Sensory and Objective Measurement of Sandiness in Dulce de Leche, a Typical Argentine Dairy Product

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Sensory and Objective Measurement of Sandiness in Dulce de Leche, a Typical Argentine Dairy Product
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  Sensory and Objective Measurement of Sandiness in Duke de Leche, a Typical Argentine Dairy Product GUILLERMO HOUGH, EDGARW MARTINEZ, and ADRIANA CONTARlNl lnstituto Superior Experimental de Tecnologia Alirnentaria zy . lrigoyen 931 6500 Nueve de Julio Buenos Aires, Argentina ABSTRACT zyxwvutsr actose crystallization in Dulce de Leche can produce a sandiness defect. The objective measurement and sensory evaluation of this defect were the thrusts of this study. The technique for the mi- croscopic measurement of the number and size of crystals is described in detail. Sensory panel members found that iac- tose crystals added to Dulce de Leche produced a sandiness defect equivalent to added sand crystals. The latter are far easier to define by zyxwvu ize so they were used for magnitude estimation, where the ex- ponent of Steven’s law was approxi- mately 1. Based on this, seven-point a sensory scale for sandiness was devel- oped using Dulce de Leche mixed with sand of crystal sizes varying between 0 and 1500 pm. A double enay table is presented whereby the sensory sandiness threshold can be read as a function of size and number of crystals. With crystal size below 6 pm sandiness is not detected, even if all lactose in Dulce de Leche is crystallized. Above this size, the detec- tion threshold depends on number of crystals. (Key words: Dulce de Leche, sandiness, sensory evaluation) INTRODUCTION Dulce de Leche, a typical Argentine dairy product similar to sweetened condensed milk, was described by Mor0 and Hough (12) in a previous paper. A standard initial formulation is 10 parts of milk and 2 parts of sucrose. This zyxwv Received December zyxwvutsr   1988 Accepted July 24, 1989 is concentrated to 70% total solids by boiling at atmospheric pressure. From milk with 12% to- z al solids and 4.5% lactose, lactose concentra- tion in Dulce de Leche is 9.85 dl00 g. Consid- ering the water phase, the lactose concentration is 33 @lo0 g water. Solubility of lactose at 15 and 30°C is 16.9 and 24.8 g/100 g water, respectively (20). So even without interfer- ences, lactose in Duke de Leche is initially in a supersaturated solution. This is compounded by the simultaneous presence of sucrose (146 g/ 100 g water), which substantially reduces lac- tose solubility (14). Under these conditions, lactose crystallization is inevitable in Dulce de Leche, which leads to the prevalent defect of sandiness. Technological methods have been developed to eliminate this defect in Dulce de Leche (2, 16, 17, 18, 19) and in sweetened condensed milk (1, 4, 7, 9), which is of similar composi- tion. To evaluate experimental results these au- thors used arbitrary category scales for the sen- sory measurement of sandiness, and in some cases measured crystal size microscopically (5, 7, 9, 17). Hunziker (7) is the only investigator to re- late quantitatively the number and size of crys- tals with a sensory scale for degree of sandi- ness. Santos et al. (17) noted a contradiction in that “no sandiness” was the score for crystals smaller than 10 pm yet Hunziker recommended seeding with 400-mesh lactose, which has a 37-pm aperture. According to Hunziker (7), absence of sandiness is obtained when there are more than 4 x lo8 crystals/cm3. With this number, he calculated a corresponding 10-p crystal size; in the calculation he used a lactose solubility value, which does not coincide with the solubility of lactose in the presence of sucrose reported by Nickerson (14). Other points of Hunziker’s sandiness scale are based on an arbitrary reduction in number of crystals. 1990 Dairy Sci 73:604-611 604  MEASUREMENT OF zyxwvu ANDINESS IN DULCE DE LECHE zyx 05 zy learly, for research and quality control of sandiness in Dulce de Leche, the following aspects need developing: zyxwvut ) a sensory percep- tion scale of sandiness in relation to objectively measured size and number of crystals, and 2) sensory threshold of sandiness as a function of size and number of lactose crystals. The objec- tive of this study was to cover both these aspects. zyxwvuts ATERIALS AND METHODS Size and Number of Crystals There is no published standard for measur- ing the size and number of lactose crystals in day products. Crystalline lactose in milk prod- ucts can be detected simply by means of a polarizing microscope (15). In this study an Olympus BHI Model BH-2 (Tokyo, pn) mi- croscope was used equipped with a polarizing filter, PM-6/ENM-7 Olympus photography and ocular with calibrated scale to allow sectorizing an observed field and measurement of crystal axis. Lactose crystal numbers and size were mea- sured as follows: a microscope slide and cover were weighed. Approximately ,003 g of Dulce de Leche was then weighed onto the slide. The center of the cover slip was placed over the Duke de Leche, and a slight pressure was exerted. Care was taken not to crush crystals; if this was observed through the microscope, the slide was prepared again. The resulting sample on the slide was circular in appearance. Crys- tals were counted in each of 10 randomly cho- sen microscopic fields and then averaged to calculate: N = nf zyxwvutsr t R2/(FW) where N = number of crystals per gram of Dulce de Leche, nf = average number of crys- tals observed in F, R = radius of sample (mm), F = area of microscopic field (mm), and W = weight of Dulce de Leche sample. The same prepared slide was used for measuring crystal size. The length of the longest axis of a-lactose crystal is the most relevant to sensory analysis; therefore, this is the measure in which sizes are expressed. Within a Dulce de Leche sample, lactose crystal sizes are not uniform; therefore, averaging was required. A microscopic field was randomly placed over the slide. Magnifica- tion was selected until approximately 20 crys- tals appeared in the field. If there were too many crystals at maximum magnification, a reticulated portion of the slide ln, 1/4, etc.) containing approximately 20 crystals was used. Average size of individual measurements was taken. In some samples size stratification was clear. In these cases, the number of crystals are re- ported separately for each size. Sensory Scale Where crystal-free Duke de Leche (CFDL) was necessary, it was provided from a fresh batch by Mastellone Hnos, Buenos Aires, Ar- gentinc who used P-galactosidase to reduce lactose content and prevent sandiness. To en- sure no crystals were present, samples were first observed under the microscope with polar- ized light. Sand used for mixing with CFDL to simu- late lactose crystals was obtained from local construction stores. The sand was washed, dried, and finally placed overnight in an oven at 130°C for sterilization. To obtain different granulomeay, the sand was sieved through standard-sized screens. A 12-member panel was used for sensory evaluations. Training included selection by per- formance on taste tests and sequential analysis (8), followed by 10 d of intensive training in sensory practices (8). Evaluations consisted of four 1-h sessions where the panel evaluated different samples using as a scale a IO-cm line marked on one end “no sandiness” and “extreme sandiness” on the other. This scale was used during training and development of a consensus evaluation procedure. Also at this stage, samples of CFDL, mixed with washed and sterilized sand of different granulometry, were presented to the panel to compare sensory perception of this sandiness with that of samples with lactose The next step was to establish sensory ver- sus objective evaluation of sandiness. The fixed modules magnitude estimation method was chosen 1 1). For this, lactose crystals of defi- nite size are needed, but these are difficult to obtain. To allow lactose naturally present in Dulce de Leche to crystallize to different sizes crystals. Journal of Dairy Science Vol. 73, No. 3, 1990  6 6 zyxwvuts OUGH zyxwv T L zyxwv TABLE zyxwvutsrqp   Sieves. resulting crystal size and concentration of sand used for magnitude estimation of zyx ensory versus objective crystal size. Concentration Sieve CrYd of sand in Sensory aperture size Dulce Leche scale' (m7 loog) ... zyxwvu   0 125 115 5 1 250 307 5 3 700 622 5 6 840 895 10 9 1250 1207 10 12 I500 1490 1 15 'Standard number as presented to panelists. was impractical because of lack of size unifor- mity. Because of these difficulties, sand sieved through different-sized screens was used as a replacement. Concentration of sand in CFDL was determined by approximating the number of sand crystals to the number of lactose crys- tals of similar size present in commercial Dulce de Leche. The sieves, sand crystal sizes, and concentrations used for magnitude estimation are in Table 1 Sizes cover the range found in Dulce de Leche. Prior to the test, the panel received an orientation session (13). Magnitude estimation was done in duplicate on different Based on magnitude estimation, a sensory scale was developed. With this, samples of Dulce de Leche with different size and number of lactose crystals were measured to control scale validity. Also, for the same crystal size, sand concentration was varied to see if it signif- icantly affected sensory evaluation. These vari- ations are summarized in Table 2. days. Sandiness Threshold As mentioned, for larger crystal sizes, sand replaced lactose. For threshold sizes, solubiliza- tion of lactose during sensory evaluation would have probably influenced perception; therefore, analytical grade lactose was used. It was crushed in a mortar and then passed through a series of sieves with apertures of 37, 44, 53, and 62 p. maller apertures are not currently available. The size of resultant crystals was measured by dispersing them in CFDL n ap- proximately 1 ?b concentration, obtaining the following sizes: 45, 57, 69, 80, and 105 pm, The reason crystal sizes are larger than the corresponding aperture is because in measuring size, the longest axis is considered. For the smallest crystal size, Nestle's (Buenos Aires, Argentina) microcrystalline lactose (used for ding sweetened condensed milk) was em- ployed as a standard. For this lactose, a 1.5 mixture with CFDL provides a size and number distribution as shown in Table 3. The sensory analysis method used for threshold determination is described by Lun- dah1 et al. (10). The same testing panel de- scribed earlier was used. To determine sensory threshold four experiments were performed. Experiment 1. Sandiness threshold was de- termined using six sizes of lactose crystals; each one was mixed with CFDL in a 1.5 concentration. Crystal sizes used were 45, 57, 69, 80, and 105 pn and microcrystalline lac- tose zyxwv see Table 3). Mixtures of CFDL with lactose were done within an hour of sensory evaluation. Crystal-free Dulce de Leche was used as the control sample (10). All samples, TABLE 2. Sand concentration zyxw se to study influence of this parameter on sensory perception of sandiness. Sensory scale size Concentrations of sand used in Dulce de Leche 1 6 12 (g/lOo/g) 2.5, 5. 10 2.5. 5 1 5 10 20 Journal of Dairy Science Vol. 73, NO. 3 1990  MEASUREMENT OF SANDINESS IN DULCE DE LECHE 607 TABLE 3. Size zyxwvutsr nd number of lactose crystals zyxwvut n sample prepared with 1.5 microcrystalline lactose in crystal-free Duke de Leche. zyxwvutsrqp rystal size zyxwvutsrqpon NSdS zyxwvutsrq including control, were vigorously agitated with a spoon in a serving cup before serving, and panelists were instructed to do likewise before taking the sample to mouth. This was necessary to mix the lactose thoroughly and to ensure uniformity in consistency of all samples. The determination was done by duplicate. Experiment 2. Sandiness threshold was de- termined using 45-pm lactose crystals in six different concentrations: .I, .25, zyxwv 5 zyxwvut 75, 1, and 1.5%. Control and agitation were the same as previous experiment. Experiment 3. Experiment 3 was conducted as Experiment 2 but lactose crystals were 105 P Experiment 4 Presence of sensory sandiness was determined in Dulce de Leche seeded with microcrystalline lactose. To this purpose, Dulce de Leche was made following a classical recipe (12) and then seeded with Nestle's microcrys- talline lactose according to the procedure given by Hunziker (7) for sweetened condensed milk. After 15 d of storage at room temperature, size and number of crystals were measured micro- scopically. Also, sensory evaluation was done to see if the seeded sample was above or below threshold. The sensory scale developed from the magnitude estimation test was used. To confm the result, a paired comparison test against CFDL was performed; panelists an- swered which of the two samples presented sandiness. The seeded sample and CFDL dif- fered in color and flavor; therefore, to avoid bias, two deliberately confusing pairs were added at each session. These were: CFDL with 1.5 added lactose of 105-pm crystal size (slightly above threshold), tested against the seeded sample or against CFDL. Each panelist tested 10 pairs in total, divided in two different sessions. Subtracting the confusing pairs gave six answers per panelist. Statistical Analysis Magnitude estimation results were analyzed as described by Mori et al. (1 1) and threshold results as proposed by Lundahl et al. (10). In all cases, significance was determined at zy c.05. RESULTS AND DISCUSSION Size and Number of Crystals The microscopic method to measure size and number of crystals was replicated 11 times on the same Dulce de Leche. Average and stan- dard deviation for number of crystals were, respectively, 2.2 x lo9 and 9.9 x lo8 crystals/g. As to size, the mean was 4.1 pn and the standard deviation was .28 pm. If quadruplicate measurements are performed, the confidence interval b.05) or number of crystals is 2.05 x 109 crystais/g; for size it is zyx 5 pm. Both are considered adequate for practical purposes. Sandiness Scale The first responses from the sensory panel provided a consensus sensory technique for evaluating sandiness: one-half teaspoonful of TABLE 4. Sensory sandiness scores, and crystal sizes and numbers of different Duke de Leche samdes. Sensory scores crystal w) (no./@ Sample size ryds Mean 95 Cll A 190 4 x 104 1.7 .5 B 650 1 x 103 1.0 .8 C 78 7 x 104 .55 .39 'Confidence interval. Journal of Dairy Science Vol. 73. No. 3, 1990  608 zyxwvuts OUGH ET zyxwv L. zyxwv Figure 1 Duke de Leche zyxwvuts ith a) laclose crys(als and b) zyxwvut and crystals as seen through microscope (100~) with polarized light. Both samples were classified as equivalent sandiness by panel. Each scale division corresponds to 14.7 pm. zy Journal of Dairy Science Vol. 73. No. 3, 1990
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