Vivek
Narisetty
a,
Rylan
Cox
b,
Nicholas
Willoughby
c,
Emel
Aktas
d,
Brijesh
Tiwari
e,
Avtar Singh
Matharu
f,
Konstantinos
Salonitis
b and
Vinod
Kumar
*a
aSchool of Water, Energy and Environment, Cranfield University, Cranfield MK43 0AL, UK. E-mail: Vinod.Kumar@cranfield.ac.uk; Tel: +44 (0)1234754786
bSchool of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield MK43 0AL, UK
cInstitute of Biological Chemistry, Biophysics and Bioengineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
dSchool of Management, Cranfield University, Cranfield MK43 0AL, UK
eTeagasc Food Research Centre, Dublin D15 KN3K, Ireland
fGreen Chemistry Centre of Excellence, University of York, Department of Chemistry, Heslington, York YO10 5DD, UK
First published on 6th September 2021
Food waste is a global problem, causing significant environmental harm and resulting in substantial economic losses globally. Bread is the commonly wasted food item in the developed world and presents a severe problem for the majority of European nations. It is the second most wasted food item in the UK after potatoes, with an equivalent of 20 million slices of bread thrown away daily. Bread is a starchy material and a rich and clean source of easily extractable fermentable sugars – this is in direct contrast to lignocellulosic feedstocks where harsh physical, chemical and/or enzymatic pretreatment processes are required for release of fermentable sugars. Furthermore, these necessary lignocellulosic pretreatment methods often produce sugars contaminated with fermentation inhibitors. Therefore, bread waste presents a clear opportunity as a potential carbon source for novel commercial processes and, to this end, several alternative routes have been developed to utilize bread waste. Possibilities for direct recycling of bread waste within the food industry are limited due to the relatively short material lifetime, stringent process and hygiene requirements. Anaerobic digestion (AD) and incineration are commonly employed methods for the valorisation of bread waste, generating limited amounts of green energy but with little other environmental or economic benefits. Most food wastes and by-products in the UK including bakery waste are treated through AD processes that fail to harness the full potential of the these wastes. This short communication reviews the challenges of handling bread waste, with a focus on a specific UK scenario. The review will consider how bread waste is generated across the supply chain, current practices to deal with the waste and logistics challenges in waste collection. The presence of clean and high-quality fermentable sugars, proteins and other nutrients in bread make it an ideal substrate for generating chemicals, fuels, bioplastics, pharmaceuticals and other renewable products through microbial fermentations. We suggest potential applications for recycling bread waste into its chemical building blocks through a fermentative route where a circular biorefining approach could maximize resource recovery and environmental savings and eliminate waste to as close to zero as possible.
Better insights into the ingredient's quality, quantity, shelf-life and monitoring would also reduce wastage in manufacturing, as would clearer understanding of the bottlenecks in storage and transportation. “Just-in-time” manufacturing strategies to match requirement based on a more detailed understanding of the demand and logistics would also be advantageous in waste reduction. Brancoli et al. (2019) considered the role of sales, the pack-size, the shelf-life and the take-back agreement (TBA) on the loss rate, and the work demonstrated significant differences in wastage due to the TBA, i.e., the type of distribution system has a considerable impact on the amount of wastage and TBA liable bread tends to be wasted more than non-TBA products.7
AD is associated with low environmental savings and is not the most suitable technology to harness the full potential of surplus bread, a rich source of food-grade glucose. Recently, some innovative work to valorise bread waste has been started. For example, some breweries in the UK have started using bread waste to substitute malted barley to act as a source of sugar for fermentation in beer manufacturing. In 2017, Greencore collaborated with Toast Ale to provide bread waste from sandwich production to the Adnams brewery, where 25–28% of original malt was replaced with dried bread.10,16,17 Bute Brew Co. in Scotland proposed a similar approach in 2018, with unsold bread replacing between 20 and 25% of the malted barley used in the production of 5.1% alcohol crafted beer Thoroughbread.18 In both these cases, the proportion of malt that can be replaced is strictly limited, as the barley contains natural enzymes that can break down bread starch to fermentable sugars, meaning replacing more than around 25% of the malt is impractical. As the amount of malted barley decreases, supplementation of external enzymes for gelatinization and saccharification increases. In a study conducted by Immonen and associates, to recycle the waste bread for production of fresh wheat bread, they observed that bread waste processed or fermented with lactic acid bacteria (Weissella confusa A16), provided better softness and viscosity to the dough in comparison to the untreated bread waste. Further to fermentation, lactic acid bacteria produced dextran or β-glucan exopolysaccharides, and yielded residual glucose or fructose. Therefore, no extra sugar supplementation was required for yeast leavening. Further fermentation with LAB strains increased the acidic nature of the bread, thus improving hygiene.19 Efforts have also been initiated to use glucose from bread waste for manufacturing pharmaceutical molecules. GlaxoSmithKline (GSK), Veolia and the Biorenewables Development Centre, York are collaboratively working on a project to investigate the production of one of GSK's pharmaceutical active compounds from bread by-products instead of directly from wheat.20
First-generation biorefineries using edible feedstocks such as corn, sugarcane, vegetable oils etc. for fuels and chemicals through fermentative routes have been quite successful and generate a billion litres of biofuels every year. However, the primary concern with an exponentially growing human population is that if edible raw materials are utilized to produce fuels & chemicals, then there may be a shortage of food commodities to fulfil the demand of the global population.23,24 To avoid the food versus fuel/resource debate, the focus of biorefining studies has generally been diverted towards non-edible feedstocks. However, most non-edible feedstocks require energy-intensive harsh pre-treatments and are associated with impurities and inhibitors such as organic acids, furan, and lignin derivatives. As an alternative, bread waste is a starchy material and a clean source of fermentable sugars and proteins. Typically 100 g of bread contains around 50–70 g carbohydrate, 8–10 g protein, 1–5 g fat and traces of phosphorus.25 Sugars and amino acids can be obtained from bread via enzymatic hydrolysis, which has several outstanding advantages including mild reaction conditions, avoidance of toxic chemical usage and minimal/no risks of generation of fermentation inhibitors.26 Furthermore, the composition of bread/bakery waste is consistent and homogeneous and the obtained sugars are as good as pure sugars. All of these reasons make bread waste an attractive potential feedstock for microbial fermentative production of a broad range of products, including industrially essential chemicals and fuels with high market values (Fig. 1).27 Sugar hydrolysates from bread waste are generally devoid of growth inhibitors and can be a suitable medium for the growth of the majority of microbial chassis strains with commercial potential to produce high value-added chemicals and fuels such as 2-keto-D-gluconic acid,28 lactic acid,29 succinic acid,26 pigments,30 aromatic compounds,31 and ethanol.32 Currently, most of these chemicals are manufactured through a petrochemical-based route and are therefore associated with adverse environmental performance. On the other hand, if bread waste-based sugar was employed for fermentative production of chemical building blocks, it will result in an economical process and contribute towards a more sustainable environment.
Table 1 summarises the fermentative production of high-value products, including fuels, chemicals, enzymes and edible materials from bread waste in the last 5–10 years. Leung and associates (2012) used bread waste for fermentative production of succinic acid (SA), a top platform chemical, and observed an accumulation of 47.3 g L−1 SA with a yield of 0.55 g SA per g bread.26 This was followed by the work of Gadkari et al., who carried out a LCA of the bioprocess and found a better environmental profile and significantly lower NREU (non-eenewable energy units) in comparison to fossil-based SA production.33 Sadaf and associates employed different modes of fermentation using bread waste containing 598 mg g−1 reducing sugars for lactic acid (LA) production by several lactic acid bacteria. Like SA, LA is another platform chemical as per the revised list by the US Department of Energy. The three different strains of Lactobacillus paracasei SKL-9, SKL-11 and SKL-21 could produce 26.4, 28, and 27 g L−1 LA via simultaneous saccharification and fermentation with conversion yields of 53, 56, and 54 mg LA per g bread waste, respectively. In the case of solid state fermentation, LA accumulated by SKL-9, SKL-11 and SKL-21 was 212, 223, and 250 mg LA per g bread waste respectively.29 The latest published work by Maina et al. (2021) demonstrates the production of 2,3-butanediol (BDO) and acetoin, two commercially important chemicals with huge significant market potential, from bread waste hydrolysate by Bacillus amyloliquefaciens. The fed-batch culture with bread hydrolysate at a kLa (volumetric oxygen transfer coefficient) of 110 h−1 resulted in a mixture of acetoin and of meso- and D-isomers of BDO with a total concentration of 103.9 g L−1 while at higher kLa (200 h−1) there was a shift in metabolism and acetoin (65.9 g L−1) was the main product.34 In a recent study, Torabi et al. (2020), employed bread waste as a feedstock for bioethanol production by Saccharomyces cerevisieae. They reported high glucose yield from acid as well as enzymatic hydrolysis of bread waste. The ethanol yields achieved using glucose obtained from acid and enzymatic hydrolysis were 248 and 313 g per kg of dry bread residues.35 Like ethanol, hydrogen (H2) is also an efficient cleaner biofuel and can be produced through electrochemical, thermochemical and biological processes. In a study Han and associates (2017) saccharified bread waste through enzymatic hydrolysis and utilized the hydrolysate to produce biohydrogen. In this process 103 mL H2 per g bread waste was produced by Biohydrogenbacterium R3 through a two stage saccharification and dark fermentation strategy.36
S. no. | Feedstock | Microorganism | Product | Fermentation mode | Titer | Yielda | Productivity | Reference |
---|---|---|---|---|---|---|---|---|
a Yield: calculated per gram of waste bread saccharified for glucose production. b Yield calculated per gram glucose. | ||||||||
1 | Waste bread hydrolysate | Biohydrogenbacterium R3 | Hydrogen | Separate hydrolysis and fermentation (SHF) | 7482 mL | 103 mL g−1 | 103.91 mL h−1 | 36 |
2 | Waste bread hydrolysate | Saccharomyces cerevisiae | Ethanol | SHF | 58 g L−1 | 0.35 g g−1 | 1.21 g L−1 h−1 | 32 |
3 | Waste bread hydrolysate | Saccharomyces cerevisiae | Ethanol | SHF | 33.9 g L−1 | 0.25 g g−1 | 0.36 g L−1 h−1 | 35 |
4 | Waste bread hydrolysate | Saccharomyces cerevisiae | Ethanol | SHF | 100 g L−1 | 0.35 g g−1 | 10 g L−1 h−1 | 45 |
5 | Waste bread | Lactobacillus paracasei | Lactic acid | Simultaneous saccharification and SSF (solid-state fermentation) | 28 g L−1 | 0.056 g g−1 | 0.58 g L−1 h−1 | 29 |
6 | Waste bread hydrolysate | Actinobacillus succinogenes | Succinic acid | SHF | 47.3 g L−1 | 0.55 g g−1 | 1.12 g L−1 h−1 | 26 |
7 | Bread crumbs | Thraustochytrium sp. AH-2 | Lipids | Submerged fermentation | 390 mg L−1 | 0.03 g g−1 | 2.32 mg L−1 h−1 | 46 |
8 | Waste bread hydrolysate | Bacillis amyloiquefaciens | BDO + acetoin | SHF | 103.9 g L−1 | 0.39b g g−1 | 0.87 g L−1 h−1 | 34 |
9 | Waste bread | Rhizopus oryzae | α-Amylase | SSF | — | 100 units per g | — | 38 |
10 | Waste bread | Rhizopus oryzae | Protease | SSF | — | 2400 units per g | — | 38 |
11 | Waste bread | Aspergillus awamori 2B.361 U2/1 | Glucoamylase | SSF | — | 114 units per g | — | 27 |
12 | Waste bread | Aspergillus awamori 2B.361 U2/1 | Protease | SSF | — | 83.2 units per g | — | 27 |
13 | Waste bread | Enzymatic hydrolysis (α-amylase + glucoamylase) + biotransformation (glucose isomerase) | Glucose–fructose syrup | Sequential hydrolysis and enzymatic biotransformation | — | 0.45 g g−1 (glucose) + 0.4 g g−1 (fructose) | — | 47 |
14 | Waste bread hydrolysate | Aspergillus sp. | Protease | Submerged fermentation | — | 117 units per g | — | 39 |
15 | Waste bread hydrolysate | Aspergillus sp. | Glucoamylase | Submerged fermentation | — | 8 units per g | — | 39 |
Bread waste has been recycled for manufacturing edible products. There are examples where breweries have started to utilize unused bread waste in the beer brewing process.17,18 For example, Jaw Brewery Limited in Scotland uses bread waste from Thomas Auld and sons Ltd (Aulds bakery) to create low-alcohol beer.18 In another study, Gmoser et al. (2020) enhanced the value of stale bread by transforming it into a nutrient enriched product through solid state fermentation using edible filamentous fungi Neurospora intermedia and Rhizopus oryzae. The protein content increased from 16.5% in stale sourdough bread to 21.1% in the final fermented product with a improved amino acid profile. In addition, an increment in dietary fiber, minerals (Cu, Fe, and Zn), α-linolenic acid, vitamin E and ergocalciferol (D2) was noticed.37 Besides biochemicals and biofuels, industrial enzymes such as α-amylase, glucoamylase, and proteases have also been accumulated on bread waste via solid state fermentation. Benabda and Associates (2019) used fungus Rhizopus oryzae to produce hydrolytic enzymes α-amylase (100 U g−1) and protease (2400 U g−1) from bread waste.38 Similarly, Haque et al. (2016) made use of filamentous fungus Monascus purpureus to generate hydrolytic enzymes glucoamylase (8 U g−1) and protease (117 U g−1) from bread waste.39
Ethanol has several interesting properties as a biofuel and offers a number of advantages, such as a high-octane number, high heat vaporization, and combustion efficiency. In addition, bioethanol is less toxic and readily biodegradable and contains negligible amounts of sulphur.40 Currently, it is blended with petrol at up to 10% levels in many countries, including the UK. The demand for ethanol has also increased during the current COVID19 pandemic due to its application as a sanitiser and disinfectant. Recently, the UK Government has decided to ban all new petrol and diesel-based cars from 2030 to cut down carbon emission to meet the zero-emission goal by 2050. Bioethanol production uses energy from only renewable sources and contributes towards a carbon-neutral environment. It is anticipated that the demand will go further up in future because bioethanol is a low carbon fuel and can play a key role in decarbonisation of the transport sector. Therefore, it can make a significant contribution to achieving the renewable energy requirements and zero emission targets of the UK by 2050. However, ethanol production is currently not promising in the UK. Bioethanol production started in 2007 in the UK and increased from 29 million litres in 2011 to 645 million litres in 2017,41 but has plummeted in recent years, with plants regularly decommissioned for extended periods. All the ethanol in the UK comes from edible feedstocks such as sugar beet and wheat. The actual UK bioethanol production has generally been significantly lower than production capacity. Some of the bioethanol plants in the UK have ceased production for prolonged periods (Crop Energies, Teesside and Vivergo, Hull) due to low prices elsewhere, leading to high levels of importation of bioethanol. These two production plants have a production capacity of 820 million litres, constituting about 85% of the UK's installed bioethanol production capacity.42 Long-term sustainability of bioethanol production in the UK can be achieved through utilization of waste biomass rich in renewable and fermentable carbon such as bread waste and can also help in mitigating environmental impacts and align with the EU policy.43 There have been a few studies where ethanol has been generated using bread waste as feedstock, and an ethanol yield of ∼350 g per kg of bread has been achieved.32 Based on this, around 102200 tonnes or 129.5 million litres of ethanol can be manufactured from annual UK bread waste and this would meet 15–20% annual demand for bioethanol in the UK. This will not only solve the problem of bread waste management but also generate extra revenues for bakeries and make the UK independent in terms of bioethanol production. Moreover, bread waste can also be utilized to produce other platform and fine chemicals, fuels and enzymes (Table 1), and the technology can be implemented to different kinds of similar single-source wastes. Thus, we propose recycling bread waste into chemical building blocks or ethanol through fermentative routes with a circular biorefining approach to maximize resource recovery, and environmental savings and eliminate waste to zero levels.
Currently, diverse microorganisms like bacteria (Actinobacillus succinogenes),26 yeast (Saccharomyces cerevisiae),32,35 and fungi (Aspergillus terreus)44 have been employed as microbial cell factories for bioconversion of starchy feedstocks like food, bakery and bread wastes into valuable products such as succinic acid, ethanol, and itaconic acid. Further, rational and process intensification approaches can be implemented to sustain the economic feasibility. Understanding the efficiency of bread waste as a sustainable feedstock, Dr Vinod Kumar’s group at the School of Water, Energy and Environment, Cranfield University, United Kingdom, is developing a circular bioeconomy approach towards valorising waste bread for the production of high value-added chemicals and fuels.
One final word of caution when using bread waste as the feedstock would be to consider the balance of GHG emissions through a complete life cycle analysis (LCA). Whilst it is the case that the use of waste as a feedstock diminishes the waste deposition at landfill sites and therefore, a reduction in amount the GHG emissions. But accepting the TBA agreement results in additional transportation for the collection of bread waste which may further contribute to increased GHG emissions. Of course, regardless of the disposal route, bread waste must likely be transported, but careful attention must be paid to the levels of transportation required for different processing routes. The use of bread waste for fermentation must be carefully monitored, and since specific processes such as ethanol fermentation produce carbon dioxide as a by-product, care must be taken to not add to the process carbon footprint though this CO2 production. Although fermentative CO2 is relatively pure, however, it could be relatively quickly contained and captured from the process. Throughout the world, various research groups in academia and industry are investigating technologies that can absorb CO2 from the atmosphere and either store it long term, use it directly in products such as carbonated beverages or convert it to long-chain alcohols and alkenes, which constitutes synthetic fuels. Hence bread waste can be a suitable and sustainable feedstock to produce value-added chemicals, biofuels, and synthetic fuels.
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