Alkaline cooking and stable isotope tissue-diet spacing in swine: archaeological implications

Abstract

In this study we examine the effects of alkaline cooking on carbon and oxygen stable isotopic ratios of mineralized tissues from nine pigs raised on monotonous mixed C3/C4 vegetarian diets. Two sources of collagen (humerus and mandible) and two sources of apatite (humerus and enamel) were analyzed. Within each diet group, humerus and mandible collagens were found to record equivalent δ¹³C and δ¹⁸O ratios; however, enamel apatite was found to be enriched over bone apatite by 2.3‰ in carbon and 1.7‰ in oxygen. Alkaline cooking was found to slightly, but significantly increase the Δ¹³C_{collagen-diet} and Δ¹⁸O_{collagen-diet} of bone collagen. A similar trend towards enrichment was observed in bone and enamel Δ¹³C_apatite-diet and Δ¹⁸O _apatite-diet, but the differences were not significant. Observed isotopic shifts were consistent with increased nutrient utilization of the alkaline-cooked maize as compared to raw maize. In addition, a reexamination of the relationship between diet and tissue carbon isotopic values suggests that species and alimentary type should be considered when interpreting ancient diets.

Introduction

Stable isotope analysis is now a widely used analytical tool in archaeology. Body tissue isotopic ratios of carbon and oxygen have been shown to record biologically meaningful information that may persist in the archaeological record for hundreds or thousands of years (Koch et al., 1997, Trueman et al., 2004, Wang and Cerling, 1994). This analytical approach has contributed to our knowledge of the dynamic history of past human societies on topics that include subsistence strategies, ecological change, and mobility patterns (Rose, 2008, West et al., 2006, Schoeninger and DeNiro 1983, Budd et al., 2004). Mineralized tissues (bones and teeth) are the most commonly analyzed body tissues for stable isotope analysis. Within these tissues, bone collagen and bone and tooth apatite can be analyzed for organic and inorganic sources of carbon and oxygen. Each of these isotopic proxies provides a semi-independent line of evidence for the investigation of complex topics such as paleodiet (Hu et al., 2006, Finucane et al., 2006) and migration history (e.g., White et al., 2002, Quinn et al., 2008). Accurate reconstruction of dietary or migratory history, however, requires a precise understanding of the multiple isotopic fractionations that accompany the conversion of dietary inputs (food and water) into consumer tissues. Controlled diet experiments on model organisms are essential to refine our understanding of the complex factors that influence isotopic enrichment or depletion of biological tissues during this process.

Previous experimental stable isotope studies have focused primarily on rodents (Mus musculus and Rattus norvegicus) to model the incorporation of dietary isotopic ratios into consumer tissues (Ambrose, 2000, Ambrose and Norr, 1993, Tieszen and Fagre, 1993, Jim et al., 2004). Rodents, however, differ markedly from humans with respect to metabolic rate, digestive physiology, and feeding habits (Baker, 2008, Blaxter, 1989). Recently, researchers have turned to swine as an alternative animal model (Hare et al., 1991, Howland et al., 2003, Passey et al., 2005). Like humans, swine are large-bodied and omnivorous, and the two species share similar digestive physiology, including a simple, monogastric gastrointestional tract (Miller and Ullrey, 1987, Tumbleson, 1986).

In addition to digestive and metabolic differences between species, environmental factors, such as aridity (Ambrose, 2000), canopy cover (van der Merwe and Medina, 1991; Cerling et al., 2004), and nutritional stress (Hobson et al., 1993, Trueman and McGill, 2005) have also been shown to influence apparent tissue-diet isotope fractionation. In this study, we sought to explore whether cultural factors, such as cooking, could also influence observed tissue-diet isotope spacing.

In the Americas, populations subsisting on a staple diet of maize (Zea mays) frequently practice a specialized form of cooking known as alkaline cooking (Katz et al., 1974). Alkaline cooking is an intensive food preparation technique that involves soaking and boiling dried maize kernels in an alkaline water solution (FAO, 1992). In Mesoamerica (Mexico, Guatemala, Belize, Honduras) and parts of the U.S. Southwest, where the alkali agent employed is calcium hydroxide (lime), this cooking technique is called nixtamalization (Katz et al., 1974).

The effects of alkaline cooking have been studied extensively in nutritional and food science literature (FAO, 1992). Although alkaline cooking has been shown to decrease the total nutritional content of maize (Bressani et al., 1958), it enhances its apparent nutritional value, as measured by increased protein efficiency ratios (Bressani, 1990) and weight gain in experimental animals (e.g., Craviato et al., 1952, Kodicek et al., 1956, Laguna and Carpenter, 1951, Pearson et al., 1957, Squibb et al., 1959, Braham et al., 1966).

The potential for alkaline cooking to affect the isotopic values of maize agricultural populations has been previously noted (Marino and Deniro, 1987, Wright, 1994). However, these studies measured the effect of alkaline cooking on the stable isotope ratios of maize itself, rather than the changes induced in consumer tissues. For example, after noting that selective loss of maize fiber and increased digestibility of essential amino acids could shift the isotopic composition of maize and/or alter its digestibility, Wright (1994) conducted nixtamalization experiments on maize, but found that alkaline cooking had no significant effect on the δ¹³C of the alkali-treated maize. This confirmed the results of previous research by Marino and Deniro (1987) that nixtamalization had little effect on the δ¹³C isotope ratios of laboratory grade alpha cellulose and cellulose from maize kernels. The results of these studies were interpreted to mean that nixtamalization had no significant isotopic impact on maize-based diets. However, nutritional studies have indicated that alkaline cooking changes tertiary protein structure, insoluble fiber content, and phytic acid activity, which are all factors that primarily affect digestion and bioavailability of nutrients and minerals (FAO, 1992, Wacher, 2003). The potential downstream isotopic effects of these changes have not been investigated.

In this study, we employed a multi-proxy approach to investigate the effects of alkaline cooking on isotopic discrimination in the mineralized tissues of swine. Nine pigs were raised on experimental diets containing either unprocessed (raw) or nixtamalized maize. After 13 weeks on the experimental diets, the pigs' mineralized tissues were analyzed for δ¹³C and δ¹⁸O. In order to assess whether the effects of alkaline cooking were tissue specific, both apatite and collagen were measured. In addition, two sources of apatite (bone and enamel) and collagen (humerus and mandible) were investigated to determine if apatite and collagen represent isotopically homogenous tissues throughout the body.