The History of Optical Analysis of Milk - ACS Publications

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The History of Optical Analysis of Milk: The Development and Use of Lactoscopes C. Millán-Verdú,* Ll. Garrigós-Oltra, G. Blanes-Nadal, and M. Domingo-Beltrán Escola Politècnica Superior d’Alcoi, Universidad Politècnica de Valencia, Plaça Ferràndiz i Carbonell, 2, Edifici Ferràndiz, 03801 Alcoi, Alacant, Spain; *cmillá[email protected]

Scholars have begun using historiographical methods to explore a problem in the history of science: the study of scientific instruments and the analysis of relations between their manufacturers and scientists (1–4). These methods are generating a new vision of the history of science, suggesting that scientific activity results from the combined action of different procedures and practices of a diverse nature in very concrete contexts (5, 6). Chemistry is a particularly interesting case as has been recently noted by Holmes and Levere (7). This paper seeks to bring a completely new perspective on the beginnings of absorptiometric analysis at a time in history when photometry was the only theoretical position. The improvement of primitive methods led the way to technical developments outside the field of spectroscopy, which were capable of quickly providing answers to some of the

problems that had emerged during the nineteenth century. These problems were related to the analysis of foods and, more particularly, to the fat content in milk, which was subject to various kinds of adulterations during this period of history. The laws that govern the decrease in intensity of a light beam traveling through a substance were formally studied by both Pierre Bouguer (8) and Johann Heinrich Lambert (9), who held different viewpoints. Bouguer treated the material studied as an optical system, whereas Lambert tended towards a more structural explanation of the material, suggesting that the absorption of light is produced by an even distribution of absorbent particles. From this theoretical background there later came applications in the field of industry. Around 1825, the possibility of relating the intensity of the coloration of a

Table 1. Synopsis of the Development of Lactoscopes in the 19th Century

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Inventors

Inventors’ Occupations

Date of Article Publication

Alfred Donné

Physician

1843

First instrument designed to determine the specific gravity of milk, this lactoscope used diaphaneity to ascertain if a volume of milk had been fraudulently diluted by water. See Figure 1.

Based on the principles of de Labillardiere’s colorimeter and Payen’s decolorimeter

Alfred Vogel

Physician

1863

Like the Donné lactoscope, users of Vogel’s lactoscope looked through a volume of milk at a light source. See Figure 2.

Based on the lactoscope of Donné; eliminated the mechanism separating the glass plates

Rheineck

Pharmacist

1871

Unlike previous lactoscopes, Rheineck’s instrument allowed the opacity of the milk to be observed without reference to a light source. See Figure 3.

Measurements made using a movable platinum thread and calibrated scale

F. Heusner

Pharmacist

1877

This lactoscope was eminently practical, requiring no particular volume of sample and no addition of water. See Figure 4.

Examined the color of the milk sample with reference to a standard liquid without measuring fat content

J. Feser

Pharmacist

1878

Feser’s “galactometer” was the simplest of all the optical instruments designed for accuracy. See Figure 5.

Chiefly a graduated glass rod inside a glass cylinder, this instrument measured the percentage of milk fat in the sample

Heeren

Pharmacist

1881

Heeren’s “Pioskop” allowed users to make a general determination of the richness of a small volume of sample milk by comparing it to reference colors on the glass.

Examined the color of the milk sample with reference to six reference colors

Mittlestrass Brothers

Milk Producers

1881

The Mittlestrass lactscope was constructed in such a way that the exact point of opacity could be determined. See Figure 6.

Based on the lactoscopes of Donné and Vogel, and Müller’s colorimeter

E. Aglot

Chemist

1893

Presented as “Aglot’s colorimeter”, this instrument was intended to measure precipitates in a liquid, not just milk fat. See Figure 7.

Identical to the Mittlestrass lactoscope, except for the addition of a plate between the mirror and the light source

Salient Features of the Instruments

Modifications Relative to Earlier Instruments

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solution with its concentration aroused the interest of industrial chemists. After the success of the colorimeter of Houtou de Labillardiere and of the decolorimeter of Payen (10–15), these chemists began to explore the possibility of designing more exact and more practical instruments.1 Following from this line of thought came experiments and conclusions about the necessary thickness of previously transparent substances before they cease to be transparent. The concept of diaphaneity was introduced by Horace Benoît de Saussure and was first used as a tool for atmospheric studies; specifically, as a means of measuring the necessary thickness of a layer of air for a solid object to become invisible (16). The next step would be to relate the concept of diaphaneity to solutions to measure the richness of a compound dissolved in a solution. At that time in history milk was a basic food substance and could be subjected to various adulterations, the most common of which was the addition of water. The analysis of milk could be carried out by chemical procedures or by measurement of density. Chemical analysis was an elaborate procedure that was further complicated by the perishable nature of the product. Industry needed simple, practical procedures since the person carrying out the test most likely would not be a trained scientist. The tests for density were not sufficiently reliable as the milk could be adulterated in such a way that it would not affect the density. It is possible to add water to the milk and decrease the density and then skim the milk to create a corresponding increase in density. Thus, a combination of these two actions could remain undetected when only density was tested. Monitoring fraud in food and medical products was left in the hands of pharmacists and physicians, and so it is not strange that it was a physician who first designed an instrument for the optical analysis of milk that could be quickly and easily used to detect fraud in milk production and sale. Over the course of fifty years scientists with various backgrounds and different goals would continue to develop and improve upon instruments to optically analyze milk. Table 1 provides an overview of the scientists involved and the significant features of their instruments. Donné’s Milk Fat Optical Analysis In 1843 Alfred Donné presented to the Academie des Sciences of Paris a letter reproduced in Comptes Rendus (17) accompanied by twelve lactoscopes built by the optical scientist Jean Baptiste Soleil. The lactoscopes were dedicated to the measurement of the concentration of fat in milk. The diaphaneity (the thickness of a layer of a substance through which a source of light can no longer be seen) was determined. Donné thought that the thickness of the required milky layer was related to the richness of fat in the milk. In his letter Donné provided some details on the apparatus before presenting a report on the same subject, which he also sent to the Académie des Sciences. This information2 was summarized in a report (18) by Pierre Siguier who then presented it to a commission made up of Louis Jacques Thénard, Eugène Chevreul, Jean Boussingault, Henri Regnault, and Siguier himself. In the report he gave a detailed description of the apparatus3 and some experiments to demonstrate their reliability. A sketch of the apparatus (19) appears in Figure 1.

Figure 1. The Donné lactoscope (20).

The apparatus is composed of two glass plates placed in parallel in two tubes. The milk is introduced by means of a funnel placed in the space between the tubes. One of them has a 1-mm thread so that it can rotate in the tube. One of the tubes is calibrated in 50 equal divisions and the other tube has a fixed mark. Each division corresponds to a distance of 0.02 mm. The instrument is placed in a dark room at a distance of 1 m from a candle. The measuring process consists of coinciding the mark of one tube with the zero of the other tube and then one of the tubes is rotated until the light is no longer perceptible. In Siguier’s summary (18), there was also included some criticism on the apparatus’ design and measuring procedure by the permanent secretary of the Académie des Sciences, François Aragó.4 Aragó based his criticism on the following points: When the human eye is used as the main observation instrument, the results will depend to some extent on the eyesight of the person making the observation. The use of a candle or a petroleum-based lamp as a source of illumination will also impose great restrictions when varying the light intensity from 100% to 16%, in the first case, and from 100% to 60%, in the second case. Another criticism of the method is based on the premise that the opacity of milk is caused not only by its richness in fat, but also by the presence of casein. Aragó (who also based his criticism on the structural properties of milk) pointed out that the fat present in milk causes opacity because it is in the form of white globules held in suspension. These globules can be present in different proportions according to size and when trying to establish the degree of opacity caused by fat, it is possible to find that in two different types of milk with the same richness in fat, the lactoscope produces different levels of opacity. These criticisms eventually led to the abandonment of this procedure. Aragó’s criticism concluded with the following statement: Quant à l’instrument de M. Dien, reproduit par M. Donné, il exige une foule d’attentions délicates, minutieuses, dont ce médecin ne semble pas s’être doute.

English translation: With regard to M. Dien’s instrument, reproduced by M. Donné, this demands a considerable quantity of meticulous and delicate attentions, for which this doctor doesn’t seem to be qualified.

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It is evident that Aragó’s language was offensive, discrediting, and implicitly underrating the medical profession. This fact can only be understood if the medical profession is interpreted as being antagonistic to the scientific activity of physicists. This position was neither shared by any member of the evaluating committee nor by any other person participating in the debate. Magendie began the defense of Donné in the following way: Je ne veux pas, dit-il, intervenir dans la discussión physique soulevée à l’occasion du Rapport; elle me paraît d’ailleurs épuisée, la Commission ne contestant aucune des objections qui viennent d’etre faites et convenant, au contraire, que l’instrument proposé n’est pas un instrument de précision; mais elle soutient que cet instrument peut rendre service entre les mains des personnes qui ont intérêt à connaître les qualités du lait.

English translation: I don’t want to intervene in the physical discussion that has arisen from the occasion of the report, besides I think this discussion has run its course. The commission didn’t answer the objections that were made. However, it agreed that the instrument was not a precision instrument, but that it could provide a good service to those people interested in carrying out a qualitative analysis of the milk.

Regnault, Chevreul, and Flourens were of the same opinion as Magendie. This inevitably leads us to conclude that in the mind-set of some scientists in the middle of the nineteenth century there existed doubts about whether the practice of some disciplines could be considered a science or not. These doubts had already been expressed about chemistry at the end of the eighteenth century (21). In a similar manner, chemistry was again subject to such doubts at the end of the nineteenth century (22). Evidently all the major players except for Aragó defended the utility of the apparatus, notwithstanding the accuracy or the validity of the theoretical base on which the design and the measuring processes of the apparatus were developed. In the summary carried out by Siguier, the existence of a proportional relationship between the thickness of milk necessary to make the light of a specific source disappear and the richness in fat was suggested. Although Siguier highlighted this fact, it was not mentioned by Donné in his report. The use of this instrument led to the creation of

A

B

C

equivalency charts for different types of milk; among these charts was one designed by the chemist and agricultural scientist Jules Raiset. Jules Raiset, on his estate at Ecorcheboeuf, carried out a series of studies on the composition of milk. An equivalency chart resulted that became so popular during the first years of the apparatus use that Chevallier included it in his Dictionnaire des Altérations et Falsifications des Substances Alimentaires (20). Finally, attempts were made by Donné and the commission to establish a correlation between the proportion of water added to a sample of milk and the reading of the lactoscope. The following equation was proposed, a +b x where y is the concentration of fatty matter and x is the separation between plates of the lactoscope. The correlation results were found to be unsatisfactory. y =

The Vogel Lactoscope Employing Donné’s ideas, Alfred Vogel designed a new instrument. His lactoscope had one major difference from that of Donné: the elimination of the mechanical system for separating the glass plates. This modification removed the inherent possibility of error caused by the system for measuring longitude, which could call into question the accuracy of the findings. The first presentation of Vogel’s lactoscope was made in an information leaflet produced by the Enke Company of Erlangen, Germany. Later, accounts of the lactoscope began to appear in reviews in various journals (23–25) together with the results of some experiments carried out by Vogel and the astronomer Von Seidel, relating the readings of the lactoscope to the results obtained by chemical analysis. As can be seen in Figure 2, Vogel’s lactoscope is made up of a receptacle A, formed by two, parallel glass plates 5mm apart. The receptacle is placed in a wooden box with darkened walls and containing a light source. Water, 100 cm3, is introduced into flask B and 3 cm3 of the milk under investigation is added to the flask using a pipet, C. This mixture is poured into receptacle A and then is inspected to see if the flame is visible. If the flame is visible, the liquid is extracted, poured again into flask B, and an additional 3 cm3 of milk is added. This process is repeated until the flame is no longer visible. Von Seidel, when comparing this method with other chemical analysis methods, derived the following equation R =

23.2 + 0.23 q

where R is the richness in fat and q is the amount of milk added. Later Designs of Lactoscopes

Figure 2. The Vogel lactoscope (34) with components (A) receptacle, (B) flask, and (C) pipet.

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In the years following the appearance of Vogel’s lactoscope, new apparatuses were designed that tried to exploit its strong point: its practicality. These developments culminated with the appearance of the most technically advanced of all the instruments designed to carry out optical analysis of milk, the Mittelstrass lactoscope.



The lactoscope proposed by Rheineck (26) was based on Donné’s idea of detachable plates. As can be seen in Figure 3, one of the ends, a, remains fixed, while the other end, b, can be separated by means of a micrometric screw. In the slight space between the glass plates, a small quantity of milk is introduced, which by cohesion occupies the volume existing between the plates. The layer of milk that the luminous beam will cross increases in thickness across the apparatus moving from a to b. The lower plate contains a platinum thread traveling from one end to the other. This thread serves as the reference that the observer views from above to find the point at which opacity takes place. Close to the thread there is a calibrated scale that allows this position to be measured. If the milk had been adulterated, the point of opacity would be closer to the movable end, b, than a sample of nonadulterated milk.

Figure 3. The Rheineck lactoscope (26) with a fixed end, a, and a movable end, b.

Figure 4. The Heusner lactoscope (27): (left) side view and (right) top view.

A

B

Figure 5. The Feser lactoscope. The figures were obtained from (A) ref 29 and (B) ref 34.

In 1877 Heusner (27, 28) presented a new instrument for the optical analysis of milk. Heusner’s lactoscope was quick and practical to use without the pretense of obtaining exact measurements of the fat richness in a milk sample. As can be seen from Figure 4, it was formed by two glass sheets the size of a watch glass between which are two cavities that are able to store a small quantity of milk. In one of the cavities, a substance is placed whose coloration and opacity is similar to the milk that is taken as the milk standard and in the other cavity the milk to be to studied is placed. The observation consists of verifying if the opacity is the same in the two samples of milk to assess if the sample of the milk being studied had been altered. To aid in the observation, a pattern of squares that can be observed through both milk samples is recorded on the back of one of the glass plates. A year later, J. Feser (29) presented his lactoscope (galactometer). This apparatus was the simplest of all the optical instruments used for the analysis of milk that aimed for accuracy (Figure 5). It consisted of a glass rod on which there were six lines 5-mm apart in the center of a glass tube. There were two versions of this instrument: a more basic version in which the central rod was welded to the bottom of the outer tube, and another more advanced version with a collapsible base in which the central rod was coiled to a metallic cover. The external cylinder had a double scale. The scale on the left indicated the volume of water that was added and the scale on the right indicated the percentage richness in fat of the analyzed milk. The procedure for using this apparatus was as follows: 4 cm3 of milk was introduced and enough water to bring the solution to 20 cm3. With this proportion, the central rod was invisible when lowfat milk was not used. Water was added 20 cm3 at a time until the central rod started to became visible. Finally, in order to achieve greater accuracy, water was introduced in smaller quantities until the rod became visible. The Heeren lactoscope was displayed in 1881. We have not been able to find any diagrams of this instrument but we have found a detailed description (30). The Heeren “Pioskop” consisted of a black rubber disk, in the center of which was an elevation onto which a drop of milk was placed. A sheet of glass was placed on the drop of milk so that the more fat that the milk contained, the less the black rubber disk could be seen. Depending on the fat content of the milk, the color that was observed through the glass was blue-gray varying in shades between almost clear to dark. On the edge of the glass sheet, there were six different colorations that corresponded to six different samples of milk with differing fat contents, which had been chemically analyzed. The approximate determination was carried out against this scale. The Mittelstrass brothers (31) made the single biggest innovation. They patented an apparatus that picked up the ideas of Donné and Vogel and the design of Müller’s colorimeter (1853). Figure 6 shows that lactoscope. The diagram on the right shows the original design and the diagram on the left shows a later version in which the candle has been substituted by a petroleum lamp since the light generated by the latter is more uniform. Mittelstrass’ apparatus used milk diluted by water instead of pure milk and because of this the determination of the point of opacity required a separation between the plates in the receptacle container of several millimeters, instead of tenth-parts of a millimeter as in Donné’s


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Figure 6. The Mittelstrass lactoscope (left) later version (34) and (right) original design (31).

apparatus. The apparatus consists of two cylinders both sealed at the bottom by glass plates that are introduced one inside the other. The milk to be analyzed was diluted with water (2 cm3 of whole milk, or 4 cm3 of lowfat milk in 100 cm3 of water) and was introduced into the external cylinder. The light was directed at the bottom of the cylinder by a pivoting mirror. The internal cylinder could be moved up or down by means of a micrometric screw and from the displacement, the observer could read the scale on the same cylinder. The zero of the scale coincided with the bases of both cylinders being in contact. By reading the scale of the internal cylinder, the observer could find the thickness of the layer of milk that the luminous beam might cross. By moving the internal cylinder, it was possible to determine the thickness of the layer of liquid necessary for the observer to be unable to perceive the light source. In the most updated version, a grill was placed in front of the lamp so that a group of dark lines was visible instead of a single luminous point. Consequently, the determination of the exact point of opacity of the solution was easier. The Measurement of the Precipitate in Suspension G. Deniges states in his publication Précis de Chimie Analytique (32) that Donné was the first person to use diaphanometry in the analysis of milk to determine its fat richness. However, in the same publication, he states that milk also contains several other substances that could cause opacity. Moreover, the fat suspension is formed by globules of different sizes, which could also affect the measurement of opacity. The measurements that were carried out never achieved the level of accuracy required to guarantee the reliability of the procedure. This short introduction was employed by Deniges to present Aglot’s colorimeter, which was shown by Aglot to Académie des Sciences (33) in 1893 and had as its purpose the measurement of precipitates by an optical method. Aglot’s report was defended by the professor of mineralogy at the Sorbonne, Charles Friedel. The apparatus (Figure 7) is structurally identical to the Mittelstrass lactoscope, the only difference being the introduction of a 766

plate between the mirror and the light source. The objective of this plate was to reduce the intensity of the light to enable a greater variety of precipitates to be studied. This apparatus is based on the same theoretical principles that Donné had defended fifty years earlier. However, it was now used for the measurement of a greater number of precipitates without the structural complication of the fat suspension in milk. Possibly this fact, and the employment of more precise measurements, offers an explanation as to the lack of criticism of this apparatus. A description and a diagram of the colorimeter is found in Deniges’ publication (32). The Aglot apparatus consisted essentially of a cylinder, C, closed in its lower part by a glass plate, G. The cylinder is placed on a concentric tray, V, which has a larger diameter. This tray is topped by a fixed cover. In the center of the cover there is a sleeve through which a tube, T, slips smoothly. This tube expands towards the top and is enclosed at the bottom with a glass plate, G1. The precipitate being studied is placed between the two glasses G and G1. The tube can be displaced by a rack and pinion system. By means of a rule equipped with a nonius the distance between the glasses G and G1 to a precision of tenths of a millimeter can be measured. This measurement is the thickness of the liquid layer that is increased until the light of a petroleum lamp reflected in the mirror, M (inclined at an angle of 45⬚ from the horizontal, after crossing the enameled sheet E), is no longer visible.

Figure 7. The Aglot colorimeter (32).



Conclusions Physicians and pharmacists responsible for monitoring fraud in food and medical products in 19th century Europe applied principles emerging from advances in colorimetry to develop lactoscopes. These instruments were used to optically evaluate the diaphaneity of milk samples as a proxy for the richness, and thus quality, of the milk. Critiqued by physicists of the time as inaccurate, some lactoscopes were subsequently developed to higher standards, often without regard for practicality of use. This historical process is particularly interesting in two regards. Firstly, technological and theoretical advances in photometry were applied to a practical problem, yielding more accurate results over time. Although the correlation of the behavior of milk and the theoretical conjectures about the diffusion of light was fairly inaccurate, a stronger relationship was detected between the two properties as the instruments used for measurement were improved. Remarkable results were achieved using the Mittelstrass lactoscope. We assessed the accuracy of the lactoscopes in accordance with the primary sources consulted: Siguier and Chatellier give data for Donné’s instrument and Seidel for Vogel’s. A lineal regression of the data with regression coefficients above .9 was obtained. Secondly, the ongoing development of lactoscopes revealed a division within scientific communities, with physicists and other “exact scientists” claiming greater credibility and authority relative to physicians, chemists, and other “inexact scientists”. This division was evident among the members of the French scientific community in the second half of the nineteenth century as has been documented in the case of the Donné lactoscope. Notes

4. 5. 6. 7. 8. 9.

10. 11.

12. 13.

14.

15. 16.

1. It is important to remember that these chemists were more accustomed to the world of trade. 2. In addition to a detailed description of the lactoscope and instructions for use, the information included a practical procedure to recognize the adulteration of milk with water, a summary about the effect of low temperatures on milk, and information on the storage of milk at low temperature in order to conserve or transport it. 3. Alphonse Chevallier also offered a description of the apparatus in the Dictionnaire des Altérations et Falsifications des Substances Alimentaires (20) published in 1852 in Paris and translated into Spanish in 1854 (Vol I) and 1855 (Vol II), by Ramón Ruiz Gómez. Baudrimont carried out an elaborate version of Chevallier’s work but also took steps to improve the accuracy of the measurements (19). 4. Aragó accused Donné of plagiarism when he affirmed that a very similar instrument had recently been built by M. Dien. Charles Dien (Paris, 1809–1870) was an industrialist and geographer who produced a large number of astronomical maps and published many diverse works on astronomy (16).

Literature Cited 1. Payen, J. Archives Internationalles d’Histoire des Sciences 1986, 36, 84–161. 2. Clarke, T. N.; Morrison-Low, A. D.; Simpson, A. D. C. Brass and Glass. Scientific Instrument Making Workshops in Scotland; National Museums of Scotland: Edinburgh, 1989. 3. Brenni, P.; Dragoni, G; McConnell, A.; Turner, G. L. E.

17. 18. 19.

20.

21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

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