Chocolate in Mesoamerica: A Cultural History of Cacao

5. The Determination of Cacao in Samples of Archaeological Interest

5 The Determination of Cacao in Samples of Archaeological Interest W. Jeffrey Hurst Although the use of cacao by Mesoamerican peoples has been discussed and documented since the early Colonial period, archaeologists have been hindered from discovering the breadth of its usage in rituals and daily life by its rare recovery from archaeological contexts. In 1989 the author was approached with the challenge of chemically determining whether cacao was present in residues from vessels recovered from Classic period tombs in Río Azul, Guatemala. Success in this endeavor has opened a new area of research into the manner in which cacao was used in pre-Columbian times and has shown that cacao was consumed in Mesoamerica at least a thousand years earlier than was previously known. In this chapter I will discuss the results of analyses on cacao samples from a range of contexts and explain the methods for chemically determining the presence of Theobroma cacao in decayed macrofossils and residues. Cacao contains more than 400 discrete chemical compounds with the list continuing to grow as new analytical techniques allow researchers to detect smaller quantities of compounds. Although cacao contains a plethora of compounds , some of these, while not totally unique to cacao, are rare enough and occur in quantities that allow them to be used as marker compounds. Marker compounds are those chemical entities that are unique to the commodity in question. One group of these compounds is the methylxanthines. The methylxanthine class contains a number of compounds based on the purine nucleus. The primary members of this class are caffeine (1,3,7-trimethylxanthine), theobromine (3,7-dimethylxanthine), and theophylline (1,3-dimethylxanthine); caffeine is the member of the class that occurs primarily in coffee. The methylxanthines have also been called purine alkaloids. Cacao is the only Mesoamerican commodity in which theobromine is the primary methylxanthine, so this com- W. Jeffrey Hurst 106 pound was used for primary analytical studies. Before the advent of modern Liquid Chromatography/Mass Spectrometry (LC/MS) interface techniques and micromethods, it was necessary to perform this type of determination using wet chemical techniques that can require a substantial sample or techniques based on Gas Chromatography (GC) that require derivatization. Liquid Chromatography /Mass Spectrometry is a method in which compounds are separated from one another based on chemical differences by liquid chromatography, and the resulting compounds are identified by mass spectrometry. The data from the mass spectrometer provides information about the molecular identity of the compound. The combination of these two techniques is then used to chemically identify unknown materials. The term “micromethods” can refer to a number of analytical techniques and in this case means that determinations can now be accomplished using much smaller quantities of material than did previous methods. Gas Chromatography is another separation technique in which it is necessary for compounds to be in a volatile physical form, requiring the researcher to chemically convert compounds to a form that is suitable for the GC. Four studies on samples of archaeological interest have been conducted using High Performance Liquid Chromatography (HPLC) with Thermospray MS, Capillary Electrophoresis (CE), and Atmospheric Pressure Chemical Ionization (APCI) LC/MS. The samples came from diverse locations in Central America, including Belize and Honduras. In this chapter I will outline the experimental protocols and techniques that have been used in each of these studies. Collection of Archaeological Samples for Analysis Samples were collected for analyses in a variety of ways. In ideal circumstances, preserved uncontaminated residues were recovered from tomb vessels. The recovery of vessels from largely uncontaminated contexts is rare, however, but the analysis of vessels filled with sediment or small fragments of vessels is feasible and has been shown to have productive results. Some excavators submitted samples of the lowest layer of sediments resting in vessels recovered from excavations . Others actually scraped a thin layer of ceramic from the inside bottom of the vessels. In a third case, sherds were boiled in water to extract the chemical components of foods formerly housed in the vessels. In some cases the author has been able to retrieve the chemical components of cacao from previously washed vessels. Cacao Chemistry The chemistry of cacao is complex. Fifteen compounds that occur in the unfermented cacao bean are listed in Table 5.1. We currently know that at least The Determination of Cacao in Samples of Archaeological Interest 107 462 different volatile compounds occur in cacao. A portion of...