 kg quantities of milk which were tempered to 30°C and inoculated with 1% (w/w) of bulk starter.  After ripening for 30 minutes, standard rennet (Chr. Hansen’s Laboratory, Reading, England) was added (0.02% [v/w] ).  The rennet was diluted ten–fold immediately prior to use.  The milk was stirred for 3 minutes after addition of the rennet and the curd cut into approximately 10 mm cubes, when it had reached a satisfactory degree of firmness.  The latter was judged subjectively.  After a 5 minute healing period, stirring was initiated and the temperature raised from 30° to 39°C in approximately 45 minutes.  After scalding for one hour, the curds were pitched and the whey run off when the TA of the curd reached 0.2% LA (w/v).  Curd was cheddared until the TA reached 0.6–0.7% LA (w/v).  At this point the curd was milled using a Clarilac cheese curd mill and salted (2% w/w).

The salted curd was weighed into 20 kg cheese moulds and pressed overnight (approximately 17 hrs) at 150 kN/m2.

After pressing, samples were removed for chemical and microbiological analysis and the curd blocks cut into 8 approximately 2 kg portions.  The latter were vacuum packed in barrier bags of nylon/polythene construction supplied by Metal Box, Portsmouth, England.  Cheeses were ripened at 7°C.

Sampling and Analysis

A summary of the analyses undertaken is shown in Table 1.  The stored milk was agitated twice daily throughout storage and was further agitated for 30 mins prior to sampling and cheese manufacture.  The pasteurised milk was also sampled for chemical and microbiological analysis.  Whey from each vat was collected in a large container in order to obtain a representative sample of chemical analysis.  The pressed curd was sampled immediately after removal from the process.  The blocks were cut into 8 approximately 2 kg portions.  One portion was retained for samples and the remaining portions packed and placed in the ripening store.  A core was removed aseptically from the sample portion for microbiological analysis and cross–sectional blocks of approximately 0.5 kg were also removed for chemical analysis and hardness and colour measurements.  This procedure was repeated at monthly intervals throughout ripening with the samples being selected randomly.

Table 1.  Microbiological and Chemical Analysis

  

ANALYSIS

Chemical

Microbiological

Raw Milk

·   TS

·   Fat

·   SNF

·   TA

·   Total N

·   Casein N

·   pH

·   Psychrotrophs

·   Mesophiles

·   Proteolytics

·   Lipolytics

Stored Milk

· As for raw milk

·   As for raw milk

Pasteurised Milk

·   As for raw milk

·   Mesophiles

Whey

·   As for raw milk

·   As for raw milk

Pressed 

·   TS

·   Fat

·   Salt

·   FFA

·   Total N

·   PTA Soluble N

·   pH

·   Colour

·   Hardness

 

Curd during ripening (monthly up to 6 months)

·   PTA Soluble N

·   FFA

·   pH

·   Colour

·   Hardness

·   Grading (1, 3,  6 months)

 

Microbiological Analysis

Dilutions of milk and whey were prepared using quarter–strength Ringers solution.  The initial dilutions of curd and cheese were prepared using 2% sodium citrate.  Subsequent dilutions were prepared using quarter–strength Ringers solution.  Total viable counts were made using pour plates of Yeastral Milk Agar (YMA) (BS 4285: 1968).  Poured dried plates of YMA were used to enumerate psychrotrophic bacteria (IDP Standard 101: 1981).  Spread plates of Tributyrin agar and 30% milk agar were used to enumerate lipolytic and proteolytic bacteria respectively (Harrigan and McCance, 1976).  Plates were incubated at 30°C for 3 days.

Chemical Analysis

All milk samples were subjected to chemical analysis on the same day as the relevant cheeses were produced.  Whey samples, in sealed plastic containers, were stored overnight at refrigeration temperatures and analysed together with pressed curd samples on the following day.

Curd samples taken during the ripening period were wrapped in non–permeable plastic bags and stored at –20°C pending analysis.

In milk and whey samples, total solids (TS), fat, titratable acidity (TA), total nitrogen (Total N) and casein nitrogen (casein N) were determined by the British Standard Method (1963a).  pH values were determined by British Standard Method (1963b).

In all curd samples, TS, fat, salt, total N and pH were determined by the British Standard Method (1963b).  Free fatty acid (FFA) determinations in the extracted fat of curd samples were carried out as described by Pearson (1976).  Phosphotungstic acid–soluble amino nitrogen (PTA–sol N) levels were determined by the modified Habeeb method as described by Jarrett et al. (1981).

Hardness

The force (kg) required to crush 1 cm cubes of cheese was measured using an Instron Universal Testing Instrument.  Samples were maintained at 7°C during testing and 10 cubes from each sample were assessed.

Colour

Cheese colour was compared using a Hunterlab Tristimulus Colorimeter.  Three sets of readings of L, a and b values were obtained for each sample.  All readings were obtained from freshly exposed surfaces of cheese.

Grading

Cheeses were graded at 1, 3 and 6 months by a commercial cheese grader using the grading scheme shown in Table 2.

TABLE 2:   Grading Scheme

 

Maximum Score

Grade

Points

Flavour and Aroma

45

Extra selected

≥ 93

Body and Texture

40

Selected

≥ 85 – ≤92

Finish

10

Grade

 ≥70 – ≤84

Colour

5

No qrade

< 70

TOTAL

100

    

Statistical Analysis

Analysis of variance and regression analysis were undertaken using a Genstat computer programme (Lawes Agricultural Trust, Rothamstead Experimental Station).

RESULTS AND DISCUSSION

Average cheese composition throughout the project is shown in Table 3.  Salt and salt–in–moisture moisture values were lower than expected and fell below the levels recommended by Giles and Lawrence (1973).  Other values were within the specifications recommended by these workers.

 Table 3.   Cheese Composition

 

MEAN

Moisture

36.63%

Moisture in non–fat substance

55.48%

pH

5.18

Fat

33.97%

Fat in dry matter

53.61%

Salt

1.39%

Salt in moisture

3.82%

 

The variations in yields are shown in Figure 3.  There was a highly significant reduction in yield of cheese made from stored milk (p < 0.001).  The reduction in yield could not be accounted for by variations in

Effect of milk storage on cheese yield

 Figure 3. The effect of storing milk at 3°C and 7°C on the yield of Cheddar cheese. Each point is the mean of 7-replicates.

moisture content.  Adjustment of the cheese yields to 38% moisture gave a similar reduction in yield between the unstored and stored milk (p < 0.001) (Figure 4).  Numerically, there was greater reduction in yield after storage at 3°C than 7°C and the yield decreased with time of storage at both temperatures but these differences were not statistically significant.  There was also no interaction between temperature of storage and time of storage.  This evidence would suggest that storage of milk at both 3° and 7°C for periods of 3

Effect of milk storage on moisture-adjusted cheese yield

 Figure 4. The effect of storing milk at 3°C and 7°C on the moisture-adjusted (38 %) yield of Cheddar cheese. Each point is the mean of 7-replicates.

days or less gives a significant decrease in yield.  In a collaborative study with Loughry College, Dr D Johnson and Dr A Gilmour found significant reductions in yield after one day of storage of milk at 7°C.  This reduction in yield could mean a considerable loss in revenue to the cheesemaker (Table 4).  For example, storage at 7°C for 3 days prior to cheesemaking could lead to a loss of £7.72 per tonne of whole milk assuming a cheese price of £2,000/tonne.  Ali et al. (1980) suggested that changes may occur in the structure of the casein

 

Table 4.  Potential loss in revenue due to storage of milk at 3° and 7°C 

 

Mean yield of cheese per tonne of milk (kg)

Yield reduction per tonne of milk (kg)

Lost revenue per tonne of milk*

Control

103.23

3°C/3 days

98.87

4.36

£8.72

3°C/5 days

97.66

5.57

£11.14

7°C/3 days

99.36

3.87

£7.72

7°C/5 days

98.47

4.76

£9.52

*Assuming a price of £2,000/tonne

micelle during refrigerated storage which may lead to reduced yields of cheese.  Other workers have suggested that lipolytic and proteolytic enzymes produced by psychrotrophic bacteria may be responsible for reductions in yield and poor quality.  Analysis of the raw milk prior to cheesemaking did not reveal any variations in the levels of Total N, casein N or fat but highly significant reductions in these components were observed after pasteurisation and prior to cheesemaking (p < 0.001).  The difference between raw and pasteurised milk samples may be due to the sampling procedures used.  The pasteurised milk samples were obtained when the milk was at a temperature of 30°C and would have required several hours to reach refrigeration temperatures.  This may have permitted changes to occur in the milk leading to reduced levels of Total N, Casein N and fat.

Total solids and fat recovery were calculated and are shown in Figure 5.  Recovery decreased with storage of the milk.  The loss of total solids can be partly accounted for by the fat losses.  It might have been expected

Recovery of fat and total solids in Cheddar cheese manufactured from milk stored at 3° and 7°C

Figure 5.  Recovery of fat and total solids in Cheddar cheese manufactured from milk stored at 3° and 7°C

that the whey would have contained higher levels of total solids and fat to account for the lower recovery rates.  There was a numerical increase in fat levels in the whey on storage but this was not statistically significant.  This might be accounted for the methods used to estimate fat levels.  These may not have detected degraded fat in the form of FFA.  Total N, casein N and TS were all significantly lower in the whey.  This may have been due to the sampling procedure which did not account for whey expressed during cheddaring and pressing of the curd.  It is possible that this whey contained a much higher level of total N, casein N and TS.

No significant reductions in the fat levels in the pressed curd were detected but levels of FFA were significantly higher in the stored milk curd (p < 0.001) (Figure 6).  The levels of PTA–

Levels of free fatty acids in the pressed curd produced from milk stored at 3°C and 7°C

 

Figure 6. Levels of free fatty acids in the pressed curd produced from milk stored at 3°C and 7°C

sol N were also significantly higher in the pressed curd produced from stored milks (p < 0.001).  This would indicate that fat and protein was being degraded during storage possibly as the result of lipolytic and proteolytic enzymes produced by psychrotrophic bacteria.  Levels of psychrotrophic bacteria found in the stored milks were consistent with this hypothesis (Figure 7).  It would therefore appear that the reduction in yield may partly be

 

Levels of psychrotrophic bacteria in milk stored at 3° and 7°C.

Figure 7.  Levels of psychrotrophic bacteria in milk stored at 3° and 7°C. Each point represents the mean from 7 cheesemaking trails conducted over 1-year.

due to enzymatic degradation of fats and proteins during storage.  We have no evidence to indicate that physico–chemical changes occurred in the milk protein during storage but the results of other workers would indicate that these changes occur and can result in reduced yields (Ali et al. 1980).

Cheese quality was assessed by a commercial grader at 1, 3 and 6 months.  The grading results at 3 months are shown in Figure 8.  There was a significant difference between the controls and the stored milks (p < 0.001).  There was also a significant reduction in quality between 3 and 5 days of storage (p < 0.001).  All variations in scores were the result of variations in flavour and aroma, and body and texture.  Scores for

 y-51 Effect of storing milk at 3 and 7C on the yield and quality of Cheddar cheese

Figure 8.   Grading scores (3 months) of Cheddar cheese produced from milk stored at 3° and 7°C.

 colour and finish did not vary.  A similar pattern was observed at 6 months (data not shown) but there was also a significant improvement in the quality of the cheese produced from milk stored at 3°C for 3 days.  Kristoffersen (1985) recommended storage of milk at 37°C for 3½–5 hrs prior to cheesemaking to produce a fuller flavoured cheese.  This incubation period permits enzymatic changes in the milk.  Assuming a Q10 of 2 for chemical reactions responsible for flavour, it is possible that storage at 3°C for 2–3 days would result in a similar effect.

The psychrotrophic counts in the milk prior to cheese manufacture are shown in Figure 7.  The counts reached 1 x 106 cfu/ml after approximately 3 days of storage.  There was a significant difference between 3° and 7°C (p < 0.001), growth being faster at 7°C and reaching higher levels after 5 days of storage.  The relationship between the grade score and the psychrotrophic count is shown in Figure 9.  There was a highly significant degree of correlation (p < 0.001).  The total viable count, proteolytic count and lipolytic counts all showed similar increases during storage of the milk. 

 

Fig-5y Effect of storing milk at 3 and 7C on the yield and quality of Cheddar cheese

Figure 9: Relationship between the psychrotrophic count in the raw milk and the grade score at 3 months.

 There was poor correlation between the proteolytic count and the grade score at 3 months but there was some correlation between the grade score and the lipolytic count (p < 0.05) and the grade score was highly correlated with the TVC (p < 0.001) (Figure 10).  A simple calculator is available using the regression equation given on Figure 10 to predict the grade value of Cheddar cheese using the TVC. calculation  Similar correlations were found between the grade score at 6 months and the psychrotrophic count (r = 0.41), the lipolytic count (r = 0.56) and the TVC (r = 0.65).  These results emphasise the importance of the psychrotrophic count, lipolytic count and TVC of the raw milk as indicators of the suitability of milk for cheese manufacture.

 

Fig-6y Effect of storing milk at 3 and 7C on the yield and quality of Cheddar cheese

Figure 10: Relationship between the grade score at 3 months and the total viable count of the raw milk

The probability of achieving a selected grade cheese (i.e. ≥ 85 points) when the TVC of the raw milk is < 1 x 106 cfu/ml was 0.86 whereas the probability decreases to 0.33 when the TVC is > 1 x 106 cfu/ml.

Muir and Phillips (1984) indicated that significant levels of FFA may occur in raw milk when the psychrotrophic count is > 1 x 106 cfu/ml.  The concentration of FFA in the pressed curd are shown in Figure 6.  The levels were higher in the stored milk curds (p < 0.001) and the levels increased with time of storage (p < 0.001).  The concentrations were also higher at 7°C than 3°C (p = 0.01).

During ripening the levels of FFA increased markedly in the stored milk cheeses (Figure 11) with highly significant differences between times and temperatures of storage (p < 0.001).  There was also a highly significant correlation between the grade score at 3 months and the FFA levels in the pressed curd (r = 0.69) (p < 0.001) (Figure 12).  Correlation was poor between the FFA in the pressed curd and the grade score at 6 months (r = 0.36).

These results indicate the importance of free fatty acids in cheese flavour.  Other workers have indicated that low levels of FFA are desirable in Cheddar cheese but at higher levels off–flavours become apparent (Woo et al., 1984).  These results would also suggest that poor quality may be due to the action of extracellular lipolytic enzymes produced by psychrotrophic bacteria.  FFA profiles of a sample of the cheeses were consistent with this theory (McNeill, Personal Communication).

PTA–sol N levels were used as a measure of protein degradation. There was a highly significant increase in PTA–sol N in the pressed curd with storage of the milks (p < 0.001) and the PTA–sol N was also found to be correlated with the grade score at 3 months (r = 0.48) (p < 0.01) (Figure 6) and 6 months (r = 0.52).  The levels of PTA–sol N increased greatly during ripening from 1.12–1.63 mg/g in the pressed curd to 10.73 – 12.92 mg/g in the cheeses at 6 months.  This is consistent with the results of other workers (Aston et al., 1983).  There were no significant differences in the levels of PTA–sol N between the controls and stored milk cheeses during ripening.  This evidence would suggest that the proteolytic activity of psychrotrophic bacteria is less important than their lipolytic activity.

Several workers have noted increased levels of moisture retention in cheeses made from refrigerated milk (O’Leary et al. 1983).  We found an arithmetic increase in moisture levels but it was not statistically significant.

There was no significant difference in hardness between the pressed curd produced from unstored milk and that produced from stored milks.  The curd produced from milk stored for 5 days was significantly harder than that produced from milk stored for 3 days (p < 0.01).  It is difficult to explain this effect when we consider that moisture levels were only slightly higher in the stored milk cheeses and no reduction in fat levels were detected.  During the ripening period (6 months) there was a significant difference in hardness between the controls and the stored milk cheeses (p < 0.001), the latter being softer.  A significant difference was also found between cheese made from milk  after 3 and 5 days of storage (p = 0.037), the latter being softer.  This may be due to the higher levels of lipolysis in the stored milk cheeses (Figure 11) and the slightly higher moisture content.

 

Fig-7and-8y Effect of storing milk at 3 and 7C on the yield and quality of Cheddar cheese

Figure 11: Changes in the concentration of free fatty acids in Cheddar cheese manufactured from milk stored at 3° and 7°C. Figure 12: Relationship between the free fatty acid concentration in the pressed curd and the grade score at 3 months.

 

The cheese grader did not detect any differences in colour of the cheeses.  Observations using the Hunterlab Tristimulus Colorimeter did not reveal any statistically significant differences between the pressed curds produced from unstored and stored milks although the L values were slightly lower for the stored milk curds indicating that these cheeses were slightly darker in colour.

Total cheesemaking time (starter–mill) was unaffected by milk storage.  The average time was 4 hrs 48 mins.  The rennet–to–cut period and run–to–mill period were also unaffected but the cut–to–run period was significantly shorter for the stored milk (p < 0.05).  This may have occurred due to the development of some acidity during storage of the milk.  A significant difference was found between the TA of the stored milk and the unstored milks.

CONCLUSIONS

  1. Raw milk storage at 3° and 7°C for 3 days or more reduces the yield of Cheddar cheese.
  2. Raw milk storage at 7°C for 3 or more days adversely affects the quality of Cheddar cheese.  Cheese quality is also reduced by storage at 3°C for 5 days.
  3. Storage at 3°C for 3 days improves the quality of Cheddar cheese.
  4. Evidence suggests that poor quality is largely due to the activity of lipolytic enzymes produced by psychrotrophic bacteria.
  5. Proteolytic activity of psychrotrophic bacteria may also be important in cheese quality.
  6. Cheese quality is correlated with the psychrotrophic count, lipolytic count and total viable count of the raw milk and the FFA in the pressed curd.
  7. It is recommended that milk for Cheddar cheese manufacture should have a TVC of < 1 x 106 cfu/ml and preferably a count  of < 1 x 105 cfu/ml at the time of cheese manufacture.

RECOMMENDATIONS

  1. Fresh milk should be used for the manufacture of Cheddar cheese in order to obtain maximum yields and satisfactory cheese quality.
  2. Where prolonged storage is necessary, milk should be cooled to 3°C or less and maintained at that temperature until it is used for cheese manufacture.
  3. Other methods of preservation may be useful for storage of milk prior to cheese manufacture.  These methods include thermisation, addition of lactic acid bacteria, gas–flushing and activation of the lactoperoxidase system.  Further work should be undertaken to evaluate these methods.

ACKNOWLEDGEMENTS

We are grateful to the scientific staff of the Food Division, Mr E Espie, Mr J Dooey, Mr E Slaine, Mr A Reid, Mr P McTeague, Mr J Marks, Mr W Currie, Mr P Devlin, Mrs M Bell and Mrs L Lawrence for their valuable assistance in this project.  We are also thankful to Dr D Kilpatrick (Biometrics Division) for his assistance with statistical analysis, Mr G McNeill (The Agricultural Institute, Moorepark Research Centre, Fermoy, Co Cork) for providing FFA profiles of cheeses and Mr J Thompson (NIDCO) for grading of the cheeses.

 

REFERENCES

ALI, A.E., ANDREWS, A.T. and CHEESEMAN, G.C. (1980).  J. of Dairy Research47, 371–382.

ASTON, J.W., DURWARD, I.G. and DULLEY, J.R. (1983).  The Aust. J. of Dairy Technol., 38, 55–59.

BRITISH STANDARDS INSTITUTION (1963a).  The chemical analysis of liquid milk and cream.  (BS 1741).

BRITISH STANDARDS INSTITUTION (1963b).  Methods for the chemical analysis of cheese.  (BS 770).

BRITISH STANDARDS INSTITUTION (1968).  Methods of microbiological examination for dairy purposes.  (BS 4285).

BURLINGAME–FREY, J.P. and MARTIN, E.H. (1984).  J. of Food Protection, 47, 16‑19.

COUSIN, M.A. and MARTH, E.H. (1977).  J. of Dairy Science, 60, 1048–1056.

CREAMER, L.K., BERRY, G.P. and MILLS, O.E. (1977).  N.Z. J. of Dairy Sci. and Technol., 12, 58–66.

FOX, P.F. and STEPANIAK, L. (1983).  J. of Dairy Res., 50, 77–89.

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HARRIGAN, W.P. and McCANCE, M.E. (1976).  Laboratory methods in food and dairy microbiology.  London Academic Press.

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Readers may also wish to consider:

 Weatherup, W., Mullan, W.M.A. and Kormos, J. (1988). Effect of storing milk at 3°C and 7°C on the quality and yield of Cheddar cheese. Dairy Industries International, 53 (12):16, 17, 25. 

Weatherup,W. and Mullan,W.M.A. (1993) Effects of low temperature storage of milk on the quality and yield of cheese. Proceedings of IDF Seminar on Cheese Yield and Factors Affecting Its Control, Cork, Ireland, pp. 85–94.

 Some of these publications can be downloaded from www.reserachgate.net .

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