Viscoelastic Properties of Cheddar Cheese: Effect of Calcium and Phosphorus, Residual Lactose, Salt-to-Moisture Ratio and Ripening Time

Abstract

The viscoelastic properties of eight different types of Cheddar cheeses prepared with two levels of calcium (Ca) and Phosphorus (P) content, two levels of residual lactose content and two levels of salt to moisture ratio (S/M) ratio were studied in a STRESSTECH viscoanalyzer. The elastic (G′) and viscous (G″) modulus were measured at 0, 1, 2, 4, 6, and 8 months of ripening during heating the cheese samples from 30 to 70°C. Low levels of Ca and P content (0.53 g Ca and 0.39 g P /100 g cheese) in the Cheddar cheese resulted up to 20.9% and 15.9% lower elastic and viscous modulus respectively, compared to Cheddar cheese prepared with high levels of Ca and P content (0.67 g Ca and 0.53 g P/100g cheese) during ripening up to 8 months. Low levels of residual lactose (0.78 g/100g) in the Cheddar cheese resulted in 39.1 and 78.1% lower elastic and viscous modulus, respectively, compared to Cheddar cheese with high levels of residual lactose (1.4 g/100g) during ripening up to 8 months. In the same way, low levels of S/M ratio (4.8) in the Cheddar cheese resulted in 40.7 and 40.5% lower elastic and viscous modulus, respectively, compared to high levels of S/M ratio (6.4) during ripening up to 8 months. Upon heating from 30 to 70°C, the elastic and viscous modulus of the eight different types of Cheddar cheeses reduced up to 91.7 and 95.1%, respectively, during ripening. Cheddar cheese recorded maximum elastic modulus at the end of 8 months of ripening, and maximum viscous modulus at the end of 4 months of ripening.

Keywords:

• Cheddar cheese
• Ca and P
• Residual lactose
• Salt to moisture ratio (s/m)
• Viscoelastic properties

INTRODUCTION

The consumption of Cheddar cheese in USA is increasing continuously over the years. Cheddar cheese is used as an ingredient in different prepared foods to improve its color, flavor, taste, texture and nutritional qualities.[1] The protein network structure inside the Cheddar cheese changes continuously during ripening and affects the texture and rheological properties. There are various factors affecting the texture and rheological properties of Cheddar cheese during manufacturing and ripening. During ripening of Cheddar cheese, the level of Calcium and phosphorous, residual lactose and salt-to-moisture (S/M) ratio present at the time of manufacturing influence the pH development in the cheese and ultimately affects the rheological properties. Many researchers have studied the effect of buffering salt like Ca on Cheddar cheese quality and reported that reduced Ca content results in faster softening and better flow properties during melting.[2–5] The level of residual lactose present in the Cheddar cheese affects the pH development and extent of proteolysis and affects the cheese quality.[6–10] The level of salt to moisture ratio (S/M) also has significant effect on the pH, water activity, hydration of protein and extent of proteolysis during ripening and significantly affects the cheese quality.[8–12]

Small amplitude dynamic testing is widely used for characterization of food materials including cheese. Most of the food materials including Cheddar cheese is viscoelastic in nature and exhibit both elastic (Hookian solid) and viscous (Newtonian fluid) behavior.[13] Dynamic testing is a fundamental test and can be used to characterize the viscoelastic nature of cheeses.[14,15] Small amplitude (<1%) sinusoidal strain is applied on to the cheese and the resulting elastic and viscous modulus are measured.[16] In dynamic testing, the rheological properties of cheese are studied without affecting the protein network structure. Dynamic rheological test differentiates the cheese characterization in a better manner compared to melt profile test and stress relaxation test etc. to evaluate the functionality of cheese.[1] Previous research with Cheddar cheese prepared from reduced fat milk had resulted in increased viscoelasticity as indicated by the increased elastic modulus (G′) and viscous modulus (G″).[17] Nolan et al.[18] observed that the dynamic rheological properties of Mozzarella cheese viz. G′ and G″ provided the basis for distinguishing between imitation and natural cheeses. They also reported that G′ and G″ of cheeses had reduced as the calcium content was decreased and storage period was increased. In another study by Ustunol et al.[19] reported that the dynamic complex modulus correlates well with the meltability of Cheddar cheese determined by Arnott test. Tunick et al.[20] reported that dynamic viscoelastic properties can be used as a tool for preventing mislabeling. Proteolysis during ripening results in reduction in G′ and G″, and G′ and G″ measured at higher temperature had shown significant difference with difference in fat content in Cheddar cheese.[1]All these research works showed that dynamic rheological testing can be used as an effective tool for characterization of cheese due to changes resulting from variations in cheese composition, as well as ripening process.

Viscoelasticity of cheese is related to composition, manufacturing methods and storage conditions and can be used as a tool for characterization of different cheeses.[14] During Cheddar cheese manufacturing, variations in calcium (Ca) & phosphorus (P), residual lactose, and S/M ratio can occur due to source of milk and cheese making protocol adopted. The stage of lactation, seasonal variations and concentration technique used significantly affect the buffering salt like Ca & P content, residual lactose content and S/M ratio in the cheese.[21–24] Much research has been carried out on the individual effect of Ca and P content, residual lactose content, and S/M ratio on the Cheddar cheese quality, but no work has been done on the combined effect of changing the levels of Ca and P content, residual lactose content, and S/M ratio level on the viscoelastic properties of Cheddar cheese. Hence, this study was undertaken to study the effect of varying levels of Ca and P content, residual lactose content, and S/M ratio during cheddar cheese manufacturing on the viscoelastic properties Cheddar cheese during ripening up to 8 months.

MATERIALS AND METHODS

Cheese Manufacture and Storage

Three replications of the following eight different types of Cheddar cheese were manufactured[9] at the Department of Food Science and Nutrition, University of Minnesota and tested at the Department of Agricultural and Biosystems Engineering, South Dakota State University.

1. High Ca and P, high lactose, high salt to moisture ratio (HHH).

2. High Ca and P, high lactose, low salt to moisture ratio (HHL).

3. High Ca and P, low lactose, high salt to moisture ratio (HLH).

4. High Ca and P, low lactose, low salt to moisture ratio (HLL).

5. Low Ca and P, high lactose, high salt to moisture ratio (LHH).

6. Low Ca and P, high lactose, low salt to moisture ratio (LHL).

7. Low Ca and P, low lactose, high salt to moisture ratio (LLH).

8. Low Ca and P, low lactose, low salt to moisture ratio (LLL).

Eight different types of Cheddar cheese were prepared from 2950 kg of raw whole milk pasteurized at 73°C for 16 s. The milk was divided equally into four 908 kg cheese vats. Color was added at 6.61 mL/100 kg of milk. In two of the vats, in order to get high lactose treatments, lactose was added at 2.5 kg/100 kg of cheese milk; where as no additional lactose was added in the other two. Calcium chloride was added at 19.8 mL/100 kg of milk to obtain high Ca and P treatment in two of the vats, where as no calcium chloride was added in the other two vats to obtain the low Ca and P treatments. Starter culture was added at 27 mL/100 kg of milk for the low Ca and P treatments and 54 mL/100 Kg of milk for the high Ca and P treatments. In the high Ca and P treatments, rennet was added at 9.9 mL/100 kg of milk simultaneously with starter culture, where as in low Ca and P treatments, rennet was added at 9.9 mL/100 kg after the cheese milk reached pH of 6.2. The time to cut the coagulum was determined by subjective assessment of the coagulum and clarity of the whey.

A different cooking protocol was followed in the four vats. In the high Ca and P, high lactose treatment the curd whey temperature was raised to 37°C in 30 min and maintained at 37°C for another 30 min. In the high Ca and P, low lactose treatments, the curd whey temperature was raised to 37°C in 30 min. Upon reaching 37°C, half of the whey was drained and replaced with pH adjusted wash water containing calcium and phosphorus and cooked for another 30 min. In the low Ca and P and high lactose treatment, the curd whey temperature was raised to 38°C in 30 min and maintained for another 30 min with continuous stirring. In the low Ca and P and low lactose treatment, the curd whey temperature was raised to 38°C in 30 min and half of the whey was replaced with potable water and cooked for another 30 min at 38°C. On completion of cooking, a stirred curd cheese making procedure was followed in all treatments. Upon reaching pH of 5.4, the cheese curd was removed form each vat and divided into two equal parts. For low S/M ratio treatments, salt was added at 2.25% of the weight of the curd and for high S/M ratio treatment, salt was added at 3.5% by weight of the curd. The salted curds were hooped, pressed, and vacuum packed in polyethylene bags. The Cheddar cheese was transported to the dairy plant of South Dakota State University on the same day under refrigerated storage conditions.

The Cheddar cheese was removed from the vacuum seal, cut in to 6 equal parts, vacuum sealed (Sipromac, Model 620A, Leybold, S.S, France) in polyethylene pouches (Cryo Vac Vacuum pouches, Cedar Rapids, IA) and stored at 5 ± 1°C in the dairy plant, South Dakota State University. Cheese samples were removed from the curing chamber at the end of 0, 1, 2, 4, 6 and 8 months of ripening and tested for viscoelastic properties.

Compositional Analysis

Compositional analysis was performed on cheeses after 1 wk of ripening. Moisture of the cheeses was analyzed gravimetrically, by drying 1.5 g cheese at 100°C in a forced draft oven (Lindberg/Blue M, Asheville, NC) for 24 h. Fat content was determined by using the Mojonnier ether extraction method.[25] Total protein in cheeses was determined by measuring total nitrogen in the cheeses using the Dumas combustion method (Leco Tru Spec N analyzer, Leco, St. Joseph, MI)[26] and converting it to protein by using a multiplication factor of 6.38. Salt content in cheeses was determined using a Corning Chloride Analyzer 926 (Ciba Corning Diagnostics, Medfield, MA), based on the Volhard test.[27] Total Ca in cheeses was measured by using an atomic absorption spectroscopy procedure.[28] Total P in cheeses was determined colorimetrically.[29] Lactose content was determined at day 1, using a HPLC-based method suggested by Zeppa et al.[30] with suitable modifications.[9]

Dynamic Viscoelastic Measurements

The dynamic viscoelastic measurements were carried out using STRESSTECH rheometer (ATS Rheosystems, Bordentown, NJ and RHEOLOGICA Instruments AB, Lund, Sweden). The rheometer had provisions for automatic gap setting, differential pressure quantitative normal force on the sample and automatic inertia compensation. The rheological measurements were made using parallel plate geometry in the rheometer. The fixed bottom plate had a diameter of 29.95 mm and provided with heating and cooling system to vary the temperature from 15 to 500°C. The detachable upper plate had a diameter of 29.95 mm and connected to the moving head of the Rheometer. The rheometer had provisions to vary the frequency of the upper plate from 1 × 10–⁵ to 2 × 10² Hz.

Cheese samples were prepared by cutting a thin slice (2.0 mm) using a food slicer (model MS 1043-W, The Rival Co., Kansas, MO). Cylindrical specimens of 28.5 mm diameter were cut using cork borer. The samples were placed in a Petri dish and transferred to a refrigerator (8–10°C) until testing.

To maintain high degree of uniformity among the measurements, the samples were loaded in between the parallel plates at a constant normal force of 20 N. The peripheral surface of the sample was oiled to prevent loss of moisture during testing. Preshear was applied on the sample at a constant shear rate of 11 Hz for 20.0 s. After preshear, the sample was allowed for 60 s for the temperature of the cheese to reach the testing temperature of 30°C. During preliminary experiments, we found out that the elastic and viscous modulus values were linear in the range of 0.1 Hz to 50 Hz at shear stress of 750 Pa for all the eight different types of Cheddar cheeses. The actual temperature sweep test was carried out at a constant frequency of 1 Hz, and at a constant strain of 0.5% throughout the study. The temperature of the sample was adjusted to 30°C at the beginning of the experiment and raised to 70°C in 40 min. For each sample, the measured parameters were elastic modulus (G′) and viscous modulus (G″). The measurements were made in triplicate for the eight different types of Cheddar cheeses at the end of 0, 1, 2, 4, 6, and 8 months of ripening.

Statistical Analysis

Three replications of eight different types Cheddar cheeses were manufactured and the viscoelastic properties were measured in triplicate at 0, 1, 2, 4, 6, and 8 months of ripening. The viscous and elastic modulus data were analyzed by students t test at p = 0.05 to find out the effect of low and high levels of buffer content, residual lactose content, and S/M ratio on the viscous and elastic modulus at different ripening times using SAS version 8.0.

RESULTS AND DISCUSSION

Cheese Composition

The chemical composition of the eight different types of Cheddar cheeses is given in Table 1. The total calcium and phosphorus content of the four cheeses with low Ca and P treatment were significantly lower than the four cheeses with high Ca and P treatment. The lactose content of the four cheeses with low lactose treatment was significantly lesser than the four cheeses with high lactose treatment. The moisture content of all the high S/M ratio treatment cheeses was significantly lower than the low S/M ratio cheeses. There was no significant difference in the protein content of the eight different types of the cheeses. The fat content of the Cheddar cheeses was found to be varying from 33.32 to 35.93%. The S/M ratio of the four cheeses with high S/M ratio treatment was significantly higher than the four cheeses with low S/M ratio. This indicated that the manufacturing methods adopted was very effective in adjusting the levels Ca and P content, lactose content and S/M ratio in the Cheddar cheese. However, significant differences in moisture content need to be considered when interpreting the effects of Ca and P, residual lactose and S/M ratio on cheese dynamic viscoelastic properties.

Table 1 Chemical composition of cheeses expressed as % by weight of cheese

	Treatments
	HHH	HHL	HLH	HLL	LHH	LHL	LLH	LLL
Moisture (%)	32.1 ^a	33.8 ^b,c	33.1 ^a,b	35.2 ^d,e	34.1 ^b,c,d	35.9 ^e	34.4 ^c,d	37.6 ^f
Fat (%)	35.9 ^a	35.0 ^a,b	35.7 ^a,b	34.8 ^a,b,c	34.5 ^b,c,d	33.6 ^c,d	34.7 ^a,b,c	33.3 ^d
Protein (%)	26.4	25.6	26.0	25.3	25.2	24.8	25.3	24.5
Salt (%)	2.04 ^b,c	1.68 ^d	2.14 ^a,b	1.73 ^c,d	2.28 ^a,b	1.63 ^d	2.47 ^a	1.75 ^c,d
S/M (%)	6.37 ^a	4.98 ^b	6.48 ^a	4.92 ^b	6.71 ^a	4.53 ^b	7.18 ^a	4.65 ^b
Total Ca (%)	0.69 ^a	0.67 ^a	0.66 ^a	0.64 ^a	0.55 ^b	0.54 ^b	0.52 ^b	0.51 ^b
Total P (%)	0.48 ^a	0.48 ^a	0.47 ^a	0.46 ^a	0.41 ^b	0.40 ^b	0.38 ^b	0.38 ^b
Lactose (%, d1)	1.52 ^a	1.35 ^c	0.32 ^d,e	0.11 ^e	1.64 ^a,b	1.41 ^b,c	0.49 ^d	0.27 ^e
^a,b,c,d,e,fMeans in a row with common superscript do not differ (P ≥ 0.05).

Effect of Calcium and Phosphorus Content on Viscoelasticity

Higher Ca and P content in the Cheddar cheese resulted in significantly higher (p < 0.05) elastic modulus at cheese temperature of 30°C at 0 month (15.7%), 1 month (20.9%) and at 2 months (8.7%) of ripening (Table 2). After 2 months, even though the elastic modulus of high Ca and P cheeses were higher than the low Ca and P cheeses, there was no significant difference between them (Table 2). In another study, using the same eight different types of Cheddar cheeses, Upreti et al.[9,10] observed that lowering Ca and P level in Cheddar cheese, resulted in higher rate of proteolysis and chymosin activity resulting in reduced cross linkage of Ca and P in the para casein matrix with low Ca and P cheeses. This reduced cross linking of Ca and P in the low Ca and P cheeses might have resulted in lower elastic modulus with low Ca and P cheeses. The viscous modulus at a cheese temperature of 30°C of high Ca and P cheeses was significantly higher at 0 month (15.8%) and 1 month (15.9%) of ripening (Table 3). After 2 months, even though the high Ca and P cheeses had higher viscous modulus compared to the low Ca and P cheeses, there was no significant difference (p < 0.05) between them. The elastic modulus of high and low Ca and P cheeses were higher than the viscous modulus of the corresponding high and low Ca and P cheeses throughout the ripening period indicating all the Cheddar cheese remained more elastic throughout the ripening process (Table 2 and 3). The same trend was observed by Guinee at al.[4] and Joshi et al.[14] with cheeses containing reduced buffering salt like Calcium. Joshi et al.[14] have reported that with reduced buffering salt like calcium, there was greater degree of hydration and protein swelling and might have resulted in the higher elastic modulus and lower viscous modulus of Mozzarella cheeses.

Table 2 Effect of calcium and phosphorus content, residual lactose content, salt-to-moisture ratio, temperature and ripening time on the elastic (G′) modulus (kPa) of Cheddar cheese

		Elastic modulus (kPa)
Storage period	Treatment	30°C	40°C	50°C	60°C	70°C
Month 0	High Ca and P	71.228 ^a	34.435 ^a	12.386 ^a	5.152 ^a	2.861 ^a
	Low Ca and P	61.583 ^b	32.403 ^a	11.540 ^a	4.415 ^a	2.607 ^a
	High lactose	66.736 ^a	34.142 ^a	12.494 ^a	4.305 ^a	2.315 ^a
	Low lactose	56.075 ^b	32.696 ^a	11.432 ^a	4.262 ^a	2.153 ^a
	High S/M	77.636 ^a	38.800 ^a	13.701 ^a	4.591 ^a	2.250 ^a
	Low S/M	55.175 ^b	28.038 ^b	10.225 ^b	3.976 ^a	2.218 ^a
Month 1	High Ca and P	94.092 ^a	45.420 ^a	19.498 ^a	9.039 ^a	4.710 ^a
	Low Ca and P	77.856 ^b	39.050 ^a	16.170 ^a	7.061 ^a	3.695 ^a
	High lactose	95.508 ^a	48.194 ^a	21.269 ^a	9.428 ^a	4.768 ^a
	Low lactose	76.439 ^b	36.376 ^b	14.399 ^b	6.671 ^b	3.637 ^a
	High S/M	95.358 ^a	48.956 ^a	21.739 ^a	10.068 ^a	5.372 ^a
	Low S/M	76.589 ^b	35.514 ^b	13.929 ^b	6.031 ^b	3.032 ^b
Month 2	High Ca and P	76.369 ^a	38.406 ^a	15.701 ^a	6.395 ^a	3.182 ^a
	Low Ca and P	70.247 ^b	33.256 ^b	12.978 ^b	5.350 ^b	2.683 ^b
	High lactose	80.836 ^a	38.722 ^a	15.584 ^a	6.325 ^a	3.078 ^a
	Low lactose	65.781 ^b	32.393 ^b	13.094 ^b	5.421 ^b	2.787 ^a
	High S/M	80.725 ^a	39.100 ^a	16.145 ^a	6.626 ^a	3.348 ^a
	Low S/M	65.892 ^b	32.561 ^b	12.533 ^b	5.120 ^b	2.517 ^b
Month 4	High Ca and P	91.694 ^a	43.408 ^a	16.596 ^a	5.938 ^a	2.614 ^a
	Low Ca and P	91.472 ^a	40.950 ^b	14.086 ^b	4.916 ^b	2.270 ^a
	High lactose	110.306 ^a	49.706 ^a	18.201 ^a	6.281 ^a	2.821 ^a
	Low lactose	72.861 ^b	34.653 ^b	12.482 ^b	4.573 ^b	2.064 ^b
	High S/M	100.508 ^a	46.528 ^a	17.658 ^a	6.258 ^a	2.887 ^a
	Low S/M	82.658 ^b	37.831 ^b	13.024 ^b	4.595 ^b	1.997 ^b
Month 6	High Ca and P	97.536 ^a	41.881 ^a	14.771 ^a	5.056 ^a	2.347 ^a
	Low Ca and P	96.759 ^a	37.978 ^a	11.155 ^b	3.480 ^b	1.482 ^b
	High lactose	110.231 ^a	44.961 ^a	14.303 ^a	4.725 ^a	2.167 ^a
	Low lactose	84.061 ^b	34.897 ^b	11.623 ^b	3.812 ^b	1.663 ^b
	High S/M	101.811 ^a	39.964 ^a	14.415 ^a	5.130 ^a	2.467 ^a
	Low S/M	92.481 ^b	39.894 ^a	11.511 ^b	3.406 ^b	1.363 ^b
Month 8	High Ca and P	118.453 ^a	46.178 ^a	16.822 ^a	5.628 ^a	2.299 ^a
	Low Ca and P	115.116 ^a	39.906 ^b	11.712 ^b	3.447 ^b	1.524 ^b
	High lactose	130.053 ^a	49.558 ^a	16.124 ^a	5.343 ^a	2.397 ^a
	Low lactose	93.516 ^b	36.525 ^b	12.410 ^b	3.733 ^b	1.427 ^b
	High S/M	118.555 ^a	46.117 ^a	16.329 ^a	5.356 ^a	2.259 ^a
	Low S/M	105.014 ^b	39.967 ^b	12.205 ^b	3.719 ^b	1.565 ^b
^a,bSuffixed with common letters had no significant difference at P < 0.05 between high and low levels of Ca and P, lactose, and S/M ratio with students ‘t’ test at each month.

Table 3 Effect of calcium and phosphorus content, residual lactose content, salt-to-moisture ratio, temperature and ripening time on the viscous (G″) modulus (kPa) of cheddar cheese

		Viscous modulus (kPa)
Storage period	Treatment	30°C	40°C	50°C	60°C	70°C
Month 0	High Ca and P	23.383 ^a	13.004 ^a	5.094 ^a	2.440 ^a	1.708 ^a
	Low Ca and P	20.178 ^b	11.840 ^a	4.994 ^a	1.741 ^b	1.075 ^a
	High lactose	22.424 ^a	12.916 ^a	5.336 ^a	2.150 ^a	1.423 ^a
	Low lactose	20.136 ^b	11.927 ^a	4.751 ^a	2.030 ^a	1.360 ^a
	High S/M	25.450 ^a	14.521 ^a	5.793 ^a	2.186 ^a	1.427 ^a
	Low S/M	18.111 ^b	10.322 ^b	4.294 ^b	1.994 ^a	1.356 ^a
Month 1	High Ca and P	29.832 ^a	16.213 ^a	7.904 ^a	4.184 ^a	2.831 ^a
	Low Ca and P	25.736 ^b	14.626 ^a	6.957 ^a	3.615 ^a	2.412 ^a
	High lactose	30.915 ^a	17.365 ^a	8.646 ^a	4.335 ^a	2.718 ^a
	Low lactose	24.653 ^b	13.473 ^b	6.215 ^b	3.465 ^a	2.525 ^a
	High S/M	30.444 ^a	17.451 ^a	8.645 ^a	4.508 ^a	3.059 ^a
	Low S/M	25.124 ^b	13.387 ^b	6.215 ^b	3.293 ^b	2.184 ^b
Month 2	High Ca and P	24.340 ^a	13.939 ^a	6.553 ^a	3.289 ^a	2.207 ^a
	Low Ca and P	23.175 ^a	12.914 ^a	5.927 ^a	3.019 ^a	1.953 ^a
	High lactose	26.024 ^a	14.505 ^a	6.670 ^a	3.210 ^a	2.131 ^a
	Low lactose	21.491 ^b	12.348 ^b	5.810 ^b	3.098 ^a	2.029 ^a
	High S/M	25.650 ^a	14.285 ^a	6.651 ^a	3.217 ^a	2.168 ^a
	Low S/M	21.865 ^b	12.568 ^b	5.829 ^b	3.091 ^a	1.998 ^a
Month 4	High Ca and P	51.878 ^a	24.975 ^a	9.862 ^a	3.986 ^a	2.192 ^a
	Low Ca and P	49.589 ^a	24.494 ^a	9.239 ^a	3.688 ^a	1.998 ^a
	High lactose	61.725 ^a	29.474 ^a	11.254 ^a	4.338 ^a	2.342 ^a
	Low lactose	39.742 ^b	19.994 ^b	7.847 ^b	3.336 ^b	1.847 ^b
	High S/M	53.233 ^a	26.261 ^a	10.493 ^a	4.174 ^a	2.320 ^a
	Low S/M	48.233 ^b	23.208 ^b	8.609 ^b	3.500 ^b	1.868 ^b
Month 6	High Ca and P	42.931 ^a	16.529 ^a	6.710 ^a	3.100 ^a	1.938 ^a
	Low Ca and P	32.003 ^a	15.788 ^a	5.769 ^b	2.385 ^b	1.344 ^b
	High lactose	38.867 ^a	18.296 ^a	6.881 ^a	2.830 ^a	1.658 ^a
	Low lactose	32.067 ^a	14.020 ^b	5.599 ^b	2.655 ^a	1.624 ^a
	High S/M	44.333 ^a	16.457 ^a	6.576 ^a	3.093 ^a	2.207 ^a
	Low S/M	30.600 ^b	15.859 ^a	5.904 ^a	2.393 ^b	1.256 ^b
Month 8	High Ca and P	32.892 ^a	6.351 ^a	5.312 ^a	3.543 ^a	1.981 ^a
	Low Ca and P	31.883 ^b	5.678 ^a	3.764 ^b	2.439 ^b	1.419 ^b
	High lactose	37.184 ^a	6.245 ^a	4.955 ^a	3.323 ^a	1.953 ^a
	Low lactose	27.591 ^b	5.784 ^a	4.121 ^b	2.659 ^b	1.448 ^b
	High S/M	34.671 ^a	6.688 ^a	5.046 ^a	3.273 ^a	1.860 ^a
	Low S/M	32.102 ^b	5.341 ^b	4.031 ^b	2.709 ^b	1.541 ^b
^a,bSuffixed with common letters had no significant difference at P < 0.05 between high and low levels of Ca and P, lactose, and S/M ratio with students ‘t’ test at each month.

Effect of Residual Lactose Content on Viscoelasticity

The Cheddar cheese with high lactose content resulted in significantly higher elastic and viscous modulus compared to the cheddar cheese with low lactose content throughout the ripening process (Table 2 and 3). The elastic modulus of Cheddar cheese with high lactose content at 30°C was 19.0%, 24.9%, 22.9%, 51.4%, 31.1%, 39.1% higher compared to the Cheddar cheese with low lactose content at 0, 1, 2, 4, 6 and 8 months of ripening, respectively (Table 2). In the same way, the viscous modulus of Cheddar cheese with high lactose content was 11.4%, 25.4%, 21.1%, 55.3%, 21.2%, and 34.7% higher at a cheese temperature of 30°C compared to the viscous modulus of Cheddar cheese with low lactose content at the end of 0, 1, 2, 4, 6, and 8 months of ripening, respectively (Table 3). In another study, using the same 8 different types of Cheddar cheeses, Upreti et al.[9,10] observed that during ripening higher rate of proteolysis and chymosin activity with low lactose cheeses. The increased rate of proteolysis and chymosin activity with low buffer cheese might have resulted in weakening of protein network structure, and resulted in reduced elastic and viscous modulus of Cheddar cheese. At any ripening time, the elastic modulus of both high and low lactose Cheddar cheeses were always higher than the viscous modulus at a temperature of 30°C indicating that the Cheddar cheese exhibited more elastic behavior throughout the ripening process. A similar trend was observed by Venugopal and Muthukumarappan,[1] Subramanian et al.,[31] and Ustunol et al.,[17] during dynamic testing of Cheddar cheese.

Effect of Salt-to-Moisture Ratio on Viscoelasticity

Changing the levels of S/M ratio had significant effect on the viscous and elastic modulus of the Cheddar cheeses (p < 0.05). The elastic modulus of Cheddar cheese at a cheese temperature of 30°C with high S/M ratio was 40.7%, 24.5%, 22.5%, 21.6%, 10.1%, and 12.9% higher compared to elastic modulus of Cheddar cheese with low S/M ratio at the end of 0, 1, 2, 4, 6, and 8 months of ripening, respectively (Table 2). In the same way, the viscous modulus of Cheddar cheese at a cheese temperature of 30°C with high S/M ratio was 40.5%, 21.1%, 17.3%, 10.4%, 44.9%, and 8.0% higher compared to the Cheddar cheese with low S/M ratio at the end of 0, 1, 2, 4, 6, and 8 months of ripening, respectively (Table 3). In another study using the same eight different type of Cheddar cheeses, Upreti et al.[9,10] observed that Cheddar cheese with low S/M ratio had higher proteolytic activity compared to the Cheddar cheese with high S/M ratio. Varying the salt content in the cheese had significant influence in the ionic strength and brings about changes in the protein matrix bonding.[32] The increased proteolytic activity and the changes occurring in the protein matrix bonding due to ionic change in the Cheddar cheeses with low S/M ratio might have resulted in the lower elastic and viscous modulus. All the low S/M ratio cheeses had higher moisture content and might have affected the hydration and swelling of protein matrix and resulted in significantly higher viscous modulus. As expected, the elastic modulus of all the low and high S/M ratio cheeses were higher than the viscous modulus of corresponding low and high S/M ratio cheeses (Table 2 and 3) indicating that the Cheddar cheese remained more elastic throughout the ripening process.

Effect of Temperature on Viscoelasticity

Continuous reduction in elastic modulus of the Cheddar cheese was observed during heating from 30°C to 70°C at all ripening times (Table 1). In the same way the viscous modulus of the Cheddar cheese also reduced continuously upon heating from 30°C to 70°C at all ripening times (Table 2). The elastic modulus of eight different treatments of Cheddar cheeses was reduced by 94.9 to 97.1% upon heating from 30 to 70°C at the time of manufacturing (0 month). The reduction in elastic modulus of the same eight different types of Cheddar cheese upon heating from 30 to 70°C during the eighth month was 98.1 to 98.7% (Table 2). The viscous modulus had reduced by 92.5 to 94.7% for the eight different treatment of Cheddar cheeses upon heating from 30 to 70°C at the time of manufacturing (0 month) and the corresponding reduction in viscous modulus was from 93.9 to 95.5% after 8 months of ripening (Table 3). Upon heating the Cheddar cheeses might have lost its elastic solid characteristics due to melting of fat and turned into viscous fluid. This might have caused the cheese matrix to soften and resulted in reduced viscoleastic properties. The same trend was observed by Joshi et al.,[14] and Venugopal and Muthukumarappan[1] during dynamic testing of different types of cheeses. The elastic modulus of Cheddar cheese with low Ca and P content, lactose content, and S/M ratio was always significantly lesser at all cheese temperatures and at all ripening times compared to the elastic modulus of the Cheddar cheese with corresponding high levels of Ca and P, residual lactose, and S/M ratio except month 0 (Table 2). At month 0, even though, the high levels of buffer and lactose content had resulted in higher elastic modulus, there was no significant difference between them. In the same way, all Cheddar cheeses with low levels of Ca and P, residual lactose, and S/M ratio resulted in lower viscous modulus at all temperatures and all ripening times compared to corresponding high levels of Ca and P, residual lactose and S/M ratio, even though there was no significant difference between some of them (Table 3).

In another related study using the same eight different cheeses, during melt profile analysis, the softening temperatures were in the range of 48.7 to 51.9°C and the melting temperatures were in the range of 63.5 to 63.9°C at month 0.[33] During dynamic testing the elastic and viscous modulus decreased at a rapid rate up to softening temperature of cheese (Figure 1). Further increasing the temperature above softening point, the reduction in elastic and viscous modulus became bare minimum, and the curves became more flat. No reduction in elastic and viscous modulus was observed above melting temperature. In the same way, at 8 months of ripening, the softening and melting temperatures of eight different types of Cheddar cheese were in the range of 43.3–48.8°C, and 60.5–61.4°C, respectively. In dynamic testing at 8 months of ripening, the elastic and viscous modulus reduced at rapid rate up to softening temperature (Figure 2). Between, softening and melting temperatures, the rate of decrease in elastic and viscous modulus reduced to a bare minimum and above melting temperature, no drop in elastic and viscous modulus were observed. This kind of result was observed throughout the ripening period.

Figure 1 (a) Effect of calcium and phosphorus, lactose and S/M ratio on the elastic modulus of Cheddar cheese at Month 0; (b) Effect of levels of calcium and phosphorus, lactose and S/M ratio on the viscous modulus of Cheddar cheese at Month 0; s: High lactose high salt to moisture ratio, h: High lactose low salt to moisture ratio, Δ: Low lactose high salt to moisture ratio, e: Low lactose low salt to moisture ratio.

Figure 2 (a) Effect of levels of calcium and phosphorus, lactose and S/M ratio on the elastic modulus of Cheddar cheese at Month 8; (b) Effect of levels of calcium and phosphorus, lactose and S/M ratio on the viscous modulus of Cheddar cheese at Month 8; Solid line –; High Ca and P; dotted line – Low Ca and P; s: High lactose high salt to moisture ratio, h: High lactose low salt to moisture ratio, Δ: Low lactose high salt to moisture ratio, e: Low lactose low salt to moisture ratio.

Effect of Ripening Time on Viscoelasticity

The elastic modulus measured at 30°C for all the cheeses found to increase up to one month and decreased during second month. After two months, the elastic modulus was found to increase continuously throughout the ripening period and reached maximum at 8 months of ripening (Table 2). The viscous modulus measured at 30°C for all the treatments of Cheddar cheese was found to increase continuously during ripening and reached maximum value at 4 months of storage. After 4 months, the viscous modulus decreased as the ripening time was increased (Table 3). Joshi et al.[14] observed that the viscous and elastic modulus of the Mozzarella cheese to decrease when the storage period was increased from 1 to 30 days. Ak and Gunasekaran[35] also observed lowering of elastic and viscous modulus for a storage period of 6 weeks in Mozzarella cheese. The results of our experiments did not show decreasing trend in viscous and elastic modulus during ripening of Cheddar cheese. In another study, using the same 8 different types of cheeses, during texture profile analysis, three distinct regions were observed based on hardness.[34] During initial period up to one month, the cheese became soft and between 1 and 6 months, the cheese became firm characterized by increase in hardness value, and again turned soft after 6 months. The resilience of the cheeses decreased during initial periods of ripening and reached minimum at 4 months. After 4 months, the resilience was found to increase during ripening. These complex changes occurring in the texture of Cheddar cheese during ripening might have resulted in characteristic changes in the elastic and viscous modulus of Cheddar cheese. The 8 different types of Cheddar cheese attained maximum G′ and G″ values at the end of 8 and 4 months, respectively.

In other studies by Upreti et al.,[9,10] during testing of the same eight different types of Cheddar cheeses all the low levels of Ca and P, residual lactose and low S/M ratio cheeses had higher proteolytic activity compared to the corresponding high levels of Ca and P, residual lactose and S/M ratio cheeses. In this study, Cheddar cheese with low levels of Ca and P, residual lactose and S/M ratio exhibited lower elastic and viscous modulus throughout the ripening time (Table 2 and 3). With higher proteolytic activity, the cheese might have become soft and resulted in reduced viscoelastic properties. Ustunol et al.[17] also observed decreased elastic and viscous modulus due to higher proteolytic activity during dynamic testing of reduced fat Cheddar cheese.

CONCLUSIONS

Varying the levels of Ca and P content, residual lactose content, and S/M ratio had significant effect on the viscoelastic properties of Cheddar cheese. pH development in Cheddar cheese during ripening depends on the level of Ca and P content, residual lactose content, and salt-to-moisture ratio present in the Cheddar cheese at the time of manufacturing, which ultimately affects the viscoelastic properties of Cheddar cheese during ripening. The Cheddar cheese prepared with high levels of Ca and P content resulted in up to 20.9% higher elastic modulus and up to 15.9% higher viscous modulus compared to the Cheddar cheese prepared with low levels of Ca and P content during ripening up to 8 months. The Cheddar cheese prepared with high levels of lactose content resulted in up to 39.1% higher elastic modulus and up to 78.1% higher viscous modulus compared to the Cheddar cheese prepared with low levels of lactose content during ripening up to 8 months. The Cheddar cheese with high levels of S/M ratio had resulted in up to 40.7% higher elastic modulus and up to 40.5% higher viscous modulus compared to the Cheddar cheese prepared with low levels of S/M ratio during ripening up to 8 months. Upon heating from 30°C to 70°C, the elastic modulus reduced up to 98.7%, and the viscous modulus reduced up to 95.1% during ripening for 8 months. The Cheddar cheese had reached maximum elastic modulus at 8 months of ripening and maximum viscous modulus at 4 months of ripening.

ACKNOWLEDGMENT

We would like to thank Dairy management Inc. (DMI), at Rosemont, IL, and the Agricultural Experiment Station, South Dakota State University, Brookings, SD for the financial support for carrying out this research work. We also express our sincere thanks to Dr. Lloyd Metzger and Dr. Praveen Upreti of University of Minnesota for their help in manufacturing the cheeses for testing and compositional analysis.