Introduction

Lactic acid (2-hydroxypropanoic acid) is a naturally occurring hydroxycarboxylic acid that was isolated first in 1780 from sour milk [1]. Lactic acid (LA) is a commodity chemical used in pharmaceuticals, bio plastics, and food, home and personal care products [2]. The homopolymer of lactic acid, poly (lactic acid) (PLA), is a biodegradable, transparent and inexpensive alternative to petroleum based plastics that are persistent in the environment. When used as a replacement for conventional plastic and properly composted, PLA may reduce landfill waste, plastic/micro plastic pollution and greenhouse gas emissions [3]. These characteristics have increased global demand for lactic acid from 130,000-150,000 metric tons in 1999 to 1.1 million metric tons in 2016 and demand is expected to grow further by 16.2% between 2017 and 2025 [4]. While industrial scale chemical synthesis results in racemic mixtures of lactic acid, fermentation processes may produce optically enriched lactic acid. Polymer and food applications require the highly enantioenriched isomer of lactic acid (typically >90% L-lactic acid), making fermentation from corn starch-derived sugar the preferred method of production. Developing carbohydrate-rich feedstocks from non-food sources abates concerns about diverting food feedstocks to non-food products [5]. Producing lactic acid from waste streams such as food waste, industrial byproducts (e.g., glycerol), or lignocellulosic biomass (e.g., corn stover, pine wood and other agricultural biomass) rather than corn starch has the potential to lower production costs. It has been reported that food waste can be converted selectively to lactic acid in excellent yields [6-8]. Another abundant and inexpensive waste feedstock for lactic acid is a byproduct of almond production. Almonds are an important crop in California where more than 2.4 billion pounds of shelled nuts are produced annually comprising more than 80% of the global production [9]. Both the shell and outer leathery hull of the almond are byproducts of kernel production. The hulls comprise nearly 53% of the fresh weight of the almond fruit or approximately 9 billion pounds [10]. Almond hulls are used primarily as cattle feed for nearby dairies. However, price volatility for the hulls is a concern, such as when prices dropped 50% between 2012 and 2015 due to low milk prices [11]. As a result, value-added markets are being sought [11]. Almond hulls contain 37.3% fermentable sugars by weight, [12] making them an attractive source of cheap carbohydrates for fermentation. Production of bioethanol and biomethane from almond hulls have been demonstrated, [12] which raises the possibility that they could be utilized for lactic acid fermentation. The objective of this study was to assess almond hulls as a feedstock for lactic acid production by comparing them with alternative feedstocks including food waste, chemical feedstocks, and lignocellulosic waste.

Materials and Methods

Nonpareil almond (Prunus dulcis (Mill) Webb cv. Nonpareil) hulls were obtained from a commercial almond processing facility located in Chowchilla, CA. Lactobacillus rhamnosus (ATCC® 10863™) was purchased from the American Type Culture Collection (ATCC, Manassas, VA). Cellulase from Trichoderma reesei and cellobiase from Aspergillus niger (Novozyme 188) were purchased from Sigma-Aldrich (St. Louis, MO) for pretreating lignocellulosic waste. Other reagent grade chemicals were purchased from Sigma-Aldrich.

Feedstock preparation Almond hull: The almond hulls were ground in a Wiley mill (Model 4, Thomas Scientific, and Ramsey, MN) to pass through a 5-mm screen. Ground almond hulls were prepared in two ways; as a lignocellulosic feedstock (see below) or as steep liquor. For almond hull steep liquor, ground almond hulls (330 g) were suspended in 2 L of tap water and mixed with an overhead laboratory mixer. The almond hull suspension was heated to 80°C and mixed for 75 minutes then allowed to cool while covered. After cooling to room temperature, the suspension was filtered through a 1.4 mm screen and the filtrate was centrifuged at 4,000 rpm for 5 minutes. The supernatant was collected, and the solid pellet discarded. The supernatant was stored (5°C) until needed for fermentation experiments.

Food waste: Food Waste No.1 was collected randomly from a household trash bin. The waste mainly consisted of lasagna, ground beef, chicken salad, and rice. Food waste No.1 was blended prior to fermentation. Food waste No.2 was a defined waste that was formulated with the intent to emulate the composition of American food waste (Table 1) [13]. The food components were homogenized in an industrial Waring blender until a thick paste formed. The paste was blended with water (1:1).